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(Chapt. 6.

On Natural Selection

How will the struggle for existence, which we have discussed in the last chapter, act? Annually during thousands on thousands of generations, multitudes have been born more than can survive to maturity. The least possible weight will have turned the balance which shall have lived & which dies. Look at the young in the same litter or nest, something must determine which shall live & procreate its kind. If two beings were absolutely alike in all respects, during the whole course of their lives, it might be truly said to be chance, which of the two should come to maturity & procreate their kind.

But such absolute identity can hardly be predicated of any living beings; & certainly, as we have seen in the fourth chapter, there is a considerable amount of variability in nature.

A large proportion of the variation, which does occur, may be quite unimportant for the welfare of the any particular organism, & such variation would not in the least be affected by the struggle for existence.

On the other hand, any variation, however infinitely slight, if it did promote during any part

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of life even in the slightest degree, the welfare of the being, such variation would tend to be preserved or selected. I do not say that it would always be invariably selected, but that an individual so characterised would have a better chance of surviving.

If we reflect on the infinitely numerous & odd variations in all parts of the structure of those few animals & plants, on which man have may be said to have experimentised on by domestication, it would be & again on the many, though slight variations which have been noticed in a state of nature, it would be most strange if in the course of thousands of generations, none of them poss not one variation added to the welfare of some varying organic being; We should bear in more especially if in thinking of this we should bear keep in mind how multifarious, singular, & complex the relations for each living being are in habits & structure to other organic beings & to climate, both for securing food & escaping various many dangers, during the various stages of life. Again we should bear in mind that whole treatises have been written, descr showing what numerous, what trifling, what strange peculiarities are inherited, or tend

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to be inherited, that is appear in some of the offspring or reappear in their descendants. Any variation therefore, which from slightly profiting an individual was selected. An individual, therefore, which from having some slight profitable variation, was preserved or naturally selected, would in most many cases, tend to transmit the new, though slight modification to its offspring.

Moreover the causes, which from their extremely complex nature we are forced generally to call mere chance, which produced the first variation in question would under the same conditions often continue to act; & assuredly these causes would be eminently likely to act on individuals having some inherited tendency however slight, in this same direction: so that the cause of the variations & inheritances would act & react on each other, thus giving fresh & fresh opportunity for natural selection to seize on & preserve the useful whatever modification of structure or habit, or constitution, was in any degree useful.

(a) text

I can hardly imagine any modification change in structure or con habits &c so slight that it might not be useful to an individual

[3v]

(a) p 3 text

(On the other hand, any modification if in the slightest degree injurious would be rigidly destroyed. In the struggle for existence, during the long course of generations, individuals thus characterised, would have a very poor chance of surviving. Even if most the inju injurious modification from the nature of the conditions, or from a strong principle of inheritance, appeared again & again, they again it would be rigidly rejected again & again.)

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of a species, & be thus hence be selected. It seems at first to be simple chance which individual insect shall fall a prey to a birds; yet birds are guided by their eye-sight; & if all individual insect who alarmed shammed death a little longer or most effectually when it might; & we so often see leaf-eating insects are green or those living on bark, mottled-brown, we may believe that a slight change in the shade of colour, which would tend to be inherited, might in the long run cause such individuals t better to escape destruction & leave offspring with the same inherited shade tint. Colour is thought an any trifling unimportant character by naturalists; but when we see we remember that the increase number only our game birds seems to be more influenced by that destruction of beast of prey, than from any other one cause, we must see the importance of the ptarmigan being white, we see, or as it has been metaphorically fancifully* said that "the ptarmigan is being is lichen in summer & snow in winter that show, the red-grouse is heather, & black-grouse peaty earth"; & when we remember the main check to the increase of our game birds, is owing to birds & beasts of prey, I can see no reason to doubt that in birds varying in colour as does the red-grouse, that the finest tints of colour might be selected owing to such birds individuals suffering less. Such selection would no perhaps

[4v]

* See, also, some good remarks on the colour of those birds giving them a better chance of escape from birds of prey by Mr. C. St. John in his Tour in Sutherland-shire, vol. 2, p. 179.—

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be more frequently readily be effected with birds & insects when they invaded a new district, or slightly changed their habits, which certainly occurs, as we see with insects attacking our exotic plants. I observe in many German & French pigeon-books, that people are cautioned not to keep white flying pigeons, as they suffer much the most from hawks. Nor let it be said that the occasional destruction of individuals of a particular colour could have no influence on the colour of the whole body mass; for it is well known how effective is the destruction of any lamb with a tinge of black in keeping the flock pure.

Again take a beast of prey, pressed for food owing to the destruction of by a dearth of the animals on which it feeds; what a trifle will determine which shall survive; the least superiority in power of scent, a shade of colour so as to be less conspicuous, (I have noticed that a prowling white-piebald cat is manifestly far easier seen by birds than a tabby), the power of springing an inch further may well determine its success,

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when success life depends on success: in such cases one meal lost may be the turning point; here it may be truly said that the last straw breaks the horse's camel's back. (a) Or again look at the surprisingly large annual destruction of shrews by cats & other animals either by mistake or for sport, as shown by the number found killed but not devoured on our gravel-walks: supposing for the moment that this destruction is a main check to the increase of shrews, can may we doubt not believe that any an individual born by chance with an inheritable stronger odour, & so a little more repugnant to the prowling cat beast of prey would tend to escape would have a better chance of escaping; & from this one individual others still more offensive might be selected, till a shrew was made formed with as an odour intolerable ordour as insufferable to man & beast, as that of some foreign allied animals.

[For readability, the following paragraph which Darwin crossed is left undeleted.]

Hence I cannot doubt that the most trifling characters if at all influential for the welfare of the being would be steadily tend to be selected; & all modification in opposite direction rigidly destroyed.

A sudden or great variation most rarely, some will

[6v]

(a) And success will depend not only on the vigour of the moment, but often on the condition in which the animal has been able to keep itself during several previous months.

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(a) p. 7

Natural selection will may act at any time period time of life; for variations appearing at any time one period tend, as we have seen, to reappear at the corresponding time period: thus the peculiarities in the caterpillar or coccoon of the silk-moth are inherited; & any modification in a caterpillar or coccoon useful to it, might be naturally selected & made permanent. (a) So again the embryo might be modified by selection in relation to the parental mother's womb: in Yorkshire according to that excellent writer Marshall, big-buttocked calves were selected, but until they were found to destroy many cows during calving, & thus a deviation of this kind, if left to nature, would be soon eliminated: on the other hand if this deviation were useful in any way to the embryo, or to the calf after its birth, no doubt in the course of time the parental structure might become modified by selection to allow of such t such births of these constructed young; for facility in parturition is undoubtedly hereditary.(B)

So again any modification in either sex separately, whether useful to that sex alone, or in functional relation to the other sex, or to the flock or to the young, might be selected & become attached to that sex alone; for

[Stauffer noted that "Folios 7b to 7k are missing".]

[7av]

(a) and so it might be through however many stages of existence, or alternations of generations to use Steenstrup's perhaps not very correct expression,* any animal might may pass. Thus also,

(* See the wonderful facts given in Steenstrup's most interesting work, translated and published by the Ray Society.)

[7av2]

(B) In the short-faced Tumbler pigeons, the beak has been rendered so short by long-continued selection, that Mr. Eaton* (A Treatise on the Almond Tumbler. 1851 p. 33) says he is "convinced that better head & beak birds have perished in the shell than ever were hatched, the reason is that this amazingly short-faced bird cannot reach the shell with its beak, & perishes in the shell if the Fancier does not extricate it." But by long-continued selection a shell thinner at the right end might be per be naturally obtained, for we know that the eggs [of the] common Hen often vary in thickness.

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say never, occurs in nature; but if it did, & were usef profitable it of course would be selected; but small modifications, let them appear ever so trifling, if in the least influential on the welfare of the being, I can see no reason to doubt after the most careful consideration to doubt would ha tend to be preserved or selected. They would, also, tend to be inherited; & slight modification might thus be added to slight modification in any given direction useful to the animal;—just as in our domestic animals & plants modifications useful to man have been added together & rendered permanent by artificial selection. (a)

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creation of each living thing endowed with a small limit of variability, or with the theory of a great amount of slow modification; & it will be the object of this work in the latter chapters to make this comparison. But for the present, in order to explain my principles, I must assume that there is no limit or no close limit to variation during the long course of ages.

From what we have seen in the first chapter the main cause of variability seems to lie in a change of the conditions of existence, perhaps aided by abundant food. That many countries have suffered great changes during the same geological period, that is within the period of existence of the majority of the same species, no geologist will dispute. Reflect for a moment on the vast changes of climate & of the level of the sea, during the glacial epoch,—a mere sub-division of one geological period. & consider Now let us

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take the case of a country subjected to some slight climatal or other change; the proportional numbers of its inhabitants will be altered & now better adapted organic beings better adapted to the new climate will flow in from the surrounding countries, as they certainly did into Europe during the glacial epoch. But if the country were cut off by some impassable barrier, from the adjoining warmer or colder or dryer countries, as the case might be, or if one supposed country was an island, then new beings could not immigrate, & tha fewer of the old inhabitants (supposing the change not to be so great to exterminate all them) would be exterminated for there would not be new ones beings to take their place; but the majority would suffer in some degree & then would decrease in numbers; but some few, which under the old conditions had just were on the confines of their ranges were previously just able to reach so far south (supposing for the moment that the change was from warmer to colder) under the new conditions would be favoured & would increase in numbers. Bearing in mind how intimately each organism is related to other organisms, & even to the proportional individual numbers of each, for one organic being in large numbers may well be far more influential for good or evil to another, than if in small numbers, there can, I think, be no doubt, I think

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that in our imaginary country the selections of nearly every organ inhabitant would be seriously disturbed both by the change of climate, & more especially from by the changed proportions of the other inhabitants & by destruction of some few. (a) Moreover the changed conditions of existence would tend to make some of the tame organic beings more variable than heretofore. Under such circumstances, it seems to me that it would be quite extraordinary, if at least in some few at least of va the slightly varying organisms, no profitable variations better fitted for the new & complex combination of conditions did not occurred. (B) If such did occur, I cannot doubt but that it would be slowly though steadily selected; & the variety thus selected would gaining strength & increase in numbers.

Under one imaginary case, which undoubtedly the above circumstances, which though imaginary must repeatedly have occurred in the world's history, the conditions would so probably be most favourable for some rapid selection & consequent modification of forms; nevertheless I think we may conclude that there does not exist a land in which the process may not be going on slowly.

Everywhere organic beings present individual differences,

[10v]

inhabitants; for we have before seen, that the severest struggle, leading even to the extermination of one, often lies between closely allied & therefore very similar species of the same genus.—If any such profitable modification did

[pencil lines, partially erased:] for we have seen that the several slight changes to its [illeg] elsewhere [3 words illeg] has become species of the same genus

[10vv]

(a) Each being would be placed under conditions, such as the world had never exactly seen before.

(B) A very slight modification would often suffice to give some advantage between the struggling [text excised] [see 10v]

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& some few more marked variations. No country can be named in which all the inhabitants are perfectly adapted to its conditions of existence: this may seem a b rash assertion, but I think it can be fully justified. Each being in its native country no doubt is adapted to its conditions of existence as perfectly as the other coinhabitants, in proportion to the average number of the individuals of its kind; but not one country, still less not one island can be named which does not possess many organic beings naturalised thoroughily well naturalised as far as we can judge.*a Now does this not show, that in the natural polity of each land there are were places open, which could be filled by other beings more perfect, not by any ideal standard, but by actual proof, in relation to the previous inhabitants & to the climatal conditions of that land? Nor let it be said that individual differences are so slight that the most careful selection could make no sensible change by adding them up during a long course of ages; for man, even during mere scores of years, has certainly thus acted on differences so slight

[11v]

So has Bunbury in Linn Transact

[11vv]

*a M. Alph. De Candolle has insisted strongly on this fact of the ⸮ universality of naturalised plants & has drawn the foregoing inference from it. The number of naturalised plants in Europe & N. America is probably in great part due to great changes effected by agriculture; but I think Sir C. Lyell has con shown that the agency action of man on other organic beings though more potent, does not differ essentially from that of t any other animal when introduced a the naturally or into a new country. In the case of many plants naturalised in desert islands sterile the uncultivated parts of many islands, man has probably in no ways influenced the conditions. No one will assert that the natural cat, rat existence of the cat, rat &c in New Zealand, of introduced monkeys in Cape de Verde Isld —of horses & cattle in La Plata &c &c is due to changes effected in the natural state of their countries through man's intervention.

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as to be inappreciable except by an eye long educated. Therefore I conclude that there is no land, so well stocked with or organic beings, or so fixed with conditions so unvarying, but that in the course of time, natural selection might modify some few of the inhabitants & adapt them better to their place in the great scheme of nature. I may here add that hereafter we shall show good reason for believing that it is not the oppressed & decreasing forms which will tend to be modified, but the triumphant, which are increasing in numbers, already very [numerous] extending their range, & coming into new relations, in individuals, widely diffused in their own country & inhabiting many countries, which are most variable & so will be most apt to be modified & so become under new forms still more triumphant.

I wish so much to make it clear how as I suppose believe natural selection acts, that I must be permitted to give an imaginary example or two. Let us take the case of a wolf, a variable which preys on various animals, & let us suppose that at the period when its prey was scarcest, securing some by craft, some by strength & some by fleetness; & let us suppose that the fleetest prey had, a deer for instance had from any change whatever increased in numbers, or that the other prey had decreased in numbers during that period season of the year, when the wolf is hardest pressed for food; I can under such circumstance see no

[12A]

(Illustrations of the action of natural selection.

In order to make it clear how I believe natural selection acts, I must beg permission to give one or two imaginary illustrations.

Let us take the case

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reason to doubt that the swiftest & slimmest wolves would in the long run be preserved & selected; always providing provided that they retained strength to master their prey at this period or some other period of the year when compelled to perhaps prey on some other animals. (a) The same process would tend to modify the deer in order to escape the wolf slowly rendered fleeter; though it might happen that some other & incompatible select modification might be more important to this animal, as getting food in during some other season. Even without any change in the proportional numbers of the animals on which the wolf preyed, a single cub might be born with an innate tendency either of instinct or structure leading it to pursue certain prey; nor can this be thought very improbable seeing that of our cats, one naturally takes to catch rats & another mice, & according to the excellent observer Mr. St. John (Wild Sports & Nat. History of the Highlands. 1846, p. 40) one in wild district to brin bring home winged game, another hares & rabbits, & one another to hunt on marshy ground & almost nightly to catch woodcocks & snipes, how if any this innate slight change of habit or structure benefitted our wolf, it would must be more likely to survive & procreate many young, than the other wolves; & some of its young would

[13v]

(a) I can see no more reason to doubt this, than that the Breeder can greatly improve the fleetness of his greyhounds by long-continued & careful selection.—

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probably inherit the same tendency, tend to be preserved & thus a new variety possibly formed from & thus a new variety might be formed, which would either supplant or coexist with the parent form.

Or again with our wolves, those inhabiting in a mountainous district might readily be led chiefly to hunt different prey from those on the lowlands; & from the continued selection of the best fitted individuals in the two sites two varieties might slowly be formed, which would, cross & blend where they met, but to this subject of intercrossing we shall soon have to return; as we shall likewise to the effect of mere hereditary habit: now it is asserted that in I may add that according to Mr. Pierce there are two varieties of the wolf in the Catskill Mountains in the United States, there are two varieties of the wolf one of which the shepherds one with a light grey-hound like form which pursues deer, & the other more bulky with shorter legs & which more frequently attacks the shepherds flocks.)

(If the individual numbers of a plant depended chiefly on the wide dispersion of its seed, so that some might fall on a proper site, I can see no any plant which had its seed furnished with pappus a little better adapted to carry it, be wafted; or with pulp more agreeable to Birds, would have a better chance of being dropped where it could germinate & producing reproduce its kind; & I can see no reason why nature should not thus select the most dispersable seed than that gardener should be able to go on selecting varieties having some more & more differences in seed, pod, or fruit.

