Origin of Species Variorum

Comparison with 1869

group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. Small and broken groups and sub-groups will finally .. disappear. Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which as yet have suffered least extinction, will for a long period continue to increase. But which groups will ultimately prevail, no man can predict; for we well know that many groups, formerly most extensively developed, have now become extinct. Looking still more remotely to the future, we may predict that owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that, of the species living at any one period, extremely few will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on Classification, but I may add that as according to this view .. extremely few of the more ancient species have transmitted descendants, and as all the descendants of the same species form a class, we can understand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms. Although .. few of the most ancient species have left modified descendants, yet, at .. remote geological periods, the earth may have been almost as well peopled with .. species of many genera, families, orders, and classes, as at the present day.

On the degree to which Organisation tends to advance. —

Natural selection acts, as we have seen, exclusively by the preservation and accumulation of variations, which are beneficial under the organic and inorganic conditions of life to which each creature is at each successive period exposed. The ultimate result will be that each creature will tend to become more and more improved in relation to its conditions of life. This improvement will, I think, inevitably lead to the gradual advancement of the organisation of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organisation. Amongst the vertebrata the degree of intellect and an approach in structure to man clearly come into play. It might be thought that the amount of change which the various parts and organs undergo in their development from the embryo to maturity would suffice as a standard of comparison; but there are cases, as with certain parasitic crustaceans, in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva. Von Baer's standard seems the most widely applicable and the best, namely, the amount of differentiation of the different parts (in the adult state, as I should be inclined to add) and their specialisation for different functions; or, as Milne Edwards would express it, the completeness of the division of physiological labour. But we shall see how obscure a subject this is if we look, for instance, to fish, amongst which some naturalists rank those as highest which, like the sharks, approach nearest to reptiles; whilst other naturalists rank the common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like, and differ most from the other vertebrate classes. Still more plainly we see the obscurity of the subject by turning to plants, with which the standard of intellect is of course quite excluded; and here some botanists rank those plants as highest which have every organ, as sepals, petals, stamens, and pistils, fully developed in each flower; whereas other botanists, probably with more truth, look at the plants which have their several organs much modified and somewhat reduced in number as being of the highest rank.

If we look at the differentiation and specialisation of the several organs of each being when adult (and this will include the advancement of the brain for intellectual purposes) as the best standard of highness of organisation, natural selection clearly leads towards highness; for all physiologists admit that the specialisation of organs, inasmuch as they perform in this state their functions better, is an advantage to each being; and hence the accumulation of variations tending towards specialisation is within the scope of natural selection. On the other hand, we can see, bearing in mind that all organic beings are striving to increase at a high ratio and to seize on every ill-occupied place in the economy of nature, that it is quite possible for natural selection gradually to fit an organic being to a situation in which several organs would be superfluous and useless: in such cases there might be retrogression in the scale of organisation. Whether organisation on the whole has actually advanced from the remotest geological periods to the present day will be more conveniently discussed in our chapter on Geological Succession.

But it may be objected that if all organic beings thus tend to rise in the scale, how is it that throughout the world a multitude of the lowest forms still exist; and how is it that in each great class some forms are far more highly developed than others? Why have not the more highly developed forms everywhere supplanted and exterminated the lower? Lamarck, who believed in an innate and inevitable tendency towards perfection in all organic beings, seems to have felt this difficulty so strongly, that he was led to suppose that new and simple forms were continually being produced by spontaneous generation. ↑ Science has not as yet proved the truth of this belief, whatever the future may reveal. On my theory the present existence of lowly organised productions offers no difficulty; for natural selection includes no necessary and universal law of advancement or development— it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life. And it may be asked what advantage, as far as we can see, would it be to an infusorian animalcule— to an intestinal worm— or even to an earth-worm, to be highly organised? If it were no advantage, these forms would be left by natural selection unimproved or but little improved; and might remain for indefinite ages in their present little advanced condition. And geology tells us that some of the lowest forms, as the infusoria and rhizopods, have remained for an enormous period in nearly their present state. But to suppose that most of the many now existing low forms have not in the least advanced since the first dawn of life would be rash; for every naturalist who has dissected some of the beings now ranked as very low in the scale, must have been struck with their really wondrous and beautiful organisation.