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Take Let us now take a more complex case: some plants excrete a sweet juice apparently for the elimination alone of something injurious from their sap, as in the case of the glands at the base of the stipules of some bracts from the Leguminosæ; & this juice is greedily sought by insects. Let us suppose the juice to be excreted at the inner bases of the petals, & in these in this case insects in seeking the juice would brush off pollen & be apt to carry it get dusted with pollen, & carry it on to the stigmas of other flowers of the same kind, & so cross them: this, as we have every reason to believe, would make more vigorous seedlings which would have the best chance of surviving surviving; & some of these seedlings would probably which would tend to inherit the nectar-excreting power; & those flo individual flowers which excreted most nectar would be most visited by insects, more & oftener crossed, & so in the long run would gain the upper hand. In order to increase the amount of nectar, the glands nectaries & with them the petals might become modified, as well as the position of stamens & pistils to any extent in relation to the particular insect which visited the flower; some insects like ants being of not the slightest service to the plant; others as Bees most being very useful in facilitating intercrosses. We might have taken for our example, insects devouring pollen instead of nectar, & as pollen is formed for a definite object its destruction appears at first a simple loss to the plant; yet if although if nine tenths were destroyed, but a little was occasionally or habitually carried to another plant, owing to the the many visits of the pollen-devouring

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insects, & a cross thus effected, although nine-tenths were of the pollen was were destroyed, it might well still be a great gain to the plants; & those individuals with which produced more & more pollen & had larger & larger anthers might would be selected. Indeed this process of selection of larger & larger anthers might be carried on, merely that the plant some of the pollen might escape destruction, without any indirect advantage being gained by the pollen being robbed, just like in the same manner as many plants probably produce thousands of seeds, that in order that a some few may e escape destruction.—)

(When one plant had by natural selection been rendered so attractive to insects, as that unintentionally on their part they regularly carried pollen from flower to flower; & how effectually they (a)

do this the result of Kölreuters experiment artificial fertilisation of flowers, the same number being left to insects, clearly shows; then another process might commence. No naturalist doubts the impor advantage of what has been called the "physiological division of labour"; the hence we must may believe that it would be an advantage to a plant to produce only male organs in one flower or one whole plant, & only female flow organs in another. flower If then an individual plant tended to fail, either in all in either sex in the different flowers of the or some of its flowers of the same individual, or on all the flowers on different individuals; nor does this seem if its flowers in either in female or male sex, or amongst compensation in b or in both sexes

[16v]

(a) do this, I could easily show by many striking facts,

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very improbable (a) seeing as we have in the third chapter how the two sexes products of plants are differently affected by changed conditions; then if this & seeing in naturehow many gradations there are between dioicous, monoicous & polygamous plants; then if this incipient division of labour profited the plant in the least degree, it might be increased by nat natural selection, until one plant had separated sexes.)

(Lastly let us turn to nectar-feeding insects in our imaginary case: let us suppose that the plants of which we have been slowly increasing the nectar by continued selection was a common plant, & that certain insects depended in main part on its nectar for food. Now (B)

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

bearing in mind the facts given in the third chapter, showing how eager bees are to save time with what surprising rapidity bees visit flowers, & how eager that are to save time, as shown by their cutting holes at the base of flowers, when they are at all troublesome difficult to enter (thus robbing the flowers without profiting them),

I can see no reason to doubt that an any slight accidental deviation in form the size or form of the body, far too slight to be appreciated, either of whole body or in the curvature or length of the proboscis &c might profit [illeg]a moth, fly or Bee, so that such an individual so characterised would more rapidly obtain food & so

[17v]

(a), as it can be shown that the two sexes in the same flower are sometimes rendered sterile in different degrees, when the plant is exposed to changed conditions of life, & as we see in

[17v2]

(B) I could give many facts, showing how eager Bees are to save time, & to visit the flowers as rapidly as possible—for instance their habit of cutting holes at the bases of flowers, which they can enter with a little trouble—bearing this in mind,

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have a better chance of living & leaving descendants with a tendency to a similar slight deviation of structure. (a) Thus I can understand how a flower & Bee might slowly become either contemporaneously or one after the other modified & adapted in the most perfect manner to each other.

I am well aware that the doctrine of natural selection exemplified in the above imaginary examples, is open to the same objections, which were at first proved launched out against Sir Charles Lyell's noble views on "the modern changes of the Earth, as illustrations of geology", but we now very seldom hear, the action of the coast-waves, for instance, called a niggling trifling & insignificant cause, as applied to the excavation of gigantic valleys or to the formation of the longest lines of inland cliffs. In our imaginary examples, it may also be observed that natural selection can act only by the preservation & addition of infinitesimally small inherited modifications each profitable to the preserved being; but as modern geology has almost banished such views as the excavation of a great valley by a single great diluvial wave, or cataclysms desolating the world, so will Natural Selection, if it be a true principle, banish the belief of the continued creation of new organic forms, or at any great & sudden modifications in their structure.)

[Origin, pp. 95-6]

[18v]

(a) For instance the tube of the corolla of the common red Clover & the tall incarnate clovers do not on a hasty inspection glance appear very different in length; but the Hive-bees can easily suck the nectar out of the latter, but not out of the common red clover; so that whole fields of the plant offer their precious nectar in vain to one useful Hive-bee. , on which the welfare of the community depends, in vain to our Hive-bees. On the other hand I can have elsewhere experimentally shown that the fertility of clover is depends in the closest closest manner on the visits of Bees, which by moving a flower moving parts of the corolla push the pollen on to the stigmatic surfaces. & no doubt occasionally

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of the continued creation of new organic forms, & subsequently of any subsequent, great & sudden modifications in their structure.

We must add to the effects of natural selection, the direct effect action, probably very small & almost certainly slow, of external climatal conditions; —we must not forget, & I believe this to be of very wide application, that in the modification of one part, either during the same or during an earlier period of life, other parts will be altered according to the complex & unknown laws of the correlation of structure, for instance a modifi selected modification of the larvæ would almost certainly influence mature forms;—we must allow something in the higher animals for the effect of habit & disuse, of which again the action must be always slow;—but over all their causes of change, I fully believe am convinced that Natural Selection is paramount.

Comparison of Man'snature's selection with man's selection. From the facts given in the two first chapters, it cannot be doubted that man can do, & has done, much in the modification of animals & plants by the artificial selection of variations. But he labours under great disadvantages: he selects only by the eye & all acts therefore on th external characters alone: he cannot perceive slight constitutional differences; nor the course of every

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nerve & vessel: he can by no means tell whether all parts & organs are correlated perfectly, but only so far that life & tolerable health are preserved. Far from allowing each animal & plant being to struggle for life; he protects then each to the utmost of his power, both during youth & times of dearth & from all enemies. Even when most carefully Instead of selecting steadily from generation to generation, he only occasionally selects; & his judgement is often bad or capricious: he & his successors never go on selecting for the same precise object for thousands of generations. Even when most carefully selecting he sometimes grudges to destroy an animal, imperfect in some respect, from as it comes up to his standard in some other respect. Each being is not allowed to live its full term of life & procreate its kind, according to its own capacity for living to exist. He does not always allow the most vigorous males to be the fathers of their breed. He often begins his selection with some striking abnormal form, differing widely from anything observed in nature, & of no use to the being selected. He often From migrations, changes of agriculture &c, he often unintentionally changes the conditions to which his products are

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exposed; or intentionally crosses them with individuals brought from another district or country, as was done in the darkest ages. He does not selects any peculiarity or quality which pleases or is useful to him, regardless whether it profits the being & whether it is the best possible adaptation to the conditions to which the being is exposed: nor does he regularly exercise the selected quality peculiarity: he selects a long-backed dog, or long-beaked birds & trains it to no particular course of life;—he selects a small dog or bird & feeds it highly;—a long limbed animal & exercises its fleetness only occasionally or not at all like the Italian greyhound. And lastly, to repeat, he can judge by external characters alone, & not from the perfect action & correlation of the whole organisation during the whole course of life.—

See how differently Nature acts! By nature, I mean the laws ordained by God to govern the Universe. She cares not for mere external appearance; she may be said to scrutinise with a severe eye, every nerve, vessel & muscle; every habit, instinct, shade of constitution,—the whole machinery of the organisation. There will be here no

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caprice, no favouring: the good will be preserved & the bad rigidly destroyed, for good & bad are all exposed during some period of life growth or during some generation, to a severe struggle for life. & death. Each being will live to the its full term & procreate its kind, according to its capacity to obtain food & escape danger. Nature will never select any modification without it gives some advantage to the selected being over its progenitors under the conditions to which it is exposed.

Every organ & part selected change will be fully & regularly exercised. Nature will not commence with some half-monstrous & useless form; but she will act by adding up deviations so slight as to be hardly or not at all appreciable by the human eye. Natural conditions remain constant for enormous periods, or generally change very slowly, so will, so will the consequent variability be slight, & the selection very slow. Nature is prodigal of time, & can act on thousands of thousands generations: she is prodigal of the forms of life,

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if the right variation does not occur under changing conditions so as to be selected & profit any any one being, that form will be utterly exterminated as myriads have been. No complications are too great for nature: a contingency happening once in a thousand generations may lead to the extermination of a variety: she can gradually select, either simultaneously or successively, slight changes adapting the selected variety to a score of other beings, most widely apart in the great scale of nature.

Can we wonder then, that nature's productions bear the stamp of a far higher perfection than man's product by artificial selection. With nature the most gradual, & steady, & unerring, deep-sighted selection,— the most perfect adaption to the conditions of existence,—the direct action of such conditions—the long-continued effects of habit & perfect training, all concur during thousands of generations. Here we meet with no hereditary useless monsters. All who have reared animals & plants believe that trueness is dependent on long-continued & careful selection, & on exposure to the same conditions. How incomparably truer, then, must nature's varieties, called by us species

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when strongly marked, these mean the domestic be, when compared with the varieties reared by man. Now trueness or the absence of variability, is the the the most important characteristic mark of a species in contrast with a variety, second only to the sterility of hybrids, if & not second to this in the eyes of some, as Gaertner & Herbert whose studies would naturally have led them to attribute the greatest importance to the laws of breeding. Last Finally, these If If we admit, as we must admit, that some few organic beings have once been were originally created, which were endowed with th a high power of generation, & with the capacity for some slight inheritable variability, then I can see no limit to the wondrous & harmonious results which in the course of time can be perfected by through natural selection.

It may, perhaps, be here worth notice, that amongst barbarous nations, there will be far less little intentional

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

the Breeds of animals kept by savages barbarians natives, there have been said to have to show much more of the characters of true species, than those kept by more civilised man, As

(25

(Ch. 6. Nat selection: mans & nature compared)

selection, & the animals in part great degree will be left to struggle for life by these without aid under but slight conditions nearly constant; & it has been remarked that in such cases the breeds approach much more closely in character to true species, than amongst civilised mankind nations.

Seeing what man has done in a few centuries or a few thousand years at most, I have sometimes wondered that nature considering the perfection of her means has not worked quicker, than geology teaches him us to believe she has [illeg] in the modification of organic beings. But from what has gone before, & from what will soon presently follow, we may see that there are most powerful retarding agencies always at work.

The forms produced by natural selection, if quite modified, will be called species, if only slightly different, will be called varieties; if no further variation occurs in the right direction by which the variety may be further profited, it may remain I can see no reason why it a variety may not remain in that state for during an enormous length lapse of years

(26

(Ch. 6. Nat selection)

of time; , & we have seen in the fourth chapter, that some varieties such as the land-shells in the calcareous superficial beds of P. Santo certainly are of high antiquity.

But that a variety should remain constant during whole geological periods is excessively improbable; for we have seen in our 5th Chapter in how important a manner the structural differences of each organism is most intimately related to those of the other coinhabitants of the same district; & as all these will be constantly ten are struggling for supremacy, & will hence constantly tend to be modified & become improved, if one variety & become improved if one variety lie so constant  fixed as not to vary at all in a fitting direction, & so become through natural selection adapted to those other changing organic forms to which it is related in the polity of nature, it will be exterminated.

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

Extinction

Forms produced by natural selection, if considerably different will be called species; if still more so different, genera & so on. But in these cases, besides inheritance & modification, extinction which will always play a part, will have will have played a very important part in & bring destroyed the destruction at some period of intermediate forms. To win

(26*

(Ch. 6. Extinction)

Extinction. — The general subject of extinction will be discussed in a future chapter on palæontology. But the subject is extinction must be here noticed here, as bearing in a very important manner on the theory of Selection. As man in any country improves his breeds, he neglects the less improved & these gradually disappear.

Hear Youatt* (*Cattle. Library of Useful Knowledge, 1834, p. 248, 199) on the cattle of northern Yorkshire: as at the commencement of the 18th century the ancient black cattle were the only breed. To them succeeded the long-horns, which by degrees spread over the whole of the northern & midland counties; but much valued as they were, they were after a time "swept away, as if some by some strange convulsion of nature". For they had to give way to the short-horns, & these for the last century have maintained their ground; & no doubt will do so, until some better breed if better can be formed, if better can be. So it has been with innumerable varieties of our new cultivated plants; "old sorts being fairly beaten out by new & better ones."* (* Gardener's Chronicle, 1857., p. 235)

Thus it has been, & thus it will be, with man's productions. In nature, the same species existing in two now separated areas, may might become modified in one or both, & the resultant forms might continue, whilst

(26 (a)

(Ch. 6: Extinction)

separated, to exist for any length of time. Such forms are often called by naturalists representative or geographical species, races or varieties: they are maiden knights who have not fought with each other the great battle for life or death. But, whenever from the union of the two areas, they meet, they must & come into competition, if one has the slightest advantage over the other, that other will triumph & the fall decrease in numbers & often or or be quite swept away.

But as we see in a vast number, perhaps in a large majority of cases, that the varieties of the same species, & the species of the same genus, inhabit the same country, or divisions of it not separated by impenetrable barriers, generally the varieties as well as the species will have come into competition with each other & with their parents from an early period or even from the very commencement of their formation; and as a form can be selected by nature solely from having some advantage, at least in the spot where the selection is going on, over its parent form; the parent will be almost infallibly there exterminated by its own offspring.

Hence, we may, I think, safely conclude, that

(26b

(Ch. 6: Extinction: Principle of Divergence)

natural selection (like man's selection) almost necessarily entails a nearly proportional amount of extinction;—one species whilst forming beating out another, & one even the finest variety, over the finest variety if having any kind of advantage over another, taking the place of & exterminating the less favoured & more less modified form variety. It is in each country, a race for life & death; & to win implies that others lose.)

Principle of Divergence.—This principle, which for want of a better name, I have called that of Divergence, has, I believe played a most important part in Natural Selection. To seek light, as in all other cases, by looking to our domestic productions, we may see in those which have varied been longest most from long domestication or cultivation, something closely analogous to our [scattered]principle. In nature. Every new outstanding peculiarity in means

Each new peculiarity either strikes his man's eye as curious or may be useful to him; & he goes on slowly & often unconsciously on selecting the most extreme forms. He has made the race-horse as fleet & slim as possible & goes on trying to make it fleeter; the cart-horse he makes as powerful & muscular as p as he can: he selects his Dorking-fowls for size

(26c

(Ch. 6: Principle of Divergence)

weight & disregards plumage; the Bantam he tries to get as small as possible, with elegant plumage & erect carriage: a pigeon has been born with slightly smaller beak, another with slightly longer beak & wattle, another with longer a crop that more largely a little more inflated than than usual, another with a somewhat larger & expanded tail &c; his eye is struck [illeg] by this peculiar & he goes on selecting each of these peculiarities, & he makes his several breeds of improved tumblers, carriers, pouters, &c fantails &c, all as different or divergent as he can possible from their wild-prototype original parent-stock the rock-pigeon; the intermediate, & in his eyes inferior birds, having been neglected in each generation & now become extinct. It is the same this fashions of his dress, each new fashion ever fluctuating & is carried to an extreme displaces the last; but natural living productions will not so readily bend to his inordinate inordinate caprice. (a text)

(Now in nature, I cannot doubt, that an closely analogous principle, in [illeg] must not liable subject to caprice, & effected by a widely different agency is steadily at work, through a widely different agency; & that varieties of the same species, & species of the same genus, family or order are all, more or less, subjected to this influence. For in any country, a far greater number of individuals descended from the same parents can be supported, when greatly modified or if slightly modified called, if slightly being called varieties when none modified if modified called distinct species, either of the same

[26cv]

a text / Moreover, far more fancy-pigeons will be kept, (I do not mean those kept as food) after they have become div broken up into very distinct breeds, than when fewer & more like similar birds existed; for each fancier likes to keep several kinds, or one fancier keeps one kind & another becomes famous for another breed.)

(26d

(Ch. 6: Principle of Divergence)

in different ways, in habits constitution & structure, as when slightly modified being called varieties, when none being called distinct species of the same or different genera. so as to fill as many places, as possible, in the polity of nature.) than when not at all or only slightly modified.

(We may go further than this, &, independently of the case of forms supposed to have descended from common parents, assert that a greater absolute amount of life can be supported in any country or on the globe; when life is developed under many & widely different forms, than when under a few & allied forms;—the fairest measure of the amount of life, being probably the amount of chemical composition & decomposition & decomposition within a given period. Imagine the case of an island, peopled with only three or four plants of the of the same order all well adapted to their conditions of life, & by as many three or four insects of the same order; the surface of the island would no doubt be pretty well clothed with plants & there would be many individuals of these species & of the few well adapted species if insects; but assuredly there would be times seasons of the year, peculiar & intermediate stations & depths of the soil, decaying organic matter &c, which would not be well searched for food, & the amount of life would be consequently less, than if our island

[26d]

(Ch. 6: Principle of Divergence)

Or distinct distinct genera,— can be supported, when they so as to inhabit

(26E

(Ch. 6: Principle of Divergence)

had been stocked with hundreds of forms, belonging to the most diversified orders.

Practice shows the same result; th farmers all over the world find that they can raise within the period of his their leases most vegetable matter by a rotation of crops; & they choose the most different crops plants for their rotation: the nurseryman often practices a sort of simultaneous rotation in his alternate rows of different vegetables. I presume that it will not be disputed that on a large farm, a greater weight of flesh, bones, and blood could be raised within a given time by keeping cattle, sheep, goats, horses, asses, pigs, rabbits & poultry, than if only cattle had been kept. In regard to plants this has been experimentally proved by Sinclair* (*The author of Hortus Gramineus Woburncnsis, in Loudon's Gardener's Mag. Vol. I., 1826 p 113), who found that land sown with only one only two species of grass, or one kind of grass with clover, bore on an average 470 plants to the square foot; but that when sown, with from 8 to 20 different species, it bore at the rate of about 1000 plants, "& the weight of produce in herbage & in hay was increased in proportion." It is important to observe that the same rule holds for different & not very distinct varieties of the same species when sown together; for M. L. Rousseau, a distinguished practical farmer, found that on sowing several fifteen varieties of wheat

(26f

(Ch. 6 Principle of Divergence)

separately, & the same fifteen kinds mixed together though on rather inferior land, that found on actual measurement that the latter "yielded a much heavier crop than that obtained far to on far better land on which the unmixed wheats were grown for the purpose of the comparative trial." (1) This favourable result may be accounted for by the "different constitutions of the kinds sown (* (1)Gardener's Chronicle & Agricult. Gazette, 1856, p. 859. See, also, p. 858. and 1857, p. 179)

We see on a great scale, the same general law in the natural distribution of organic beings; if we look to an very extremely small area if supposing the conditions to be absolutely uniform & not very peculiar* (a) or unfavourable for life, we seldom find it occupied by any two or three closely allied & best adapted forms, but by a considerable number of extremely diversified forms. Thus To give an example, I allowed the plants on a piece plot of my lawn three feet by four square which was quite uniform & had been treated for years uniformly, to run up to flower; I found the species exactly 20 in number, & as these belonged 18 genera & these to 8 great orders & they were clearly much diversified.* (b) To give another example of a small area but very much larger area, than the last, having singularly uniform conditions of life; namely one of the low & quite flat, cor oceanic coral-islets having a wretched soil, composed exclusively of coral-debris, but with a fine climate; for instance Keeling Atoll, on which I collected nearly every phanerogamic all th plant, & these consisted of 20 species* (c) belonging to 19 genera & to no less than 16 different orders!

(* (c) Described by the Rev. Prof. Henslow in Annals of Nat. Hist., 2. Ser., Vol. I, 1838, p. 337)

[26fv]

* (a) (p. 26 (f) Ch. 6)

Where the conditions are peculiar & small the station small as compared with the whole area of the country, on as Alpine summits; Heaths in salt-marshes, or even common marshes, lakes & rivers, &c.—a great number of individual plants are often supported, belonging to very few species: so it is with Fresh-water shells; so it is with the marine inhabitants of the arctic seas. But even in these cases, though the individuals are appear to be very numerous compared with the species, yet even in these cases, the coinhabitants belong to very different types; for instance Dr. Hooker has marked for us all the plants in Britain, which he thinks may be called truly aquatic: they are,    in number, & they belong to     genera and to     orders.— With respect to the number of individuals to the species, we shall have to return to this subject in our chapter on geographical distribution, & I will here only say that I believe it mainly, but not wholly, depends, on the manufacturing, if I may so express myself, being small in size (& sometimes in duration); that is that the number of individuals is small in comparison with the numbers of individuals of the commoner species which inhabit ordinary stations: & for we have seen in our 4th Ch, that it is common species which vary most most abound in individuals & are much differed & range farthest which oftenest present varieties, or incipient species.

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* (b) (p. 26 (f) Ch. 6)

The most remarkable exception to this rule, under conditions not apparently very peculiar, is one given by Mr. C. A. Johns (Phytologist, vol. 2, p. 908) who says that he covered with his hat, (I presume broad-brimmed) near to Lands End six species of Trifolium, & a Lotus & Anthyllis; & had the brim been a little wider hav might it would have covered another Lotus & Genista; which would have made ten species of Leguminosae, belonging to only four genera! The wretched soil of Heaths, though covered thickly with one or two species of Erica, supports very little life, as judged by their very extremely slow growth, & yet, selecting the very worst spots, I have very rarely been able to find a space two yards square, without one or two other plants, belonging to quite different orders, not to mention a good crop of Cryptogams.

To show the degree of diversity in our British plants on a small plot, I may mention, that I selected a field, in Kent, of 13 1/2 acres, which had been thrown out of cultivation for 15 years, & had been thinly planted with small trees most of which had failed: the field all consisted of heavy very bad clay, but one side sloped & was drier: there was no water or marsh: 142 phanerogamic plants were here collected by a friend during the course of a year; these belonged to 108 genera, & to 32 orders out of the 86 orders into which the plants of Britain now have been classed. Another friend collected for me all the plants on about 40 uncultivated, very poor, acres of Ashdown Common in Sussex; these were 106 in number, & belonged to 82 genera & 34 orders; the slightly greater proportional number of orders in this case being chiefly owing to the presence of water & marsh plants on the Common: the vegetation was, however, considerably different in other respects, no less than nine of the 34 orders, not being found on the field of thirteen acres in Kent.—

(26g

(Ch. 6 Principle of Divergence)

The extreme poverty of the floras of all such islets may be in partly due to their distance from other lands to their isolation & the seeds arriving from lands having different Floras, but chiefly to the poverty & peculiarity of the soil; for coral-islets, when lying close to large volcanic groups, have an almost equally poor & closely similar flora: the extreme diversity of the plants, belonging the twenty in the case of Keeling islands, belonging to sixteen orders, can, I think, only be accounted for by the fact that of all the plants of which the seeds have been borne across the sea in the later periods of the natural colonisation of the island, those alone, which differed greatly from the earlier occupants, were able to come into competition with them & so lay hold of the ground & survive.

As with plants so with insects. I may premise that entomologists divide the Coleoptera into 13 grand sections, & then into families, sub-families &c. Mr. Wollaston* (*Insecta Maderensia. 1854.) carefully collected during several visits all the Beetles on the Dezerta Grande, a desert volcanic islet about four miles long, & in widest part only three-quarters broad, lying close to Madeira; & he found 57 species, belonging to 47 genera; & these to all 13 grand sections, except two, which being aquatic forms, could not exist on this waterless islet. Again on the Savages, an extremely small volcanic isld between Madeira, & the Canaries, six beetles were collected, & these

(26h

(Ch. 6 Principle of Divergence)

belonged to six genera, to six Families, & to three of the grand Sections!* (a) As a general rule, I think we may conclude, that the smaller the area, even though the conditions be remarkably uniform, if they are not at the same time very peculiar or very unfavourable to life, the more widely diversified will its inhabitants be: in all respects for to this very diversity, the power of supporting the greatest possible amount of life number of living beings, all of which are struggling to live, will be due.

There is another way of looking at this subject; namely to consider the productions naturalised through man's agency in several countries; & see what relation they bear to each other & to the aboriginal productions of the country. Do they generally belong to the same genera, i.e. Are they closely allied to, that is do they generally belong to the same genera with, the aboriginal inhabitants of the country? Do many species of the same genus become naturalised? If we looked only to the inorganic conditions of th a country, we might have expected that species, belonging to genera already inhabiting it, & supposed on the common view to have be adapted by creation for such country, would have formed the main body of the colonists: or

[26hv]

* (a) In the volcanic Galapagos Isld in the Pacific, I carefully collected all the Coleoptera during several weeks; but omitting two probably naturalised species, I got only 24 species, which have been described by Mr. Waterhouse in Annals & Mag. of Nat. History (2d Series) vol. 16. 1845, p. 19.—The 24 species belong to 18 genera, to 17 families & to 10 out of 13 grand sections. So here again we see the same rule as in other cases in the text. Lundy Isld.

(26i

(Ch. 6 Principle of Divergence)

that several many species of certain favoured genera would have been in the successful intruders. On the other hand, the principle of diversity being favourable to the support of the greatest number of living beings would lead to the expectation, that land already well stocked by the hand of nature would support such new forms alone, as differed much from each other & from the aborigines.

Alph. De Candolle* (a) (*(a) Geographie Botanique, p. 745, 759, 803.) has f ably fully discussed the subject of naturalisation: He shows that 64 plants have become naturalised in Europe (excluding species from not neighbouring regions) during the last three centuries and a half; & these 64 species belong to 46 genera & 24 orders; s so that they are of a much diversified nature; of the genera, 21/46 are new to Europe.*(b) Again in N. America, 184 species have become naturalised & these belong to 120 genera & to 38 orders; of the genera, 56/120 are new to N. America.*(c) A list of the naturalised plants on many in Australia & on many islands would give similar, but much more striking results. The number of new genera naturalised in Europe & N. America, reciprocally from each other, is the more remarkable when we consider how much allied the two floras are; & that a very large proportion of the

[26iv]

* (b) In some respects small areas, not including in the sub-regions many indigenous many natur representative species, are best for comparing the indigenous native & naturalised productions. De Candolle gives a list of (p. 645 et seq) (in large type) of the 83 plants, which he considers as most certainly naturalised in Great Britain: these belong to 71 no less than 71 genera: & of these 31/71 are new to Britain. The indigenous genera include on an average about 2.8 indigenous species: the naturalised only 1.1.

* (c) In Dr Asa Gray seems to consider many more plants as naturalised, than does De Candolle for in his Manual of the Botany of the Northern United States (2d Edit.) he gives an a list of 260 [illeg]naturalised species plants, belonging to 162 genera, of which no less than 100 are new to America.

The naturalised genera include on an average 1.6 species: the indigenous include 2.6.—

(26K

(Ch. 6 Principle of Divergence)

naturalised plants inhabit land, cultivated nearly in the same manner, which would favour the the introduction of allied forms & many forms of the same groups. Hence, I think, we may conclude that naturalised productions are generally of a diversified nature within the same class; & as Alph. De Candolle has remarked th native floras gain by naturalisation, proportionally to their own numbers, far more in genera than in species.

If we now turn to animals, we find, though our data are very scanty, the same general fact: no where in the world have more mammals become well naturalised than in S. America (cattle, horses, [foxes] assespigs, dogs, cats, rats & mice); & yet how extremely unlike is the native mammalian Fauna of S. America to that of the Old World!

The whole subject of naturalisation seems to me extremely interesting under this above point of view, & would deserve to be treated at much greater length. It confirms the view that in natural colonisation, for instance in that of a coral-islet, diverse forms very different from the few aborigines previous occupants, would have the best chance of succeeding. It shows us, & by no other means can we form a conjecture on this head,

(26L

(Ch. 6 Principle of Divergence)

what are the gaps or still open places in the polity of nature in any country: we see that these gaps are wide apart, & that they can be best filled up by organic beings, of which a large proportion are very unlike the aboriginal inhabitants of the country. Consequently we may might perhaps from this alone infer, that natural selection, if immigration were by the preservation of the most diversified varieties & species, would in the long run tend, if immigration were prevented, to make the inhabitants, which were undergoing any modification more & more diversified; though such modified forms they would plainly retain for immense periods plainly retain from heritage the stamp of their common parentage.

The view that the greatest number of organic beings (or more strictly the greatest amount of life) can be supported on any area, by the greatest amount of their diversification is, perhaps, most plainly seen by taking an imaginary case. This view doctrine is in fact that doctrine of "the division of labour", so admirably propounded by Milne Edwards, who argues that a stomach will digest better, if it does not, as in many of the lowest animals, serve at the same time as a respiratory organ; that a stomach will get more nutriment out of flesh vegetable or animal matter, if adapted to digest either separately instead of both. More It is obvious that more descendants from a carnivorous animal could be supported in any

(26m

(Ch. 6 Principle of Divergence)

country: if some were adapted, by long continued modification through natural selection, to catch hunt small prey, & others or others large large prey the prey living either some to show either prey on plains or in forests, in burrows, or on trees or in the water. So with the descendants of a vegetable feeder more could be supported, if some were adapted to feed on tender grass & others on leaves of trees or of aquatic plants & others on hard seed bark, roots, hard seeds or fruit.—

Perhaps I have already argued this point superfluously; but I consider it as of the utmost importance fully to recognise that the amount of life in any country, & still more that the number of modified descendants from a common parent, will in chief part depend on the amount of diversification which they have undergone, so as best to fit them for fill as many & as widely different places as possible in the probity great scheme of nature. as possible

Now let it be borne in mind that all the individuals of the same variety, and all the individuals of all the species of the same genus, family &c, are perpetually struggling to become more numerous by their high geometrical powers of increase. Under ordinary circumstances each species will in the briefest period have arrived at its fluctuating numerical maximum. Nor can it pass this point, without

(26n

(Ch. 6 Principle of Divergence)

some other inhabitants of the same country suffering diminutions; or without all the descendants of one species becoming similarly modified in some respect so as that they better to fill the place of their parent species; or without (& this would be the most effectual) several varieties & then several species being are thus formed by modification, so as to occupy various new places, the more different the better, in the natural economy of the one country. Although all the inhabitants of the country will be tending to increase in numbers by the preservation through natural selection of diverse modifications; but few will succeed; for variation must arise in the right direction & there must be an unfilled or less well-filled place in the polity of nature: the process, moreover, in all cases, as we shall presently see, must be slow in an extreme degree.)

(Let us take an imaginary case of the Ornithorhynchus; & suppose this strange animal to have some an advantage over some of the other inhabitants of Australia, so as to increase in numbers & to vary: it could not, we may feel pretty sure, increase to any very great extent, only by its descendants becoming modified, so that some could live on dry land, some could feed exclusively on vegetable matter in various stations, & some & could prey

could prey on various animals, fish insects fish or the smaller animals fish or quadrupeds. In fact its descendants would have to become

(26o

(Ch. 6 Principle of Divergence)

diversified, somewhat like the other Australian marsupials, which, as Mr. Waterhouse has remarked, greatly typify in their several sub-families, our true carnivores, insectivores, ruminants & rodents. Moreover it can, I think, be hardly be doubted, that these very marsupials would, have profited by a still further division of physiological labour; that is by their structure being becoming as perfectly carnivorous, ruminant & rodent as can are our old-world forms; for it may well be doubted (not here considering the probable intellectual inferiority of the marsupialia in comparison with the other or placentate mammals) whether many marsupial vegetable feeders could long exist in free competition with true our ruminants, & perhaps still less the carnivorous marsupials with one f true feline animals. And who can pretend to say that the animals mammals of the old world are diversified & have their organs adapted to different physiological labours to the extreme, which would be best for them under the conditions to which they are exposed? Had we known the existing mammals of S. America alone, we should no doubt have thought them perfect & diversified in structure & habit to the exact right degree; but the vast herds of feral cattle, horses, pigs & dogs,

(26P.

(Ch. 6 Principle of Divergence)

at least show that other existing animals, & some of them as the equine horse & solid-horned ruminants, very different from any existing the endemic S. American mammals, could beat & take the place of the native occupants.

In Chapter Iv we have seen on evidence, which seems to me in a fair degree satisfactory, that on an average the species in the larger genera in any country oftenest present varieties in some degree permanent, and likewise a greater average number of such varieties, than do the species of the smaller genera. It is not that all the species of the larger genera vary, but only some, & chiefly those which are wide-rangers, much diffused & universally common or universally large numerous in individuals. In the same chapter we also saw that (a text)

all to a certain extent confirm the view that varieties, when in some degree permanent, do not essentially differ from species, more especially from such species, as are closely allied together. Hence I have looked at l look at varieties as incipient species.)

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

as marked distinction in character could be drawn between species closely allied together & varieties of the same species: we, also, saw that the close affinity of the species in the larger genera, & the manner in which they are clustered around other species, the distribution &c

[The pencil insertion to the above is transcribed here:]

the species in the larger genera might thought to be more closely allied together being clustered in little groups round other species, than are to species in the smaller genera, & in the close offering & grouping & other facts & in all being as certain [many words illeg] are in Europe the species in the larger genera make some appear resemble to a certain extent, varieties

[26Pv]

A text / the species in the larger genera are thought by highly competent judges to be more closely related together, being clustered in little sub-groups round other species, than are the species in the smaller genera; & this close affinity & grouping of the species in the larger genera, & the fact that there is no unfailing test by which to distinguish species & varieties,

(26Q

(Ch. 6 Principle of Divergence)

(I have lately remarked that the formation of new varieties & species through natural selection almost necessarily implies (as with our domestic productions) a large amount of much extinction of the less altered, & therefore less favoured, descendants from the same original parent-stock, whose places they seize are occupy in the struggle for life. Hence, though the larger genera may be now varying most, & must, according to our theory, have varied largely, so as to have become modified into many specific forms, yet they such large genera must have suffered a large amount of extinction, & very many intermediate & less modified forms have been wholly swept away. Nevertheless, I think we may infer that in any given country, on the whole, there will have been rather less extinction, proportionally to the whole number amount of extinction within any given period, amongst the larger genera than amongst the smaller genera. For the species which vary most & thus give rise to new species, are chiefly the very common & much diffused species, & therefore generally in a large proportion being to the larger genera the most favoured forms, which would naturally be the least liable to extinction; & such common & much diffused species tend to belong to the larger genera. Indeed it seems to me that the simple fact of a number of beyond the average of many allied species, beyond the average that is the many species of the larger genera beyond the average amount number of allied species, inhabiting any country; shows that there

(26R

(Ch. 6 Principle of Divergence)

is something in common in such groups of species, or genus genera, which is favourable to them, & which would tend to lessen the probable amount of extinction consequently that they would be suffer proportionally less liable to from extinction than the smaller genera. Therefore, I believe, from the species of larger genera tending to vary most & so to give rise to more species, & from their being probably somewhat less liable to extinction, I believe that the genera now large in any area, are now generally tending to become still larger. But what will be the end of this? for we do not find in nature genera of indefinite size, with innumerable species. so numerous as hardly to be numbered. Here in one way comes in the importance of our so-called principle of divergence: as in the long run, more descendants from a common parent will survive, the more widely they have become diversified in habits, constitution & structure so as to fill as many places as possible in the polity of nature, the extreme varieties & the extreme species will have a better chance of surviving or escaping extinction, than the intermediate & less modified varieties or species. But if in a large genus we destroy all the intermediate species, the remaining forms will constitute sub-genera or distinct genera, according to the almost arbitrary value put on these terms,—according to the number of intermediate forms which have been destroyed,—and

[26Ra]

[Diagram I.]