Nearly the same remarks are applicable if we look to the great existing differences in the grades of organisation which occur within almost every great group; for instance, to the co-existence of mammals and fish in the vertebrata,— to the co-existence of man and the ornithorhynchus amongst mammalia— or of the shark and amphioxus, which latter fish in the extreme simplicity of its structure closely approaches the invertebrate classes. But mammals and fish hardly come into competition with each other; the advancement of certain mammals or of the whole class to the highest grade of organisation would not lead to their taking the place of, and thus exterminating, fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aërial respiration; so that warm-blooded mammals when inhabiting the water live under some disadvantages compared with fishes. In this latter class, members of the shark family would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz Müller, has as sole companion and competitor on the barren sandy shore of South Brazil, an anomalous annelid. The three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in South America in the same region with numerous monkeys, and probably interfere little with each other. Although organisation, on the whole, may have advanced and be advancing throughout the world, yet the scale will still present all degrees of perfection; for the high advancement of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they do not enter into close competition. In some cases, as we shall hereafter see, lowly organised forms seem to have been preserved to the present day from inhabiting peculiar or isolated stations, where they have been subjected to less severe competition, and where they have existed in scanty numbers, which, as already explained, retards the chance of favourable variations arising.

Finally, I believe that lowly organised forms now exist in numbers throughout the world, and in nearly every class, from various causes. In some cases favourable variations may never have arisen for natural selection to act on and accumulate. In no case, probably, has time sufficed for the utmost possible amount of development. In some few cases there may have been what we must call retrogression of organisation. But the main cause lies in the circumstance that under very simple conditions of life a high organisation would be of no service,— possibly would be of actual disservice, as being of a more delicate nature, and more liable to be put out of order and thus injured.

A difficulty, diametrically opposite to this which we have just been considering, has been advanced, namely, looking to the dawn of life, when all organic beings, as we may imagine, presented the simplest structure, how could the first steps in advancement or in the differentiation and specialisation of parts have arisen? Mr. Herbert Spencer would probably answer that as soon as the most simple unicellular organism came by growth or division to be compounded of several cells, or became attached to any supporting surface, his law would come into action, namely, "that homologous units of any order become differentiated in proportion as their relations to incident forces become different." But as we have no facts to guide us, all speculation on the subject is useless. It is, however, an error to suppose that there would be no struggle for existence, and, consequently, no natural selection, until many forms had been produced: variations in a single species inhabiting an isolated station might be beneficial, and through their preservation either the whole mass of individuals might become modified, or two distinct forms might arise. But I must recur to what was stated towards the close of the Introduction, where I say that no one ought to feel surprise at much remaining as yet unexplained on the origin of species, if due allowance be made for our profound ignorance on the mutual relations of the inhabitants of the world during the many past epochs in its history.

Various Objections considered.

I will here notice a few miscellaneous objections which have been advanced against my views, as some of the previous discussions may perhaps thus be made clearer. Thus a distinguished German naturalist has recently asserted that the weakest part of my theory is, that I consider all organic beings as imperfect: what I have really said is, that all are not as perfect in relation to the conditions under which they live, as they might be; and this is shown to be the case by so many native forms yielding their places in many quarters of the world to intruding and naturalised foreigners. Nor can all organic beings, even if at any one time perfectly adapted to their conditions of life, remain so, when these conditions slowly change; and no one will dispute that the physical conditions of each country, as well as the number and kind of its inhabitants, are liable to change. It has been argued that as none of the animals and plants of Egypt, of which we know anything, have changed during the last 3000 years, so probably none have been modified in other parts of the world. The many animals which have remained unchanged since the commencement of the glacial period would have been an incomparably stronger case, for these have been exposed to great changes of climate and have migrated over great distances; whereas, in Egypt, during the last 3000 years, the conditions of life, as far as we know, have remained absolutely uniform. The fact of little or no modification having been effected since the glacial period would be of some avail against those who believe in the existence of an innate and necessary law of development, but is powerless against the doctrine of natural selection, which only implies that variations occasionally occurring in single species are under favourable conditions preserved.

It has been objected, if natural selection be so powerful, why has not this or that organ been recently modified and improved? Why has not the proboscis of the hive-bee been lengthened so as to reach the nectar in the flower of the red-clover? Why has not the ostrich acquired the power of flight? But granting that these organs have happened to vary in the right direction, granting that there has been time sufficient for the slow work of natural selection, checked as it will be by intercrossing and the tendency to reversion, who will pretend that he knows the natural history of any one organic being sufficiently well to say whether any particular change would be to its advantage? Can we feel sure that a long proboscis would not be a disadvantage to the hive-bee in sucking the innumerable small flowers which it frequents? Can we feel sure that a long proboscis would not, by correlation of growth, almost necessarily give increased size to other parts of the mouth, perhaps interfering with the delicate cell-constructing work? In the case of the ostrich a moment's reflection will show that an enormous supply of food would be necessary in this bird of the desert, to supply force to move its huge body through the air. But such ill-considered objections are hardly worth notice.