[Diagram II.]

Original species in Diag. I A. B. C. D. E. F. G. H. I. K. L. M.

Diagram III. a.10 h.10 l.10 B. C. D. E. F. G. H. I. K. L. M.10 {x20 z20 m20

Diag. IV. A20 k20 n20 E.20 F.20

Compositor: To be printed on separate page to be folded out & so all exposed. Attend to distance of capital letters from each other: the letters had better be smaller:

Attend to chains of dots & hyphens. The numbers to small letters to be the very smallest possible. The capital & other letters in each diagram to match exactly in position.— I hope the 4 Diagrams will go in length of page.—

(26S

(Ch. 6 Principle of Divergence: Descent)

according to the degree of difference between the extreme species of the original genus. Nevertheless the modified descendants from the common parent-stock, though no longer forming what is called the same genus, may still go on becoming more & more numerous, & more & more diversified.

The complex supposed supposed complex action of these several principles, namely of natural selection, divergence & extinction, may be best, [illeg]yet very imperfectly, illustrated by the following Diagram, printed on a folded sheet for convenience of reference, for the subject is sufficiently complex

(a) Let A to M represent the species of a genus, numerically large in its one country compared with the other genera of the same class in the same country, & arranged as naturally as can be done, so that A & M are the two most distinct forms in any one

all respects [pencil insertion:] for simplicity sake, we shall find of course to show [many words illeg]. The unequal distances of the letters in the diagram may represent the ordinary way in which the species, even when as in this imaginary case all are closely related together, yet stand unequally related in little sub-groups. This genus may have one, two or even more varying species.

Any of the species may vary; but it will generally be those species which are most numerous in individuals & most diffused; & this shows that such species have already some advantages over the other inhabitants of the country. From our principle of divergence, the extreme varieties of any of the species, & more especially of those species which are now extreme in some characters, will have the best chance,

[26Sv]

(a) text / This diagram will show how I suppose the manner, in which I believe species descend from each other & therefore shall be explained in detail: it will, also, clearly show several points of doubt & difficulty;

(26t

(Ch. 6 Principle of Divergence)

after a vast lapse of time, of surviving; for they will tend to occupy new places in the economy of our imaginary country. I do not mean that any of these points are of invariable occurrence, but that in the long run such cases will prevail. The extreme species A and M will differ in very many respects; but for convenience sake we may look to any one character, & suppose A the most moisture-loving plant species of plant & M the least so, or the most drought enduring moisture-loving species.

We will first take the simplest case. Let M inhabit a continuous area, not separated by barriers, & let it be a very common & widely diffused & varying plant. From the fact of M. being very common & widely diffused, it clearly has some advantages over in comparison with most of the other inhabitants of the same country; but, we will suppose, that it might become still more common, if retaining the advantages which it already has, it could endure rather still more drought. It is a varying species; & let z1-m1 represent numerous, extremely slight variations of many kinds, produced at intervals, of which m1 alone is a less more drought-enduring variety.

As m1 tends to inherit all the advantages of its parent M, with the additional advantage of enduring somewhat more drought, it will have an advantage over it, & will first become a probably first be a thriving local variety, which will spread & become extremely common & permanent variety, which will & ultimately, tend to supplant its own parent. We may now repeat the process, & let the variety m1 vary in a similar manner; perhaps

(26u

(Ch. 6 Principle of Divergence)

many thousands of generations may pass before m1 will produce another variety m2 , still more drought-enduring & yet inheriting the common advantages of m1 & M; but if this should ever occur, the same results, as before, will follow: & ultimately, by repeating the process, there may be produced m9 10 , which may either be, according to the amount of difference thus acquired, a very strongly marked variety, or a sub-species, or a good species, enduring far more drought than M & will probably with other correlated differences of structure. [illeg]In each stage of descent, there will be a tendency in the new forms to supplant its parent, though probably, as we shall see, very slowly, & so ultimately cause its extinction.

But if M had originally inhabited in an area a country separated by barriers into distinct districts, in one or more of which the varieties such as M1-10 had never originated or had never had been able to immigrate enter, M and m1 & ultimately m 10 might be living contemporaneously, but separated: (a)

(It should always, be borne in mind that there is a wide distinction between mere variations & the formation of permanent varieties. Variation act is due to the action of external or internal causes on the generative systems, causing the child to be in some respects unlike the its parent; & the differences thus produced may be advantageous or disadvantageous

[26uv] if {     } m 1-10 had been produced,

a text/

: or, again, if the varietiesm 1-10 had been produced, capable of enduring more drought, but not at the same time enduring an equal amount of moisture with the parent M, both parent & modified offspring might coexist: the parent is with (with perhaps a more restricted range) in the dryer stations, & m1-10 in the very dryest stations.) when M could not

(26v

(Ch. 6 Principle of Divergence)

to the child. The formation of a permanent variety, implies not only that the modifications are inherited, but that they are not disadvantageous, generally that they are in some degree advantageous to the variety, otherwise it could not compete with its parent when inhabiting the same countries area. The can be formation of a permanent variety must can be effected by natural selection; or it may be the result, generally in unimportant respects, of the direct action of peculiar external conditions on all the individuals & their off-spring exposed to such conditions. We shall best perceive the importance of the difference by glancing at our domestic breeds: for instance of pigeons: in our truest breeds, innumerable very slight differences are continually occurring & can be detected by measurement, but only those differences which no improve the breed in the often fanciful eyes of the Fancier are preserved rendered permanent by the birds animals so characterised being carefully tended preserved, matched & largely bred from; all other slight differences being lost, by the birds animals not being largely bred from, & from indiscriminate crossing.

If, however, two Fanciers were to go on selecting the process of selection were continued for a long time by two Fanciers, under very different conditions of climate or food, some subordinate differences would probably arise between the two lots, owing to the direct action of such conditions. Now in our diagram, the letters z1 -m1 , z2 -m2 &c under represent all sorts kinds sorts of slight successive slight variations, of which m1-10, the most drought-enduring varieties alone have been naturally selected & thus have then formed been rendered a permanent. variety [pencil insertion:] vary

This natural selection has been possible, owing to there having been

(26w

(Ch. 6 Principle of Divergence)

a place in the economy of our imaginary country, which the descendants of M, could seize on, owing to their inher from inheriting all or some of the advantages over the other inhabitants which made M a very common species, could seize on, when rendered more drought-enduring.

With respect to the process by which each new & improved variety supplants its parent, this must often have gone on in two slightly different manners, differing, however, only in degree. In those animals which are highly locomotive & of which two individuals unite for each birth, there can only seldom have arisen as we shall hereafter see, within the same continuous area, especially if of not very large size, distinct varieties, for they would become blended by such free crossing. In such cases, modifications must be effected quite insensibly by the sel natural selection of mere individual differences; just nearly in the same way as the [commonest]many of our domestic breeds throughout certain countries whole districts have been insensibly changed by artificial from their ancient condition state. So that in our diagram the letters m1-10 may represent in the case of the higher animals, not recognizable varieties, but mere ideal steps in a real, yet insensibly gradual, change of structure. In organic beings which do not cross freely (though

(26x

(Ch. 6 Principle of Divergence)

we have seen that crossing, & which are more stationary, & which are capable of propagating at a great rate, a district & recognizable a variety might easily be formed in one spot (more especially if in some slight degree isolated) & might not spread & supplant its parent-stock, until it had become developed by the continued natural selection of similar extremely slight or individual differences into a distinct & plainly recognizable variety.

(a text)

This same process must, however, also have often occurred with the most freely crossing & locomotive animals, when inhabiting an area, separated by barriers only to be passed after geographical changes or through some most rare accident, a similar process must often have occurred; for in these such cases, distinct & plainly marked varieties might have been insensibly formed in the different districts by the selection of mere individual differences; & when these districts became united, th so that the varieties could mingle & come into competition, the best variety would supplant the other varieties or the parent-stock.)

(To return to our diagram. I do not suppose the process generally to have been so simple as represented under M, where a simple variety m1-10 in each stage of descent tend to support its laws has been naturally selected. We have seen that not only more species, especially the very common species, in the larger genera in any country two present varieties in some degree permanent, but that each such species on an average tends to present a greater number of varieties, than do the species, especially the rarer species, in the larger smaller & less flourishing genera. As varieties from a species whi will tend to inherit the advantages which

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

made the parent common; if such varieties under the new selection to which they are adapted even do become common they will tend to vary, & giving rise & at first the sub-varieties of varieties & alternating perhaps to new & permanent varieties

[pencilled insertions:] in [3 words illeg] highly [illeg] species Thus tending to give rise to a multitude of varieties

[26xv]

These two processes, which differ only in

[26xVv]

a text/ I am inclined to think from the frequency of local varieties, though the subject must remain very doubtful, that this latter is a must be the commonest process process has been a very common one, for a variety would often be unable to supplant its parent, until it had become sensibly considerably modified so as to have a decided advantage over it. For instance in the imaginary case of the varieties m1-10 which are supposed to inherit all the characters of M, with the addition of enduring more drought; these varieties would have inhabit stations, where M could not exist, but in the less dry stations m1-10 would have very little power of supplanting their parent M; yet nevertheless during unusually dry seasons m1-10 would have a great advantage over M, & would spread; but in damper seasons M, would not have a similar corresponding advantage over m1-10 for these latter varieties are supposed to inherit all the characters of their parent. So it there would be is with many other peculiarities a tendency in m1-10 to supplant M, but at an excessively slow rate. It would be easy to show that the same thing might occur in the case of many other new characters thus acquired; but the subject is far too doubtful & speculative to be worth pursuing. I will only add that

(26y

(Ch. 6 Principle of Divergence)

made the parent common, these varieties will ultimately tend to be common & to vary; moreover they descend from a variable stock, & are still exposed to the conditions which made their parents vary, so hence for this cause they also, will be tend liable to vary. Hence Consequently there will be a tendency in the original varying species in such cases, after a vast number of generations to produce an almost infinite number of varieties from the first original varying species; but our principle of divergence will give a decided explain explains how the most diversified varieties will always generally have an decided advantages over the less diversified & intermediate varieties, causing their extinction & thus reducing the number of varieties living at any one time. These remarks are illustrated in our diagram under A., which species, after many generations represented by dots, is supposed to have produced new yielded many varieties varied largely, & to have produced somewhat these varieties a1 , d1 l1 in some degree permanent; of these, again after many generations & much variation; the two extreme varieties a1 and l1 , have are supposed to have produced other permanent varieties in some degree permanent; of which the extreme varieties have again reproduced others, represented finally by a10 & l10 . In the diagram I have been able to represent only one other branch proceeding from l5, & giving rise to a third parent variety h9 h10 which being the extreme form in its own branch has the best chance of surviving

(26z

(Ch. 6 Principle of Divergence)

& filling & seizing on some new place in to its own the natural economy of the country inhabited by the genus.

By continuing the process represented in the diagram, the forms marked a10, h10, l10, may be made different in any degree, till they would be universally be ranked as good species; & the number of such new forms as in would continually tend to increase.

These new species would will generally have supplanted, perhaps by a very slow process, their several parents in each stage of descent & their aborigine common original common parent A,—that is if they formed in one continuous area, or as soon as they came into competition with each other if formed in different areas. The original species A. was supposed to be the most moisture loving plant; & if for simplicity sake we imagine a1 more moisture loving & l1 less moisture loving, but inheriting some of the advantages which made A in the great & complex battle for life a very common species; & the offspring most divergent offspring of these varieties to be continually to be se naturally selected on similar extreme the same principle, a10 will have been rendered so moisture loving as to have become semi-aquatic, & l10 far less moisture loving than A; & in the third branch, h10 , about as moisture loving as A, from having for it has descended from f5 which was more moisture-loving than A. & then than A, and subsequently having has become less so. Not that I at all suppose the diversity is ever so simple as th thus confined to one point; for as a1-10 becomes moisture-loving & as l10 becomes less moisture-loving both would under the extremely complex conditions to which all organic beings are exposed, come to be exposed to new dangers &

(26aa

(Ch. 6 Principle of Divergence)

would have to gain in some other advantages over other organic beings with which they would have to compete. So that in love of moisture & in many other respects, they would a1-10, h1-10, l1-10 would come to differ or diverge more & more from each other & their original parent-stock.

A little reflexion will show the extreme importance of this principle of divergence for our theory. which from the several reasons assigned in an earlier part of this discussion I cannot doubt is highly essential effective, on one theory. I believe all the species of the same genus have descended from a common parent; & we may call the average amount of difference between the species, x; but if we look at the contemporaneous varieties of any one species, the amount of difference between them is comparatively extremely slight & may be called a. How thus can the slight difference a be augmented into the greater difference x; which must on our theory be continually occurring in nature, if varieties are converted into distinct good species? The process feebly illustrated in our diagram, I believe, explains this; namely the continued natural selection or preservation of those varieties, which diverge most in all sorts of respects from their parent-type, (but still largely inheriting those advantages which made their parents generally dominant & common species) so as to fill as

(26bb

(Ch. 6 Principle of Divergence)

many, as new, & as widely different places in the economy of nature, as possible.

A glance at Diagram 2. will perhaps render this plainer. Here everything is the same, & varieties. The varieties a1-10 , l1-10 may be here again for simplicity be looked at as more & less moisture loving plants; & everything is the same as in diagram I (the third branch h6-10 cannot be introduced) except that it is left to mere chance in each stage of descent, whether the more or less moisture loving constitute variety to varieties are preserved; & the result is, as graphically shown, that a10 & l10 differ in this respect; & so in other respects, hardly more than did the first varieties (a1 l1) which were produced.

In regard to the difference between varieties from the same species & species of the same genus, I may add that varieties differ from each other & their parents, chiefly in what naturalists call unimportant respects, as size, colour proportions &c; but species differ from each other in these same respects, only generally in a greater degree, & in addition in what naturalists consider more important respects. But we have seen in Ch. IV, that varieties do occasionally, though rarely, very slightly in such important respects; and in so far as modification differences in important physiological characters generally stand in direct relation to different

(26cc

(Ch. 6 Principle of Divergence)

habits of life, modifications however slight in such characters would be very apt to be picked out by natural selection & so augmented, thus to fit the modified descendants from the same parent to fill as many & as widely different places in nature as possible. We shall, also, see in a future chapter that a large part of the differences in structure in between species may be accounted for by the mysterious laws of correlation of growth; by which, I mean, that when one part is modified, (or the whole animal at one age, (as with the larvae of insects) other parts necessarily become altered through the correlated laws of growth. That there is no obvious & unmistakeable difference in between the differential characters of species & varieties, is plainly shown by the number of debateable forms in the best known clo countries, which are ranked by one good naturalist as true species, & by another as mere varieties.

Our principle of divergence has another very important bearing. In the diagram, A. has given rise to three new species, & M to one. and according to our views there will be constant tendency for the modified descendants of these two original species to go on increasing. The other species of the genus, B to L, are supposed in the diagram to have

(26dd

 

(Ch. 6 Principle of Divergence)

transmitted unaltered descendents. The result will be even Hence, even supposing that A & M have been supplanted as I believe will usually have been the case, by their modified & improved descendants, that the genus will have become not only more divergent in character (a10 more aquatic than A; & m10 more drought-enduring than M.) but numerically larger. The original species A to M were supposed to be closely allied, but yet to exhibit traces, as is so general, of being divided into sub-groups. The sub-groups, now after the modifications formation of the new species, will be slightly altered & increased in number; for a10 & h10 will be closely related together from common descent from f5 , & less closely yet less closely with l10 from common descent from their common ancestor A.; and and they will all differ as much, generally more from B, than did A. So again and m10 having constantly diverged from the characters of M will now stand more distant from L, than, M originally stood.

This is represented in the Diagram III. But ascending to our views, alth And from the reasons already given, I believe there will be a constant tendency in the modified descendants of A & of M to go on thus producing new & still more & more new specific forms & thus more & more modified or divergent. new specific forms.

What will be the limit to this process in nature? Though many genera are large, they do not increase include an indefinite number of species. I believe that there is no limit

(26ee

(Ch. 6 Principle of Divergence)

to the number of species capable tending to be formed from the most favoured forms in any country (or those which have any the greatest advantages over the coinhabitants), except the number of species which the country is capable of supporting; but such modified descendants, or new species, after a long period will have to be ranked not in the same genera, but in distinct genera, families or orders. For if we suppose the process illustrated in the diagram I. to have long continued & the modified descendants of A to have become greatly extremely much multiplied, as they will at the same time have diverged as much as possible in character from this common parent A & from each other, they and diversified in many ways, they will tend to take the places of & thus exterminate the original species B.C.D. &c, which originally were nearest related to them A.& were not then such common & flourishing species. & will thus tend to cause their extermination. So if M had left several modified & divergent descendants, it would have been with L, K. &c.* Continue this process, & nearly all, or we nearly all the original species (A to M) will disappear become extinct. will suppose for simplicity sake, all the original species will be exterminated.

In Diagram IV. this is represented, E & F alone now having descendents, whether or not modified. And the final result will be, that we shall have two large groups of modified descendants,

[26eev]

and the distance or difference between

[26eev]

A text/ It may be here worth observing, that although h the new species in taking the place of the old (their great uncle) may have acquired through natural selection, some of their characters; which by naturalist would this kind of resemblance would be called by naturalists that of analogy, & the real affinity of the new species would be with their true real parents: thus l10 might come to simulate some of the character of B, from occupying its place in nature yet the real affinity of l10 would be with A.—

(26ff

(Ch. 6. Principle of Divergence)

coming from the two species, generally the extreme species, (A & M) of the original genus, and differing as much as natural selection & could make them from each other within the same group & from their two parents, as much which at the first start differed much: assuredly these two new groups of new species would be ranked as distinct in different genera, which would be very distinct, if all the original intermediate species from B to L. had become extinct been exterminated, as is represented in diagram IV: but somewhat less distinct if some few of these species (as represented in Diagram IV.) had left descendants, whether or not modified.)

Now for a moment let us go back many stages in descent: on our theory the original twelve species A to M are supposed to have descended & diverged from some one species, which may be called Z, of a former genus. But now, according to the result shown given in the last paragraph, Z will have become the parent ancestor of two or three very distinct groups of new species; & such groups, naturalists call genera. By continuing the same process, namely the natural selection of generally the most divergent forms, with the extinction of those which have been less modified & are intermediate, Z may become the ancestor of two very distinct groups of

(26gg

(Ch. 6 Principle of Divergence)

genera; & such groups of genera, naturalists call Families or even Orders. But to this subject, we shall have to return in our Chapter on Classification.

I have previously remarked that there seems to be no limit to the number of modified descendants, likely to proceed from the most favoured form in any country,—the most favoured always tending to diverge in structure & take the place of & exterminate the less favoured & intermediate forms,— except the total number of species, which the country is capable of supporting. But it may be objected that as natural selection, extinction & divergence must have been going on since the dawn of Life, why have we not an infinite number of species, almost as many species, as individuals? We shall presently see that natural selection can act only with extreme slowness. Nor do we by any means know that the maximum number of species, has anywhere which any country would be best fitted to support, has anywhere been as yet produced: the fact that there is no country which does not support several, often many, organic beings naturalised by man, shows the without, as far as we know,

(26hh

(Ch. 6 Principle of Divergence)

having caused the extinction of an equal number of the indigenous productions, shows renders it probable that such countries were capable of supporting a greater number of specific forms than nature had supplied them with. Even the Cape of Good Hope, which is so extraordinary apparently the richest district in the world in different species kinds of plants has received, as I am informed by [pencil insertion:] Mr Bunbury does not know he may Prof. Haney from     to     naturalised species. Many geologists, indeed believe that the number of species in the world has gone on increasing from the earliest geological days; & but I am sorry to own that the evidence on this head seems to me quite insufficient.* a

(But if the time has not yet arrived, may it not at some epoch arises come, when there will be always almost as many specific forms as individuals?

I think we can clearly see that this would never be the case. In the Firstly, place there would be no apparent benefit in a greater amount of modification than there were would fit adapt organic beings to different places in the polity of nature; to fill; the inorganic conditions are not infinitely varied for although the structure of each organism stands in the most direct & important relation to many other organic beings, and as these latter

(26kk

(Ch. 6 Principle of Divergence)

into very many species, without all become many numerous in individuals, which see hardly ever see to be the case with all the species of the same genus or family, there will be a check amongst the less common species to their further modification: the lesser number of the individuals serving in all cases as a regulator or fly‐wheel to the increasing rate of further modification, or the production of new specific forms.)

Subject to these restraining influences, I can see no limit to the number of modified descendants, which might proceed from the most favoured forms, whatever they may see be, now living in the world. If we return to look to the future, as far into the remotest futurity as the Silurian system lies in the remote past, our views theory would lead to the conclusion that all organic beings which will live at that far distant period, will be descendants from a very few of our contemporaries; perhaps from not so few, owing to the increasing complexity of the organic world, as our existing organisms have descended from; for our animals have descended, according to our theory, from four or five parent ancestral types & our plants from perhaps apparently still

(L

[This mostly incoherent pencil draft is re-worked in folio 26mm]

The relation of all past & present organic being, may be loosely represented by the growth of a few an gigantic trees; therefore if we suppose that from each of the innumerable twigs, more numerous buds are trying sprout forth each in very driest spring, & that the outer or divergent buds & twigs & branches [illeg] one alone survive from getting air & light

have the best chance of moving growing from getting more air & light. If we burn the twigs down we find their proceeding & [illeg] from few & few lesser stem branch & thus from the main branches limbs, & stem from the main stem trunk; branches are very unequal in size.

During growth the innumerable buds & bud & twigs, have perish perished representing extinct species & genera; so here many small branches, representing genus family & some larger have representing buds the Families. Some large branch yet remain just alive with only a few smaller branches & on these only a few bu twigs, representing the group in having with few genus & few species which still exist, but are on a road to extinct Towards In lower part of the tree, where now we see only a few gnarled branches, we know still they one existed as the leader branch of (as a [illeg] covered with buds & twigs so we may believe in part true one [illeg] living & extinct families & orders, of which to find only a few fossil [illeg] on a swarmed each [illeg] force of life. As buds give rise by growth of fresh buds, what if vigorous branch out & give rise to many a diverging branch covered with twigs & buds & causing the death of many a neighbouring branch, so my generation, [illeg] of growth, I believe it has been with the great tree of life, which fill the crust of the earth with its dead fragment & still has covered over & now, cover the earth the face of it with itsmarvellous reticulation living ramifications the face of the earth branches, ramifications & still vigorously living giving ramifications.

[26hhv]

* a p 26 hh. Ch. 6.)

It might indeed be argued from the enormous list of shells, found in the eocene Paris basin, & even in the ancient Silurian system of Bohemia, as dis so admirably worked out by Barrande, that at these periods & in these places, a greater number of species existed than anywhere at the present day. But it may be doubted how far such comparisons is are in any instance fair trustworthy; for we have reason to suppose that the duration of each sub‐division of each geological stage formation is so enormous, that it is not fair include at to compare all the species found in one such sub‐division with all existing within an area at the present day. Barrande's "colonies" show, according to Sir C. Lyells explanation of them, (Supplement to Manual of Geology, 2d Edit. 1857. p. 34) what changes of climate or currents must have taken place within certain definite periods: the Glacial epoch within what may be called the present period, should teach us caution, for far lesser changes than the glacial epoch, not easily to be detected in ancient geological formations, might well bring in many & single one area alternately bring in & displace, & apparently mingle many organic beings, which never really lived [illeg]coinhabited the same area.—

(26ii

(Ch. 6 Principle of Divergence)

increase in number & complexity diversity of organisation, the conditions of the one will tend to become more & more complex, & its descendants might well profit by a further division of labour; yet all organisms are fundamentally related to the inorganic conditions of the world, which are not do not tend to become infinitely more varied. Secondly as the amount of life & number of individual beings, whether or not much diversified, also primarily depends on such inorganic conditions; if there exist in any country, a vast number of species (although a greater amount of life could be supported) the average number of individuals of each the each species must be somewhat less than if there were not quite so many species; & any species, represented by but few individuals, during the fluctuation in number to which all species must be subject from fluctuations in seasons, number of enemies &c, would be extremely liable to total extinction. Moreover, whenever the number of individuals of any species had should becomes very small, the ill‐effects, as I believe, of close inter breeding would come into play. & add another grain to Lastly we have seen in our Chap. IV & shall presently again see, that the amount of variations, & consequently of variation in a right or beneficial direction for natural selection to seize on seize on & preserve, will bear some relation within any given period, to the number of individuals living & liable to variation during such period: consequently when the descendants from any one species have become modified

(26LL

(Ch. 6 Principle of Divergence)

fewer; & if we rashly dare trust to mere analogy, all our plants & animals from some one form, into which life was first breathed.

Taking a more modest glance into futurity, we may predict that the dominant genera, now abounding with common & widely diffused species, will tend to be still more dominant for at least some considerable lapse of time, & will give rise to new groups of species, always diverging in character, & seizing on the places occupied by the less favoured forms, whether or not their near blood relations, supplanting them & causing their extermination. The great & flourishing genera both of plants & animals, which now play so important a part in nature, thus prophetically viewed become doubly interesting, for they are include conquerors the fathers are heralds of the unseen future. include the ancestors of future conquering races. In the great scheme of nature, to that which has much, much will be given.

Finally, then, in regard to our principle of Divergence, which regulates the natural Selection of all favourable variations, & causes the Extinction of intermediate & less favoured forms, I believe it to be all

(26mm

(Ch. 6 Principle of Divergence)

important as explaining why the average difference between two species of the same genus, the parents of which by our theory once existed as mere varieties, is greater than the average difference between two such varieties.

It bears on, & I think explains, the classification or natural affinities during all times of all organic beings, which seeming to diverge from common stems are yet grouped like families within the same clan, tribes, tribes within the same nations, & nations within the same sections of the human race. We shall, also, hereafter see that these views bear on palaeontology & explain why extinct forms either fall within existing groups, or, as is so frequently the case, are in some slight degree intermediate between them.) (The relation of all past & present beings may be loosely represented by compared with the growth of a few gigantic trees; that is if we suppose that from each of the innumerable twigs, innumerable buds are trying to sprout forth, & that the outer buds, twigs & branches have the best chance of growing from getting more light. The living buds & twigs may represent existing species, & all beneath their living extremities may represent extinct forms. We know that the twigs proceed from lesser branches, these from larger & these from main limbs, from the trunk, & that the several branches & limbs are of very unequal

(26nn

sizes; & this grouping of the branches may represent the natural classification of organic beings. In our living trees we can trace in the gnarled & leafless branches the connecting links; but so imperfect are our geological palæontological records, that we can only now here & then there find a form which may be called a forked branch, with its two arms pointing directed towards two now distinct groups of organisms. As we know that the gnarled branches were onc at successive periods tender twigs crowded with buds, so we may believe that group of fossil every organic class class of, whether or not now having lineal descendants on the earth, once swarmed at each stage of descent under with with diversified forms of life. of which we know have seen only a few fossil representative.

Many a smaller & larger branch, & even some main limbs have utterly perished, from being over topped by the ever-diverging budding twigs; so it has been with whole groups of organic beings. Here & there a branch is still alive, carry carrying only a few twigs & buds; & these may will represent the organic groups having few species & fewer genera, which are now on the road to extinction. As buds give rise by growth to fresh buds, & these, if vigorous, branch out & give rise to many a diverging branch with its twigs & buds, still branching out, & causing the death of many wea a feebler twig & branch on all sides all round & beneath, it so by generations I believe it has been with the great Tree of Life, which fills the crust of the earth with fragments of its dead & broken branches, & covers the face of the earth with its marvellous ever living, ever diverging & marvellous ramifications, the face of the earth.)

[pencil insertions later erased:] & water & beautiful branches

(27 & (28

(Ch. 6. Nat selection)

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

A small & very large scale. This principle of divergence, I believe, plays an very important part in the affinities or classification of all organic beings.

(Lead)

Long before By ere this, time, a crowd of difficulties will have arisen in the reader's mind, apparently overwhelming my theory of natural selection, more especially when applied to organs or beings widely apart that will always different in the same great classes. Some of these difficulties are indeed great enough almost to overwhelming crush my belief; but many, I think, are only apparent.

Is it possible to believe that the eye with its admirable correction for spherical & chromatic aberration, & with its power of adapting the focus to the distance, could have been formed from the simplest conceivable eye, by natural selection? Is it possible for the instinct of a bee, which produces a cell constructed on the highest geometrical principles, to be thus perfected? I confess that my mind recoils from such an admission; yet, seeing reflecting on the possible known gradations in so wonderful an organ as the eye amongst existing animals,—a mere

(29

(Ch. 6. Nat selection)

small fraction of those which have lived,—I can see no logical impossibility; & as far as probability goes is concerned, a safe conclusion can be drawn, as it seems to me, only from the general phenomena of organic beings, as indicative whether each being has been simply created or has been produced by the common laws of generation with superadded modification. But these questions, & likewise the general subject of instinct shall be discussed in separate chapters.

What shall we say of small & apparently trifling organs, yet most useful to the animal possessing them, as the eye‐lash, or a tail serving as a fly‐brush; could these have been produced by natural selection, which is in fact selection for life & death? But I have already shown how cautious we must should be in deciding what trifle may turn the nicely‐suspended balance of life in the great struggle for existence. Again how could a swimming animal be turned into a crawler, or a walking animal into a flyer: how

(30

(Ch. 6. Nat selection)

could they live in an intermediate state? Undoubtedly nothing can be effected through natural selection except by the addition of infinitesimally small changes; & if it could be shown that in cases like the foregoing, transitional states were impossible, the theory would be overthrown. This being so, it may be further asked, do we not find meet in certain members of a class organs, which, as far as we can see, are absolutely new creations, & which cannot be some other part or organ modified by natural selection in accordance with the laws of morphology? We shall see that such cases are surprisingly few & hard to find.

Again it has often been urged that if species were subject to change all nature would be in confusion & the limits of no species distinct; but this argument depends wholly on the assumption that the change is rapid & that many species are simultaneously undergoing change. If [illeg]species were as distinctly defined, as some authors pretend, systematic

(31

(Ch. 6. Nat selection)

natural history would be a far less difficult subject, that those authors will find if they will take up for description almost any group, especially a varying group of species; but to this subject I shall immediately recur.

So again it has been said, if species were subject to change, we should find plain evidence of such change in our collections of fossil remains; but the force of this objection, in main part, lies in the supposition that the records of geology are as ancient as the first commencement of life, & that they are far more perfect than some of our most experienced geologists have shown good reason for believing that they really are in truth. I will here only ask those who make this objection, can they believe that at some future geological epoch, fossil remains will tell that which we do not now know, namely what were the precise exact steps by which the various British breeds of sheep & oxen have descended

(32

(Ch. 6. Nat selection)

from some one or two parent stocks. It shd be remembered we do not mean forms intermediate between horse and tapir, but between both of them & some unknown common parent.

Lastly why do two species when crossed, either yield few or no offspring, & these more or less sterile, & why do those varieties which we may safely conclude are descended from a single species yield abundantly fertile offspring? To this important subject I will devote a chapter. And all the foregoing great difficulties, & some curious special cases shall be stated in detail, as fairly as I can, & be discussed. That some before few some difficulties remain to be mastered will not be thought surprising by those who will make allowance for our ignorance oh what is daily passing around us in the living world; & our incomparably greater ignorance of the many past worlds which have teemed with life.—

(33

(Ch. 6. Causes fav. & unfav. to Selection)

Causes favourable & unfavourable to Natural Selection.—

Having given a pretty full outline of my theory, it will be necessary to discuss a little more in detail as well as we can, though very imperfectly, the circumstances, favourable or the contrary to natural selection. We have seen that variability is the foundation. The variability variation, whatever its cause be, must be be inherited or tend to be inherited to be of any use. Certainly this tendency is very strong & applies to the most trifling changes; but it not seldom often fails; & the offspring instead of taking after its their parents resemble their grandfathers or more remote tendency ancestors. We see this repeatedly ⸮ (perhaps oftenest, at least most plainly, where strongly marked varieties are crossed; but in all cases it must tend to retard natural selection.

Again the variation must be in the right direction to profit the individual, otherwise it will not be selected. I do not here refer to the direct effects of climatal conditions, which may have some effect, but most for these must be quite unimportant, in producing relation to the numberless exquisite co‐adaptations of each organic being to the coinhabitants other inhabitants of the district area.

(34

(Ch. 6. Causes fav. & unfav. to nat selection)

I am inclined to believe that in the polymorphous or protean groups of species, as they have been called, mentioned in our Ch. IV which we see meet with in almost every great class of being, we see more fluctuating variability,—perhaps the very tendency being to vary being being inherited,— as one the variation being of no use in any one point direction to the being in question, & therefore with with no one particular character steadily selected, augmented & rendered nearly constant.

The expression of variation in a right direction implies that there is a place in the polity of nature, which could be better filled by a modified organism one of the inhabitants, after it has undergone some modification: the existence, therefore, of an unoccupied or not perfectly occupied place is an all important element in success the action of natural Selection.