The celebrated palæontologist, Professor Bronn, in his German translation of this work, has advanced various good objections to my views, and other remarks in its favour. If both have become fitted for slightly different habits of life or conditions, they might live together; though, in the case of animals which freely cross and wander much about, varieties seem to be almost always confined to distinct localities. But if we put on one side polymorphic species, in which the variability seems to be of a peculiar nature, and all mere temporary variations, such as size, albinism, &c., the more permanent varieties are generally found, as far as I can judge, inhabiting distinct stations, high land or low land, dry or moist districts, or distinct regions. Bronn also insists that distinct species never differ from each other only in single characters, but in many parts; and he asks, how it comes that natural selection should invariably have affected simultaneously many parts of the organisation? But there is not the least necessity for believing that all the parts have been simultaneously modified; they may have been gained one after the other, and from being transmitted together, they appear to us as if simultaneously formed. Correlation, however, will account for various parts changing, when any one part changes. We have evidence of this in our domestic races, which though they may differ greatly in some one selected character, always differ to a certain extent in other characters.

Bronn, again, asks how natural selection can account for differences between species, which appear to be of no service to these species, such as the length of the ears or tail, or the folds of the enamel in the teeth, of the several species of hares and mice? With respect to plants, this subject has been recently discussed by Nägeli in an admirable essay. He admits that natural selection has effected much, but he urges that the families of plants differ chiefly from each other in morphological characters, which seem quite unimportant for the welfare of the species. He consequently believes in an innate tendency towards perfection or progressive development. He specifies the arrangement of the cells in the tissues, and of the leaves on the axis, as cases in which natural selection would fail to act. To these may be added the numerical divisions in the parts of the flower, the position of the ovules, the shape of the seed, when not of any use for dissemination, &C. Professor Weismann, in discussing Nägelis essay, accounts for such differences by the nature of the varying organism under the action of certain conditions; and this is the same with what I have called the direct and definite action of the conditions of life, causing all or nearly all the individuals of the same species to vary in the same manner. When we remember such cases as the formation of the more complex galls, and certain monstrosities, which cannot be accounted for by reversion, cohesion, &c., and sudden strongly-marked deviations of structure, such as the appearance of a moss-rose on a common rose, we must admit that the organisation of the individual is capable through its own laws of growth, under certain conditions, of undergoing great modifications, independently of the gradual accumulation of slight inherited modifications. Various morphological differences probably come under this head, to which we shall recur; but many differences may at the present time be of high service, or may formerly have been so, although we are not able to perceive their use; and these will have been acted on by natural selection. A still larger number of morphological differences may certainly be looked at as the necessary result—through pressure, the withdrawal or excess of nutriment, an early-formed part affecting a part subsequently developed, correlation, &c.—of other adaptive changes, through which all species must have passed during their long course of descent and modification.

No one will maintain that we as yet know the uses of all the parts of any one plant, or the functions of each cell in any one organ. Five or six years ago, endless peculiarities of structure in the flowers of orchids, great ridges and crests, and the relative positions of the various parts would have been considered as useless morphological differences; but now we know that they are of great service, and must have been under the dominion of natural selection. No one at present can explain why the leaves in a spire diverge from each other at certain angles; but we can see that their arrangement is related to their standing at equal distances from the leaves on all sides; and we may reasonably expect that the angles will hereafter be shown to follow from some such cause, as the addition of new leaves to the crowded spire in the bud, as inevitably as the angles of a bees cell follow from the manner in which the insects work together.

In certain whole groups of plants the ovules stand erect, and in others they are suspended; and in some few plants within the same ovarium one ovule holds the former and a second ovule the latter position. These positions seem at first purely morphological and of no physiological signification; but Dr. Hooker informs me that of the ovules within the same ovarium, in some cases the upper ones alone and in other cases the lower ones alone are fertilised; and he suggests that this probably depends on the direction in which the pollen-tubes enter. If so, the position of the ovules, even when one is erect and the other suspended, would follow from the selection of any slight deviation in position which might favour their fertilisation and the production of seed.