I do not doubt, as previously remarked from the number of naturalised productions, that everywhere such open places ready to be filled exist; but it is obvious that such places or gaps will be more frequent, & it may be said wider, in districts favourable for life, but yet not a thickly stocked with various forms.

Districts so circumstantial can be subjected to some physical change & cut off from free immigration will be thus circumstanced;—as for instance part of a continent separated by a desert or mountain‐barrier, into which after climatal, or other changes,

(35

(Ch. 6. Isolation)

the other inhabitants of the continent cannot freely enter; or better still a volcanic island, rising from the ocean at first with few or no inhabitants, but receiving an occasional stray colonist. Now both Mr. Wollaston & Alph. de Candolle (Variation of Species, p. 70. Bot. Geograph.   ) have strongly insisted that isolated areas are the chief scenes of what they call consider, with like most naturalists, as the actual creation of new species & likewise of new varieties. It is not, I may add isolation in the abstract which seems to affect organic beings; for the very same spot may easily be isolated for one set of beings & not to another: thus Madeira is not isolated for birds for annually a birds are blown there from the mainland, & there is only one endemic or peculiar species bird & that not a very distinct species: from what we know of the habits of land‐molluscs this island must be closely isolated for them, & a large majority of the species are endemic; whereas there is not a single endemic sea-mollusc, & these, which little as we know of their means of dispersal, can hardly be so completely isolated as the land‐molluscs: have not a single endemic species: again coleoptera are seldom strong flyers, & therefore would be here more isolated

(36

(Ch. 6. Isolation)

than the other orders of insects, & Mr. Wollaston tells me that he believes that there are far more endemic species of Coleoptera than in the other orders. We have seen in the last chapter that birds birds, for instance, in the struggle for existence would be apt to come more into competition with other birds, than with other animals; & so land‐molluscs with land‐molluscs, & beetles with beetles: consequently a few beetles or land‐molluscs (whether we suppose them the remnants of an ancient population before the island was severed from the mainland, or as I think far more probable, occasional stray colonists) placed by themselves in this island would find themselves in a far more disturbed condition & with more places opened to them in their own little scheme of nature, than would those other animals, which found themselves associated with all or nearly all their old compatriots with whom they had long struggled in their native land.

Isolation by itself will apparently do nothing; we can find in England very many groups of plants on distant mountain summits, & some in the lowlands innumerable instances of plants & insects with not another individual of the same species, within a score or fifty a distance of many miles, & which we have no reason to doubt have long remained there, & yet are absolutely identical with the same species

(37

(Ch. 6. Isolation)

from elsewhere. Isolation under a somewhat different climate introduces another element of change; but the fact which must strike every naturalist is that isolation under the same climate seems to have been eminently favourable to the production of new forms. Mr Wollaston has shown how strikingly this holds in the two or three little islands, forming the Madeira group; & indeed to climatal conditions

The climatal conditions of Madeira could probably be parallelled on the shores of Europe, as closely as the habits of the most species require, judging from their ranges on the mainland; yet, as Mr. Wollaston has shown, those islets swarm with peculiar endemic Coleoptera & Land Molluscs. We see the effects, of isolation under the same climate in the numerous endemic species, both with whole groups & in the separate islets of in the Canary different islands of the Galapagos & Sandwich & Canary groups archipelagoes, & in the West Indies, as far as some of their productions are concerned. (a) In some of these cases it can be shown is known, & in other cases it is highly probable, that the inhabitants, excluding those peculiar a single one to the archipelago, are differently grouped to what they are in the mainland, & differently on the separate islets, so that a colonist would be exposed in each to a somewhat different set of competitors. But to this subject, also, we shall have to return in our chapter on Geographical Distribution: ⸮ & we shall then see notice the important fact,

[37v]

(a) p. 37

In our chapter on Geographical Distribution, I shall enter on some details showing how extremely rich isolated islands are in endemic species in relation to their area, as compared with an equal area on the most favoured mainlands. In the case of some of the above archipelagos it is known, & in

(38

(Ch. 6. Isolation)

From the foregoing considerations I conclude that the association of an organic being in any country with a different set of such organisms those beings, with which it comes into the most direct competition or dependence, as eminently in allowing favourable for natural selection for acting on whatever variations may occur, & so fill up the open seizing on & filling up new places in the economy of that country. I look at this as so important as to be second only to variability, the basis on which the power of selection rests.

Now an organic being could be particularly liable to become associated with new competitors, either when first arising by chance entering an isolated region into which few of its compatriots had entered; or when living there, after climatal or other changes had destroyed many of the inhabitants, & the isolation of the spot had checked free immigration of new & better adapted inhabitants. In this way, I think, isolation must be eminently favourable for the production of new specific forms. It must not, however, be supposed that isola isolation is at all necessary for the production of new forms;* (a) when for instance a species spreads widely it will almost go universally become associated with new competitors & there will often be some advantage gained by the selection of some modifications

[38v]

* a See on this subject some excellent remarks by Dr Hooker in his Review of A. De Candolle Geographie Botanique in a note in Hooker's Journal of Botany vol. VIII p. 81 83—83.

(38(a

(Ch. 6. Isolation)

in its structure. I do not doubt that over the world far more species of all have been produced in continuous even than in isolated areas. But I believe that on a given in relation to the area far more species have been manufactured in, small for instance, isolated islands than in continuous mainland.

The rate of at which selection can act, depending on the chance production, as we must call it, of slight favourable variations; it might well happen, that a of two forms which only continued selection might would be rendered highly [illeg], undergoing modification, the one might beat out the other, if brought soon into competition; whereas if time had been allowed the other might have gained through selection some advantage, by which it could have held its own, when thrown into competition with the first. In this way, I can believe isolation may have played an important part in allowing two varieties from the same species to be considerably modified, before they are enabled to struggle with each other for existence.)

[38av]

(Isolation, moreover, comes into play in lessening the amount of inter crossing, but here we are launched on a sea of doubt. That most the majority of animals have their sexes separated or when united require the concourse of two individuals each time for the production of young is certain; & I think it has been shown in the third Chapter that occasional crosses will take place both

[39]

Yet this an original tendency to vary in this particular manner would be strengthened

(39

yet the offspring from such crosses would have a stronger inherited tendency to vary in nearly the same beneficial favourable manner, as did the first variety; & natural selection would continue to preserve by preserving such individuals continually augment the tendency; until all the individuals had might become insensibly modified in the same favourable manner. Just in the same way as a large herd of cattle may be modified by crossing even with a single bull of an improved bad shape & by the continued selection of the crossed offspring most like the Bull; & this would be wonderfully much facilitated if the conditions of the country had any the weakest tendency occasionally to produce catt animals with of the desired for character.)

(39

(Ch. Intercrossing & Isolation)

with plants & animals far oftener than would at first be anticipated: but facts do not allow us to say that such occasional crossing is of universal occurrence. In those few cases, moreover, in which intermediate forms have been observed between two strongly marked varieties or reputed species, unfortunately we do not hardly ever know whether they are due to crossing, or to the intermedial degree of action of natural selection external conditions & of the powers of natural selection. But as two individuals of most animals & some plants habitually unite for reproduction; this crossing will obviously retard, perhaps annihilate obliterate, the process of selection by dragging back the offspring of a selected variety towards its parental type. Let us suppose a stray gravid female colonist or rather a pair of any animals to reach any a small isolated islet island; if at its first birth it produced several young of the it their offspring instantly & greatly varied & the old died, there would be no crossing, but such an improbable supposition may be quite disregarded; but if after some some few several generations when the island was well stocked, when the island was pretty well stocked some of its their offspring slightly varied in any favourable direction; these would be selected or preserved, & though they would in all such cases be apt to cross with the parent‐form;

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

we may believe that if there was a repeated partly repeated strong tendency from external causes to vary in somewhat nearly the same favourable direction, this together with tendency strengthened by selection & with are the some inherited tendency in the same line, due to crossing with the favoured variety, might overcome the retardation from crossing & a change might be effected yet by variable steps I am inclined to think that with organism

[40aAv]

[Fragment]

[text excised], where there might be neither the same tendency to vary nor the same place in the polity of nature open & ready to be filled up by the selected forms; in such cases, the retardation from crossing would be much stronger extreme.)

(40aa)

(Ch. 6. Intercrossing)

more quickly formed & hold their own against the ill effects of crossing, without being completely isolated. Though in most such cases, isolation, at least partial isolation at first, would be favourable to their natural selection.

I have just taken the case of the selection of a variety of a freely‐crossing animal, on an isolated island; if we suppose the same process to be going on, in some favourable spot, but open all round to the inroads of the parent or unaltered form, there would be crossing with the parent form, not only at first in the actual spot in question birthplace of the variety, but all round its confines, [Stauffer added the above fragment to p. 256]

(40

(Ch. Intercrossing &) Isolation)

(I am inclined to believe, that wherever very many individuals of a freely crossing & highly locomotive animal existed, the retardation of any selected modification from crossing would be so strong, that it could hardly be overcome, without indeed the was a tendency to vary in some particular direction was extremely strong. Hence I infer that some degree of isolation would generally be almost indispensable. This isolation may result from the nature of the area; or from the varieties keeping in as soon as produced, keeping to a certain extent separate; & we shall immediately see that some partial separation of varieties, in the higher animals can & does take place in nature. That isolation from locality is important with highly locomotive, freely crossing animals, I infer from the fact, that I find with birds & mammals, the varieties & close & very doubtful species, (not here considering mere monstrosities, such as albinoes &c) almost generally inhabit distinct areas.

On the other hand, with organic beings, such as most plants, which do not cross for each birth or which are not highly locomotive I can well believe are so as to cross widely with individuals over a wide area, or which when favoured can increase at a great rate, I can well believe that a small body of any selected variety might be

(41

(Ch. 6. Nat. selec. Intercrossing & Isolation)

In all these cases of crossing, we should remember the facts given in the third chapter, which convinced me that the offspring from two varieties have a greater amount of vigour & fertility, which would give them an slight inherent advantage, however slight, over the parent forms; tending thus to obliterate the variety, but on the other hand leaving descendants with some inherited tendency on which the same original cause of the variation, we may believe, would be very likely to react. I am tempted to give an illustration of the effects partial isolation & which I should attribute to isolation in regard to crossing: in Madeira there are 20 land‐birds* (*Excluding Grallatores; see Mr. E. Vernon Harcourt's excellent paper on the ornithology of Madeira in Annals & Mag. of Nat. History June 1855. I am infinitely obliged to Mr. Harcourt for having given me much valuable information on this subject.) ie of which onc are strictly inhabitants of breed in the island, & of which only one does not likewise are inhabitant of the Europe or Africa; but besides these twenty, are 26 other species occasional visitants stray species from Europe the continent have been observed. Of these stragglers about 17, as I am informed by Mr. Harcourt, appear every two or three years, & some of them almost annually, & occasionally in little flocks, which has been noticed in the case of the starling, rook &c.—This being

(42

(Ch. Intercrossing & Isolation)

the case, it seems most improbable that individuals of those species which are strictly inhabitants of the breed on the island, should not likewise be occasionally blown there from the continent, although of course it is almost impossible to prove this. Therefore I should infer that the Birds of Madeira have not suffered undergone modification, in the first place because the small island is well stocked with the same species, which have long struggled together in other & not very dissimilar lands, & secondly because, any slight tendency to change, which I believe must exist would occur as the conditions cannot be identically the same, is checked by an occasional cross with quite unaltered forms having no tendency to vary;— & finally that the crossed offspring would having greater vigour & hence a better chance of surviving.

What a contrast is presented by the Galapagos Islands, situated in a most tranquil sea, & at nearly line the distance from the mainland climate, without any storms to blow birds from the mainland, which is nearly twice as far off; in this considerably larger group we have 26 land‐birds, of which 25 are endemic or peculiar to the archipelago! Of these 26 species, 8 belong to one peculiar endemic genus Geospiza, & five others belong to sub peculiar & three sub‐genera closely allied subgenera to Geospiza; there are three closely allied mocking-thrushes, & two tyrant-flycatchers; so that according to the views, which we are considering I imagine that there were only

(43

(Ch. Intercrossing & Isolation)

14, perhaps only 11 original stray colonists, which arrived at different periods, & which had to fill the places in the cas economy of nature, occupied by 20 birds in the very much smaller island of Madeira; hence I suppose that nearly all some the birds had to be modified, I may say improved by selection in order to fill as perfectly as possible their new places; some as Geospiza, probably the earliest colonists, for as Geospiza having undergone far more than other change than the other species; Geospiza now presenting a marvellous range of difference in their beaks, from that of a gross‐beak to a wren;*a one form sub‐genus of Geospiza mocking a starling, another a parrot in the part the form of their beaks. In this archipelago, moreover, there would could be little or no little retardation, or none, from crossing with unaltered forms from the continent.

I have remarked that in animals of which two individuals unite at each act of reproduction, & more especially this some degree of separation must be if not actually necessary, yet most advantageous. This may arise from a selected variety individual with its descendants, continually selected, & inheriting a tendency towards the same & similar variation, continually selected, & reacted in by the same original cause of variation, together with an inherited, as soon as formed even into an extremely slightly different variety, tending to haunt a somewhat different station, breeding

[43v]

* Zoology of the Voyage of the Beagle Pl. 36 to 44.

(44

(Ch. 6. Intercrossing prevented)

[Darwin's 'U' indicates he had used the passage. This is the only folio marked 'U']

at a somewhat different period season, & from like varieties pairing preferring to pair with each other. from preference. To show that this is possible The following facts show that this is possible.

After matching for experiment the most distinct breeds of Pigeons, & though the birds, though paired for life, seemed to me to show plainly tendency to take notice a liking each for its own kind, so that I was led to ask Mr. Wicking, who has kept a larger stock of various breeds together than any man probably in Britain, whether he thought the different breeds, supposing that there were plenty of males & females of the same kind together, preferred to each other would prefer to match together; & he without having any theory whether unhesitatingly answered that he was convinced that they did so would. (a) It has been asserted*(1) that sheep of different breeds turned out together tend to separate, one sort taking to the more upland pasture another to the lowland pastures: in the Shetland Islands*(2) two breeds of sheep have long kept distinct, the one haunting the mountain summits, the other the lower lands. In the Falkland Islands, Capt. Sulivan assures me

[44v]

(a)

: it has, moreover, often been remarked that the Dovecot pigeon, the pare ancestor of all the breeds seems to have an actual aversion to the several fancy breeds.*

* (The Dovecote by the Revd E. S. Dixon, p. 155.)

(45

(Ch. 6. Intercrossing)

that the herds of white & brown cattle tend to keep separate, & though neither are quite pure: yet that the white the white haunt the mountains, & contrary to what might have been expected, they white breed about a month earlier than the brown. (U) which would greatly In the New Forest*(3) the herds of brown & pale‐coloured deer have long kept separate, without intermingling. We have seen in the Catskill Mountains*(4) two varieties of the wolf hunting different prey.

In the forest of N. America, Lyell Sir John Richardson*(5) (*5 Fauna Boreali‐Americana, p. 241 239, 250) says that "there are two well-marked & permanent varieties of the Caribou deer that inhabit the fur-countries; one of them confined to the woody & more southern districts, & the other retiring to the woods only in the winter & passing the summer on the Barren Grounds ":so that these annual migrations are different; the woodland variety retiring more inland in September, the other more southward.

So in Tasmania, Mr. Gould informs me that there are two very slightly different varieties of     one of which migrates & the other does not. Many instances could be given of Birds of the same species inhabiting the same country, some of which migrate and some do not & which can be distinguished by very slight differences. In all such cases there can merit would be f some tendency for varieties having such different habits to keep distinct.

(46

(Ch. 6. Intercrossing)

We have seen in ch the fourth chapter how the Common Ravens in Faroe drive away the pied Ravens, though sometimes pairing with them: the hooded & common crow haunt different districts which must check their crossing, for but when they meet they often cross; but here we have to do with repeated species se forms considered as species by most ornithologists.

[pencil insertion:] Himalaya Pheasant of 2 kinds — N. America Woodpecker

So again in India reputed species of Coracias, as I am informed by Mr. Blyth, intermix & blend on the confines of their range. as do the Carabus So do, to give one instance in insects, the Carabus purpurascens of Western Germany & the eastern C. violaceus; of th at least where they meet there is a reputed third species C. exasperatus, which presents varieties undistinguishable from the two foregoing species.*(6) (*(6) Erichson's Report in Ray Soc. Reports 1841‐1842. p. 161)

In the case of plants, as there is no reason to suppose that in the great majority cross of cases there is are crosses are more than are very frequent or at least in many cases only an occasional crosses at only occurs, there will be less retardation in natural selection from this cause; more especially as any favoured variety might rapidly increase, & hold its own, on exactly the same principle, that seed-raisers cultivate large plots of the same variety in order to get pure seed & lessen the ill‐effect of an accidental cross. If A variety might, also, easily affect a slightly different station & seed, for instance

(47

(Ch. 6. Intercrossing)

on a hill-top at a different period on a hill‐top for instance as is known often to be the case. Indeed there are innumerable instances of varieties of plants occupying particular sites or whole districts in the midst of the range of the species: thus the Centaurea nigricans, which Prof. Henslow, as we have seen has proved by culture to be only a variety of C. Nigra, holds occupies Hampshire to the exclusion of the common forms. The primrose & cowslip are sometimes found mingled though generally affecting slightly different stations. We show also Although I think there can be little doubt that crossed varieties of plants will have an advantage from their inherent vigour; yet we shall see in our Chapter on Hybridism that there are some few curious but well ascertained facts showing that one between certain varieties the pollen does not a of one variety  far from having a prepotent influence of another fertilising power on the other variety, is less influential. This leads me to remark, that although facts are greatly wanted to support the hypothesis, that sterility may supervene in between varieties slowly formed by natural selection, sterility may supervene, I think I shall be able to show in the same chapter that this is not in itself very improbable. At least I shall be able clearly to show that the difficulty in crossing species & the sterility of their off‐spring, by no means follows laws, as if simply

(48

(Ch. 6. Intercrossing)

ordained to keep species distinct. On the hypothesis of that sterility at last supervenes in between varieties formed by nature, & called by us species, there will obviously be not the least difficulty, where this has happened in keeping such varieties for ever distinct: But on this hypothesis it may be very important that two varieties during the whole early formation until converted into species should be isolated or kept apart.)