Several plants belonging to distinct orders habitually produce flowers of two kinds,—the one open and of the ordinary structure, the other closed and imperfect. In the latter the petals are almost always reduced to the merest rudiments; the pollen-grains are reduced in diameter; five of the alternate stamens are rudimentary in Ononis columnæ and in some species of Viola three stamens are in this state, two retaining their proper function, but being of very small size. In six out of thirty of the closed flowers in an Indian violet (name unknown, for the plants have not as yet produced perfect flowers), the sepals were reduced from the normal number of five to three. In one section of the Malpighiaceæ the closed flowers, according to A. de Jussieu, are still further modified, for the five stamens which stand opposite to the sepals are all aborted, a sixth stamen standing opposite to a petal being alone developed; and this stamen is not present in the ordinary flowers of these species; the style is aborted; and the ovaria are reduced from three to two. In all the foregoing plants the minute closed flowers are of high service, for they yield with perfect security, and with the expenditure of extremely little pollen, or other organised matter, a large supply of seed; whilst the perfect flowers permit occasional crosses with distinct individuals. Therefore, these changes may have been, and no doubt have been, effected through natural selection; and I may add that nearly all the gradations between the perfect and imperfect flowers may sometimes be observed on the same plant.

With respect to modifications which necessarily follow from other changes—through the withdrawal or excess of nutriment—through pressure and other unknown influences—there is space here only for a few brief illustrations. In the Spanish chesnut, and in certain fir-trees, the angles of divergence of the leaves differ, according to Schacht, in the nearly horizontal and in the upright branches. In the common rue and some other plants, one flower, usually the central or terminal one, opens first, and has five sepals and petals, and five divisions to the ovarium; whilst all the other flowers on the plant are tetramerous. In the British Adoxa the uppermost flower generally has two calyx-lobes with the other organs tetramerous, whilst the surrounding flowers generally have three calyx-lobes with the other organs pentamerous; and this difference appears to follow from the manner in which the flowers are closely packed together. In many Compositæ and Umbelliferæ, and in some other plants, the circumferential flowers have their corollas much more developed than those of the centre; and this is probably the result of natural selection, for all the flowers are thus rendered much more conspicuous to those insects which are useful or even necessary for their fertilisation. In connection with the greater development of the corolla, the reproductive organs are frequently more or less aborted. It is a more curious fact that the achenes or seeds of the circumference and of the centre sometimes differ greatly in form, colour, and other characters. In Carthamus and some other Compositæ the central achenes alone are furnished with a pappus; and in Hyoseris the same head yields achenes of three different forms. In certain Umbelliferæ the exterior seeds, according to Tausch, are orthospermous, and the central one cœlospermous, and this difference has been considered by De Candolle as of the highest systematic importance in the family. If in such cases as the foregoing all the leaves, flowers, fruits, &c., on the same plant had been subjected to precisely the same external and internal conditions, all no doubt would have presented the same morphological characters; and there clearly would have been no need to call in the aid of the principle of progressive development. With the minute closed flowers, as well as with many degraded parasitic animals, if it be assumed that any such aid is requisite, we should have to call in an innate tendency to retrogressive development.

Many instances could be given of morphological characters varying greatly in plants of the same species growing close together, or even on the same individual plant; and some of these characters are considered as systematically important. I will specify only a few cases which have first occurred to me. It is not necessary to give instances of flowers on the same plant being indifferently tetramerous, pentamerous, &c.; but as when the parts are few, numerical variations are in all cases comparatively rare, I may mention that, according to De Candolle, the flowers of Papaver bracteatum offer two sepals with four petals (and this is the common type with poppies), or three sepals with six petals. The manner in which the petals are folded in the bud is in most groups a constant morphological character; but Professor Asa Gray states that with some species of Mimulus, the æstivation is almost as frequently that of the Rhinanthideæ as of the Antirrhinideæ, to which tribe the genus belongs. Aug. St. Hilaire gives the following cases: the genus Zanthoxylon belongs to a division of the Rutaceæ with a single ovary, but in some of the species flowers may be found on the same plant, and even in the same panicle, with either one or two ovaries. In Helianthemum the capsule has been described as unilocular or 3-locular; and in H. mutabile, "Une lame, plus ou moins large, sétend entre le pericarpe et le placenta." In the flowers of Saponaria officinalis, Dr. Masters also observed instances of both marginal and free central placentation. Lastly, St. Hilaire found towards the southern extreme of the range of Gomphia oleæformis two forms which he did not at first doubt were distinct species, but he subsequently saw them growing on the same bush; and he then adds, "Voilà donc dans un même individu des loges et un style qui se rattachent tantôt à un axe verticale et tantôt à un gynobase."