(If there be organic beings in opposition to the general facts, given in the third chapter, there do exist organisms beings, of which two individuals never, or only at intervals of thousands of generations, unite or cross, then these cannot be kept uniform by intercrossing & selection cannot be thus retarded. by intercrossing.

In such cases the formation of new varieties & species will then be stopped only from want of variability, of a the absence of a new place in the polity of nature, from the want of variability, or such variability the variations not being inherited, the offspring taking after its grandfather or more remote ancestor, instead of its parent.)

[49a]

agriculturist with a large stock of animals to work on, will have a better chance of gaining a prize for the standard of perfection than will one with a male having only a few animals to select from: so again it is nurserymen, who raise large crops of our different flowers, who generally succeed in getting new & prettier varieties. As in each country all the variable forms are striving through selection to get the upper hand, so any one that will there is not have unlimited time; if others are imposed for any one; & if

(49

(Ch. number of Individuals)

(The number of the individuals of any species must form one important element in the formation of new species through natural selection. Several considerations incline me to lay considerable stress on this. We have seen in the Ch. IV, that on evidence which seems to me satisfactory, that it is actually the common species abounding with individuals & some which oftener present varieties; & I there attempt to gave the very obvious reason, that when many individuals existed there would be a better chance within a given period of variations arising, which might in some way prove beneficial to a selected variety. Just in the same way, as an

(50

(Ch. nat selection)

& any particular form if be not modified it will run a good chance of being left behind in the march race & being thus exterminated.

( (a) On the other hand the a large number of individuals of individuals will cause farmer much intercrossing of the selected form. apparently be injurious by favouring intercrossing with the selected forms. But it may be a g we have not facts enough to guide our conjectures on these complex points: it may be that generally varieties, even amongst organisms which do not freely cross, generally arise on a small spot, partially secluded isolated, in the midst of the range of the parent‐species; & that they remain there till so much modified, as to spread largely by overcoming the parent form; either sometimes crossing with it on its confines with selection continually acting on the crossed forms. (a) or such spreading till, crossing [2 words illeg] having as I believe to a to be possible cause, some sterility when crossed with its parent form has supervened.

(As perfectly isolated spots, such as islands, are apt often small, selection will be here retarded by the fewness of the individuals; but at the same time the competition will be less severe & there will be less danger of the extermination of a new variety from their being fewer forms to give rise to other new & victorious varieties or species. The greater number of open places in the polity of nature in is islands, especially if stocked at long intervals by chance colonists only at long intervals, could probably more than counteract the evil from the fewness of individual numbers;

[50v]

(a) Several facts strongly incline me to lay considerable stress on the number of individuals in favouring selection modification through natural selection.

[50v]

a) The supplanting of a parent-species by a variety, which inherits all the characters & advantages of the parent, with some superadded advantage, will generally be an extremely slow process, as already explained in a former part of this chapter; for instance  a plant a variety more capable of enduring drought [illeg] or resisting some insect enemy will have an advantage over its parent only in the dryer spot, or chiefly where the enemy abound, yet during fluctuations of seasons, or when the heat very dry or when the enemy hostile insect is unusually abundant, it will everywhere have an advantage, & tend to spread & supplant its parent.)

(51

(Ch. 6. nat selection)

Certainly, as we shall hereafter see, oceanic islands abound out of all proportion to their size area, with endemic forms, in comparison with continents; but taking an enlarged view but for reasons hereafter to be given, I suspect that the formation of species through nat sel has been slower.

Considering the whole world, from the fewness of the completely isolated spots, & from the difficulty of the subsequent diffusion of new forms when produced in their spreading from such spots, they these spots probably will not have not have played in any therein produced, such isolated spots, will probably not have played a very important part in the manufacturing of species.)

Slowness of Selection.— From the various considerations now advanced, we can see that the formation of new species must be an extremely slow process. New places in the polity of nature for the occupation of the modified descendants of any species can be formed in most cases only at an extremely slow rate. Such new places will be due to physical changes, which will act either directly on the habits or requirements of the inhabitants, or, in a more important manner, indirectly, by causing the extermination or change in proportional numbers of some of the species; by chance immigration, also, not only will new places be opened to the immigrating species, but likewise the older the economy of many of the old inhabitants may be thus most seriously affected. All these processes such changes will generally occur either very slowly or at long intervals. Secondly we require for the formation of new species, variability, & variability if is right repeated variation of

(52

(Ch 6. Slowness of Selection)

the most diversified nature, in order that changes of structure may occur if in the beneficial right direction.

Variability will depend largely on the conditions, more especially on changing conditions, to which the organic being is exposed; & the amount of variability variation will in part depend on the number of varying individuals. Selection acts only by the addition of infinitely small & numerous variations in some given & advantageous direction; & the process will be stopped by want of inheritance in any such characters & retarded by intercrossing.

(I can well believe that many will exclaim, that these causes are amply sufficient wholly to stop all selection & modification through natural selection: I do not believe so; but the result must be judged of by the general phenomena of nature. That the changes will usually be most extremely slow, I fully admit; & I am convinced that a fair view of the geological history of the world accords perfectly with an extreme degree of slowness in any modification of its inhabitants.)

[53a]

June 1858

I doubt whether I have got intermediate links yet clear. An animal rarely ranges over whole continents from climate—if it ranges to some extent then it will get into new conditions, but they will change rather abruptly, & only few cases—so we ought not to expect infinite gradation at same time only over moderate area, over which climate will let it range.

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On the absence of intermediate forms or links between species close at species of the same genus.— One of the most obvious difficulty on our theory, is if any two or more species have descended from a common parent, & have been so slowly modified by in numerous small changes, why do we not see all around us, or find embedded as fossils in the earth, innumerable varieties or the finest links closely connecting in an unbroken chain such species? This subject must be discussed here at some length, & likewise in our chapter on palæontology. That such links must, have existed on our theory, have existed, or do now exist, I fully admit. With respect to the nature of the links it is difficult always to keep clear of one source of deception, namely the expectation of finding direct links between any two species which we are considering: an example from our domestic breeds of pigeons will make what I mean clear; if we take a Pouter carrier & Fantail pigeon & look at them as two species consider their origin, we have not the least reason to expect a graduated links between them, namely birds with longer beaks slightly covered with wattle slightly pouting & at same time with tail slightly expanded; but what we should find, if we had records of every bird kept by fanciers, during the last few thousand years, would be pigeon varieties intermediate in character between pouter carriers & the rock‐pigeons, & between fan‐tails & rock‐pigeons: (a) So again, still more strong, if we look to two parent species in nature

[53v]

or beneficial kind;

[53vv]

(a) The rock pigeon, have being in its general characters intermedial between these two breeds, though not pouting having a long‐beak covered with any wattle, or having its tail at all expanded.

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remote in character; for instance the Horse & Tapir; from not having any idea, what on our theory, was their common ancestor, it is hardly possible to avoid look expecting some form the conclusion that numerous forms directly intermedial between these two must have existed: whereas it might well happen that the common ancestor was fully as unlike in general appearance either many of its characters a horse or a tapir, as these two animals are from each other, yet being in its general the some degree character organisation intermediate between them,) though, perhaps much more nearly resembling one of these two genera,) than the other.)

From what we have already seen in this chapter, it seems probable that each variety, whether found arising insensibly within the same district with the parent-form from the slow modification of the whole parent‐stock, or when formed in a separate area, or on some one spot within the same area with the its parent-form, & subsequently spreading, will tend in the long run to supplant & exterminate its parent‐stock; for ( as its its formation is due to some new advantage gained under the conditions to which it is exposed, as & it will generally largely inherit the same the advantages with common its of its parent). This process will be continually repeated. In all these cases we could obtain a chain of intermediate gradations, only by finding discovering fossil remains of extinct forms; for of those living at one time & within one area we should see only the parent‐stock and one or two varieties, slowly increasing if increasing in range & numbers which if destined to become triumphant will increase th in numbers & range & so ultimately supplant the parent; the parent, I may add,

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being ranked as the variety, as soon as its range became less than that of the conquering variety. In the cases of insensible modification we should not at any one time see within the same area, a variety recognizably different from the parent, only mere individual differences.)

(Why in those classes, of which abundant fossil remains have been are discovered, we do not find are capable of being preserved & have been abundantly discovered, we do not find innumerable links connecting recent spe with extinct species, will be most conveniently discussed in our chapter on palæontology. I think several fairly good reasons can be assigned. I will here only add that the whole force of the difficulty rests on the assumption that our geological records are not only nearly continuous in time, but at during each time period over nearly continuous in space; for otherwise local varieties, which seem at first to be so frequently local could only rarely be preserved. We should, also, remember that the definition of the term species is arbitrary; if an extinct form be found to a certain extent intermediate in character between two existing species, as is of such frequent occurrence; this may be fairly viewed on my theory as one of the intermedial links; the extinct form may have been the actual ancestor of our two species, or more probably on conditions

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more probably it may be an early & less modified or divergent descendant of the common ancestor, either in the direct line of descent of one of the two species or in a collateral & extinct line;*a but all naturalists would rank our in some degree intermediate fossil as a distinct species, without they likewise discovered every intermediate grade between it & one or both of the two living species; or but that this should be asserted obviously requires the collection of very many specimens to have been collected over, which generally must have been embedded at slightly different periods & over a wide considerable area: supposing moreover this to have been effected, as occasionally has been the case, nothing more is thought about it; it is only the often [illeg]case of two forms at first ranked by our palaeontologist as two species & subsequently thought to be proved by a second palaeontologist to be merely varieties.

Conchologists dispute now doubt whether certain sea‐shells, living on the shores of N. America & Europe should be ranked as species or varieties; when in the perfection the present day has become a miocene or eocene epoch is it probable that with the palæontologists of that far future day fr epoch will find fossilised remains intermediate links between these now living & doubtful forms. He who does not expect this, has no right, as far as I can see, to expect now to find all the fossil links between a recent & closely allied fossil shell.)

[56v]

* a p. 56 Ch. 6

What I mean may, perhaps, be best understood by turning to the Diagram printed at p….

Let a10 & l10 be two now living forms one a10 living form & l9 an extinct & with all their ancestors be extinct. If A should chance to be discovered it will be strictly intermediate, though it might in many of its characters far more resemble a10 than l10 ; if a2 were found were found, it would be a lineal ancestor of a10 , & it would be largely intermedial between a10 & l10 , for it had diverged but little from the parent‐type A. So it would be with f3 or i3, which are for they are early & collateral descendants from A, which from that as have not tran become extinct & transmitted no descendants.

either as

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Looking now to the present time alone, if we travel for instance southward over a continent; we find at fr the point whence we start many species very common, but as we travel southward some of them become, more or less abruptly, rarer & rarer, till they disappear; but as they disappear, other closely allied or representative species, apparently filling nearly the same place in the economy of nature, take their place, at first being rare, & then becoming more or less abruptly, becoming very common common. The two species, both comparatively rare, often commingle in the a narrow neutral territory which is narrow. Every [hereafter] naturalist must have met been struck with I have seen many in seve man very many instances of this such cases with amongst the birds & mammals of large continents: it is seen may be observed with plants in ascending mountains, & with shells, as discovered by the dredge, in the descending depths of the sea.* (* See Prof. E. Forbes numerical observations on this head in his Report. Brit. Association on the Aegean Sea 18' p.   ) Why in the such open neutral or border territories without any barriers driving dividing them into sub‐regions and under apparently quite intermedial conditions do we not commonly find in intermediate & graduated forms, connecting the two species, which are supposed by our theory to have originally descended from a common parent. That we do not most rarely find such take the such forms is most certain; the two species, even selecting the most locomotive & freely crossing animals, on comparison, will be found in every single respect as absolutely distinct, as if specimens had been taken from the metropolis of to each species. had [illeg] been compared. This

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for a long time, formerly appeared to me a most serious difficulty; but the difficulty is largely due, as I believe, to common yet erroneous views on several circumstances points in nature.

In the first place we should be very cautious in concluding that because a continent is now continuous, it has existed remained in this state during the whole period of existing species. How many extensive areas have been greatly elevated within the period of existing shells; & what wonderful changes of level are shown by [3 words illeg] erratic boulders now scattered over the low‐lands & mountain‐summits, & which have been borne on ice‐rafts over the sea. What an enormous amount of recent depression of level may be inferred from the structure of living coral‐reefs.

Even when we have no direct evidence, the form of the land sometimes leads to the conclusion, as in the case as at the Cape of Good Hope of the southern extremity of Africa, which is so extraordinarily rich in species, has had several hereafter to believe that it existed formed at no very remote epoch as a large archipelago of islands. It is probable that very many single volcanic islands have once within the recent period existed as a group of islets; like those forming the little Madeira group which are now repeatedly inhabited by many distinct species & distinct varieties. Even when there has been no change of level, desert tracts may formerly have intervened, where the land is now continuously fertile. If we read visit many look at some great of the larger volcanic

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(Ch. 6. Intermediate gradation)

islands, or read Mr Webb & Berthelot's account of the Teneriffe, we shall see that some of the valleys are almost as is much perfectly separated from each other for some organic beings from each other by lofty spurs as if divided by arms of the sea.

In such new isolated fragments of land, of a as larger one are soon, groups of the same species might become differently modified, for they would be associated, especially after any changes in climate &c, with different sets & different proportional numbers of competing associates; & in such cases there could be formed no fir graduated intermedial links by crossing; nor would the more important conditions of life, in relation to other organic beings, graduate insensibly away between one of the isolated fragments of land to and another. After reelevation, if the new forms had been sufficiently modified not to keep distinct cross & blend with each other, each would spread as far as it could, & would mingle in the intermediate territories with other forms proceeding from different birth‐places. On each of the once isolated spots & there alone, we ought to find & these alone if our geological records were perfect, intermedial links between the new forms and the states under which they formerly existed.

Nor should we forget the facts, already given, of varieties of the most freely crossing animals, sometimes keeping apart, or breeding at different seasons &c, which would greatly lessen or prevent the formation

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of intermediate links by crossing, though it would not often lessen the function of of such links in relation to the intermediate state of the conditions of life. Unfortunately in In those cases, in which intermediate varieties have been found, connecting two species or races or two sp closely allied species, we hardly ever know, whether they have originated from crossing or from the direct & graduated action of climate, or from natural selection having fitted those intermediate forms for intermediate conditions of life. And our ignorance on this head greatly adds to the perplexity of this whole subject.)

(Although I believe the former broken & isolated state of parts of now continuous areas, & in a lesser degree the keeping apart voluntary separation of the varieties of the higher animals, have played a very important part in the formation of species, new since become thoroughily  commingled, or only just meeting in a border territory; yet I do not doubt that many species have been formed at different points of an absolutely continuous area, of which the physical conditions seem to graduate from one point to another in the most insensible manner. But here lies a source of deception; we are so much struck with the evident manner in which the heat or moisture graduates away, in going from one point latitude to another, that we can hardly avoid

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overlooking the more important relations of organic beings to each other. We have every reason to believe, from what we see in gardens & menageries, that almost all organisms can withstand more heat, cold, moisture or dryness, than they are exposed to within their natural range; the reach definite limit to any the range of most species, being other under gradually increasing unfavourable conditions, being the presence of other competing forms better adapted to the such conditions. So that in going for instance southward, the decreasing numbers & final disappearance of any species, is not by any means wholly due to the extremely gradual change of climate, but to the sudden presence of other competing forms, or the sudden absence of others, on which our species may chiefly depend for food; & the relation of the prey or fo6d will again depend on other organic beings; all nature being bound together in an inextricable net‐work of relations.

(a)

[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

Hence as the limits of a species does depend in large part on the other organic beings, the conditions on which its presence depends will not graduate away so insensibly, as if they it had depended on climate alone; & the strictly intermediate zone with strictly really intermediate conditions not depend in chief part on the larger of other organic beings. B between any two representative species will be comparatively narrow & support proportionally few nearly in the same way as if the temperature along som one any line of latitude had

[61v]

(a) a gradual change in climate is very obvious, but the struggle for existence, is depending on many obvious contingencies & chiefly on other organic beings often far removed in the scale of nature is extremely obscure; & it is most difficult to keep this steadily in mind.

[61v]

It comes to this, that if the majority of the living forms in any country, as every one can see with care, are defined in their character and do not insensibly blend together, & have a moderately wide range then the extension of relations in range & in all other respects of any one form undergoing modification will have tend to be defined: if the relations of organic beings were had been in a wholly preponderant degree were concerned with related to climate alone, then the range & specific modification of any form undergoing modification would have been related in an [illeg] close gradation to manner to the insensibly changing climate.

(61

if all the individuals of a species were slowly changing in a body or all those in one locality; in such cases we could get evidence only by the comparison with of the living with those which had existed some considerable time previously for the new each new change is preserved solely for having some advantage over a given state, which it will tend to annihilate & the possibility of this comparison will depend on whether the orga species could be preserved in a fossil state & on the perfection of our geological records). (But if a species were over a continuous within the same common area was in the act of in giving rise to dividing into two or more species we might expect to find existing at the same time intermediate forms. Such do exist, as we have seen in this & the fourth chapter. But we We should always remember that intermediate varieties seem comparatively rare, & therefore would not likely to be discovered except in well known countries; & from this very rarity would be apt to be exterminated, &

[text excised] during the slow process of their act of separation, without

[61v]

Hence we have no reason to expect that in going southward that any one species ought to be insensibly modified in relation to the slowly changing climate, but chiefly in relation to each, new set of those organic beings, with which it comes into the most direct competition or stands in some relation; of preying on and being preyed & the zone with really intermediate conditions, will depend in chief part on the range of other organic beings. As we see that the range of most organisms is in some degree defined, the species becoming, generally within a rather over a narrow space, rare & then quite disappearing, the zone with really intermediate conditions between for any two species will generally be narrow, & therefore cannot support any vast number of intermediate varieties intermediate between such two species.)

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[For readability, Darwin's horizontal and vertical crossed lines are left undeleted.]

changed abruptly by some few degrees. Consequently the number of intermediate forms intermediate forms between the two representation, which it might have been expected, could have been supported in the intermediate zone will be less, that if that zone had been commonly very wide broad.)

(Whether we ought, on our theory, to find many cases of two species closely connected by intermediate links in the narrow zone, which is really intermediate in all the its conditions relations of the to the two bordering species, must depend on whether at the same period many species are undergoing modification & on whether intermediate varieties, when once formed are likely to endure for long periods. I am very far from supposing this to be the case, yet as to process of change is supposed to be

Every fact in geology seems to show that species change very slowly & therefore I conclude that but few species are undergoing modification at any one period; but as the process process by our theory excessively slow is excessively slow, some such cases ought to be occur in every large area. I believe that they do, & in our Ch. IV & in a former part of this chapter several such cases have been given of varieties connecting two forms, which have been considered by several naturalists as good species. The cases on record are probably few compared with those which exist in nature; for varieties intermediate between other vairieties or sub‐species or species (for there is no rule to follow in knowing what to call such forms) seem to be generally scanty in individual numbers, & hence would be observed, generally, only in well countries which have been thoroughily well explored worked.)

(The fact truth of intermediate varieties being individually rare is of importance to us. Mr

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Wollaston* (*Variation of Species p. 5) has stated his opinion that this is the case, & he informs me that it is founded from upon his observations on insects & land-molluscs; and only from& few from them (reference) his immense experience in collecting, is more able to form few naturalists have a better right to express an opinion. As Botany is a more being in advanced state than zoology, I applied to Mr. H. C. Watson & to Dr. Asa Gray for their opinions on this point head; for as from their critical knowledge of the floras of Great Britain & the United States, they & their cautious judgment, I show everyone would place great confidence in their judgment.

Both these botanists concur in this opinion, & Mr. Watson has given me a list of eight twelve nearly intermediate varieties found in Britain which are rarer than the forms, which they connect. But both these naturalists have insisted strongly on various sources of doubt in forming any decided judgement on this head.

[faint pencil lines, partly excised:] to be shortened — at p. 6 discuss intermeditate

[63v]

[For readability, the vertical lines which Darwin crossed are left undeleted.]

(a) Insensible selection, which flock [altering]—intermediates cd only be discovered in past times — it is not that a marked separate variety is formed, but whole mass slowly altered — This might be going on in several districts when [text excised] — so in separate station of one great area, if the whole [text excised] crossing or borders from chance [text excised] by spread.—

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Therefore, as it seems to me, we ought to expect to find only some few cases of intermediate varieties, inhabiting a narrow zone between the areas inhabited by any two species which they closely link together. But it may be asked, if varieties intermediate in character between two bordering species are ever once formed in such narrow intermediate zones, why do they not endure for as long a time as the species which they connect? & if they did so endure, cases of linking varieties could hardly fail to have become in the course of time with species after species undergoing modification far commoner in nature than they seem to be. I think some sufficient reasons can be assigned why they should not last for very long periods. As they inhabit a narrow zone (for we have seen zones with really intermediate conditions must generally be narrow) they cannot can hardly be, & do not seem to be, numerous in individuals, so that they would always be in some degree liable to extinction from great fluctuations in seasons, or any extraordinary increase of enemies. Morev They are, also, bordered on each side hand by forms adapted to the somewhat different physical conditions, to greater heat or cold, moisture or dryness &c, & to the coinhabitants of the bordering regions, so that if during [a] few successive seasons the temperature became higher or lower &c, they would

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be liable to invasion on either hand; & if they had not great powers of endurance or of migration, or of endurance or if any slight obstacles intervened to migration, they would be liable to be wholly extirpated. Moreover in the case of any two species having moderately wide ranges & commingling, as is so often the case, in a narrow border territory, if we suppose this border territory to have been once peopled by a chain of intermediate links, be wa case see that connecting the two bordering species, we can see that these latter from having wider ranges would be more abundant in individuals, than the intermediate forms in the narrow intermediate zone; and on the principle already explained of a large number of individuals greatly favouring the production of favourable variations, one or the other of the two bordering species would have a better chance of being modified or improved so as to seize on the place of the intermediate links, & perhaps even to invade the territory of the other bordering species.

Finally, then, I suppose, that a large number of closely allied or representative species, now inhabiting open & continuous areas, were originally formed in parts formerly isolated; or that the varieties became in fact isolated from haunting different stations, disliking each other, breeding at different times &c, so as not to cross.

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That amongst those organisms, of which two individuals rarely (or never) unite for reproduction, that varieties have arisen on some one spot & from having some advantage over their parents either during occasional times or at all times has spread (perhaps sometimes crossing on their confines) & have supplanted their parent-forms; & this would be most readily effected in small & isolated districts. That amongst organisms of all kinds, I suppose, that many species have been formed on different points of open & continuous areas, of which the physical conditions change quite insensibly, & that in such cases linking varieties have been formed, but that these would not tend to be infinitely numerous & insensibly spread over a wider space, for they would by no means be related solely to the insensibly changing climate, but in an equally or more important manner to the somewhat definite ranges of certain other organic beings. Such linking varieties (whether produced by the intermediate action of natural selection or of external influence in an inter-mediate degree, or by crossing) seem, as might have been inferred from their theoretically restricted range, not to be abundant in individuals; & hence, I believe, would be apt to be exterminated by fluctuations of seasons, extraordinary increase of enemies &c, and by the inroads of the bordering species, which

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they link together. And lastly, I believe that these bordering species would have a better chance, owing to their greater individual numbers, of being modified & improved, so as to seize on the places of the intermediate & linking varieties. I am well aware, that if I wished to treat my subject as a mere advocate, it would have been better to have slurred slurred over all these complex actions & contingencies, which apparently must affect the formation of new species, & of the relative importance of which I cannot judge; but my object is to point out all difficulties, as plainly, as lies in my power.

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(Ch. 6 Summary)

Summary of Chapter.— During the severe struggle for existence, to which all organic beings, owing to their high rate of increase, are exposed, during some period of their lives or during some shortly succeeding generations, Natural Selection acts by the simple preservation of those individuals which are best adapted to the complex contingencies to which all are related. Natural Selection can seize on plainly marked variations or on the slightest modifications, on mere individual differences even though inappreciable by the human eye, if in any way whatever advantageous to the individual, from its egg state to as late a period as the powers of generation last & can transmit any new character. As pecularities are often, probably generally, inherited at corresponding ages, it can modify the egg or seed, the larva, or pupa, without causing any change in the adult form except such as necessarily follows from correlation of growth. As peculiarities are often inherited by the corresponding sex, it can modify each sex in relation to the other; and the individuals of the male sex may be modified by sexual selection, enabling them to struggle for supremacy with other males, like natural selection modifies both sexes that they may struggle for supremacy

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with other & distinct organisms. Sexual selection will also aid natural selection in giving most offspring to the most vigorous males, under whatever conditions they live. Natural selection will scrutinize every habit, instinct, constitutional difference, every organ external & internal, will preserve the good, & rigidly reject the bad. It may pause in its work for thousands of generations, but whenever a right & fitting variation occurs, without error & without caprice its natural selection will seize on it. From the several reasons already assigned, the process in all or nearly all cases will be excessively slow. in the extreme.

The more greater the variability the better the chance of favourable variations. Some degree of variability, or use Individual differences seem to be of almost universal occurrence; a larger amount of variability apparently depends mainly on changed conditions of life. The amount of v chance of favourable variations occurring will, also, stand in some close relation to the number of the individuals of the varying species. Different External conditions will, also, act directly on the individuals differently exposed & so modify them to a certain

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limited extent: as will, also, use & disuse; but to these subjects we shall selec have to treat recur in a future chapter.

Intercrossing will retard or as prevent or retard the process of natural selection; but here we are involved in much doubt. Locomotion Those animals, which move much about & unite for each (a)

[For readability, the horizontal and vertical lines which Darwin crossed are left undeleted.]

birth, will suffer be retard must in the [lines] can be thus be kept the most uniform to their parental types, or be necessarily can altogether will may be modified not in insensibly slow rate if undergoing change will be modified in a altogether in a manner, travelled at short periods intervals. without, indeed, [2 words illeg] excepting in so far as my

It may be otherwise in those cases, in which varieties, from their very first commencement, haunt some distinct station or breed at different periods &c. Those organisms which rarely cross, & which are capable of increasing at a quick rate, may be formed on some one spot, & thence spread with little retardation from intercrossing.

The direction in which natural selection will act & its very power to effect any thing will mainly depend on there being places in the natural economy of any country not filled up, or not filled up as perfectly as possible. And this will depend on the number, nature, & relations of the other inhabitants of the region, in a far more important manner than on its physical conditions. Look

[70a]

birth will thus be kept truest to their parental type; or if undergoing change will be modified altogether in an insensible manner, without any recognizable variety being formed at any one period.

It may, however, be otherwise in those cases, in which varieties of the most freely crossing animals

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(Ch. 6 Summary)

at the woodpecker or the Bee or almost any other animal or on plants (though here the relations to other organisms, as we have seen in our last Chapter, are less plain, though not less certain) & see how clearly their structure is related to other organic beings: a woodpecker or bee may inhabit the hottest or coldest, the dampest or driest regions, yet how essentially similar is its whole organization. Hence I infer that the association of an organism with a new set of beings, or with different proportional numbers of the old inhabitants, as perhaps the most important of all elements of structural change modification.

If a carnivorous or herbivorous animal is to be modified, it will almost certainly be modified in relation to its prey or food, or in relation to the enemies it has to escape from. Change of climate will act indirectly in a far more important manner than directly, namely in exterminating some of the old inhabitants or in favouring the increase of others. The immigration of a few new forms, or even of a single one, may well cause an entire revolution in the relations

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[Emma Darwin with corrections by Darwin]

of a multitude of the old occupants. If a certain number of forms are modified through natural selection, this alone will almost certainly lead to the modification of some of the other inhabitants. Every where we see organic action & reaction. All nature is bound together by an inextricable web of relations; if some forms become changed & make progress, those which are not modified or may be said to lag behind, will sooner or later perish —

As the isolation of a region of with subsequent climatal changes or with subsequent

When a district is isolated, so that after any change in its physical conditions, new beings cannot freely immigrate, or enter only by a rare accident, the relations between its inhabitants will assuredly in time become greatly disturbed. Hence I infer that isolation would be eminently favourable to the production through natural selection of new specific forms. Isolation will also to a certain extent lessen the retarding influence of intercrossing. It will facilitate the supplanting of the parent type by its modified offspring, & lastly it will give time for a variety to be sufficiently changed so as

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not to blend with, and to hold its own against, other varieties formed elsewhere, with which it may hereafter be thrown into competition.

(As each new variety is formed through natural selection, solely from having some advantage over its parent, each new variety will tend to supplant & exterminate its successor predecessor. In regard to the intermediate links by which each new species must once have been closely connected with its parent, we could expect generally to find such only amongst fossil remains. In those cases however in which a species, ranging over a continuous area, is at the present day in the act of breaking up into two or more distinct species, we ought to find intermediate links in that narrow border territory which is really intermediate in all its organic & inorganic conditions; but we have no reason to expect to find many such cases, & we do find some. The intermediate links in such border territories, from reasons already assigned, would be liable to early extermination.

As a general rule we have seen that widely diffused species, abounding with in-

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: dividuals, & belonging to large flourishing genera, are those which vary most. Of the varieties descended from any one species, the most divergent, or those which differ most from each other & their parents in all respects, will in the long run prevail, for they will be enabled to fill more & more widely different places in the polity of nature. It follows from this that the amount of difference which at first may have been very small between any two varieties from the same species, in each successive set of new varieties descended from the first two, will steadily tend to augment as the most divergent or different will generally be preserved. From reasons already given, namely from the number of different places in the polity of any country not being indefinitely large, and from the individual numbers of each species necessarily being small where very many species exist, which will render such poor species liable to accidental extinction, and will check further modification,—the number of species inhabiting any country will not increase indefinitely; and as the most divergent are those which are the most likely to succeed, the intermediate forms, whether called varieties

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(Ch. 6 Summary)

or species of the same genus or of distinct genera, will tend to disappear.

The groups already large being those which vary most, & the principle of divergence always favouring the most extreme forms, & consequently leading to the extinction of the intermediate and less extreme, will taken together give rise to that broken yet connected series of living & extinct organisms, whose affinities we attempt to represent in our natural classifications. For all organic beings during all time seem to have been related to each other like twigs diverging from the same branch, branches from the same limb, and limbs from the same main trunk representing the common ancestor of a whole class of organisms, with many an intermediate branch and limb now lost.

Finally, then, in regard to the several contingencies favorable to natural selection, I am inclined to rank changed relations or associations between the inhabitants of a country from as opening up new places in its polity, as the most important element of success. The amount of variability, which is largely contingent on the

[76A

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(Thus far I think we may with some safely assert safety conclude that a [illeg]large tract of land, stocked with nearly similar beings species, if by subsidence converted into a group of islands, like those of the great Malay archipelago, would in the course of time be eminently favourable for the production of new forms; & we know that this such archipelagoes does abound with are known to be extraordinarily rich in species. In the course of time, after our supposed subsidence, we might expect the destruction of some species through climatal changes destroying some species; in and the in be occasional introduction of stray colonists; oscillations of level further subsidence causing and oscillations of level which when downwards would cause more destruction; elevations extending, when upwards would extend the area, & thus directly & make new stations;—

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(Ch. 6 Summary)

number of individuals, as of secondary importance; though perhaps time being given for each new variety to be perfected before being thrown into competition with other varieties, may be almost equally important. A diminished amount of inter-crossing is probably the least important element. But the subject is far too much involved in doubt for us to be enabled to weigh to strike any balance between these several contingencies.

(58 (77

(Ch. 6. Natural selection) Summary)

;all and then combined cause, combined actions would act powerfully in the relations of the organic beings inhabitants to each other, ensuring variability, & opening would thus open places in the polity of nature for natural selection to fill. Such changing conditions would also add to the variability of many of the organisms.  In such large islands, there would be plenty of individuals to act on; intercrossing, on at least on the confines would in some slight would be checked prevented; & time would be allowed for the varieties in each all the islands to be well strongly marked & perfected before being uninhabited by any one so as to have a better chance of escaping annihilation, when thrown into competition with other & more favoured varieties, formed elsewhere.

[faint pencilled words:] found in another other districts

Those organisms which spread over were originally common to the whole region, before the first great subsidence, might become converted into a new new forms, whether called varieties or species, in each separate island, or else in some of them remain unaltered, in some according to the organic conditions of life which nature of the organic being forms with which they [pencil insertion:] the parent form had to struggle in each island after it had undergone physical changes.

If we Now Let us If we now suppose our archipelago, through renewed elevation, that this one archipelago was to be reconverted into continuous land; then of the several forms produced from the same parent-species in each former island, some would probably remain on the spot to which they had been adapted, some would spread, & if only slightly different might become blended by crossing with other varieties, or they would exterminate them, or if sufficiently distinct might live & being might live commingled with them. I will only add that owing to the principle divergence to which mere allusion has as yet been only made, I

But in the case both of varieties & species, the most divergent, or those which had become most modified so as to fill some the most diverse new places, would have the best chance of surviving.)