In the case of these plants, will it be said that they have been detected in the act of progressing towards a higher state of development? On the contrary, I should infer from such characters varying so greatly, that they were of extremely small importance to the plants themselves, of whatever importance they may be to us in our classifications. Although we are quite ignorant of the exciting cause of each modification, yet it seems probable from what we know of the relations of variability to changed conditions, that under certain conditions the one structure would have prevailed over the other, and thus might have been rendered almost or quite constant. From the very fact of such differences being unimportant for the welfare of the species, any slight deviations which did occur would not be augmented or accumulated through natural selection; and they would be liable to obliteration through the occasional intercrossing of distinct individuals. A structure which has been developed through long-continued selection, when it ceases to be of service to the species, will generally become variable, as we see with rudimentary organs; for it will no longer be regulated by this same power of selection; but on the other hand, when from the nature of the organism and from a change in the conditions definite modifications have been produced which are unimportant for the welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified descendants. Hair has been transmitted to almost all mammals, feathers to all birds, and scales to all true reptiles. A structure, whatever it may be, which is common to many allied forms, is ranked by us as of high systematic importance, and consequently is often assumed by us to be of high vital importance to the species. Thus, as I am inclined to believe, morphological differences, which we consider as important—such as the arrangement of the leaves, the divisions of the ovarium, the position of the ovules, &c.—first appeared in many cases as fluctuating variations, which sooner or later became almost constant through the nature of the organism and of the surrounding conditions, as well as through intercrossing; for as these morphological characters do not affect the welfare of the species, any slight deviations in them would not be acted on or accumulated through natural selection. It is a strange result which we thus arrive at, namely that characters of slight vital importance to the species, are the most important to the systematist; but, as we shall hereafter see when we treat of the genetic principle of classification, this is by no means so paradoxical as it at first appears. Finally, whatever may be thought of this view, in none of the foregoing cases do the facts, as far as I can judge, afford any evidence of the existence of an innate tendency towards perfectibility or progressive development.

I need allude only to two other objections: a distinguished botanist, Mr. H. C. Watson, believes that I have overrated the importance of divergence of character (in which, however, he apparently believes), and that convergence of character, as it may be called, has likewise played a part. This is an intricate subject which need not be fully discussed. I will only remark that if two species of two allied genera, both produced a number of new and divergent species, I can believe that they might sometimes approach each other so closely that they would for convenience sake be classed in the same new genus, and thus two genera would converge into one; but from the strength of the principle of inheritance, and from the two parent-species already differing and consequently tending to vary in a somewhat different manner, it seems hardly credible that the two new groups would not at least form distinct sections in the genus.

Mr. Watson has also objected that the continued action of natural selection with divergence of character will tend to make an indefinite number of specific forms. As far as mere inorganic conditions are concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable diversities of heat, moisture, &c.; but I fully admit that the mutual relations of organic beings are more important; and as the number of species in any country goes on increasing, the organic conditions of life will become more and more complex. Consequently there seems at first sight to be no limit to the amount of profitable diversification of structure, and therefore no limit to the number of species which might be produced. We do not know that even the most prolific area is fully stocked with specific forms: at the Cape of Good Hope and in Australia, which support such an astonishing number of species, many European plants have become naturalised. But geology shows us, at least within the whole immense tertiary period, that the number of species of shells, and, probably, of mammals, has not greatly or at all increased. What then checks an indefinite increase in the number of species? The amount of life (I do not mean the number of specific forms) supported on any area must have a limit, depending so largely as it does on physical conditions: therefore, if an area be inhabited by very many species, each or nearly each species will be represented by few individuals; and such species will be liable to extermination from accidental fluctuations in the nature of the seasons or in the number of their enemies. The process of extermination in these cases will be rapid, whereas the production of new species will always be slow. Imagine the extreme case of as many species as individuals in England, and the first severe winter or very dry summer would exterminate thousands on thousands of species. Rare species, and each species will become rare if the number of species become in any country indefinitely increased, will, on the principle often explained, present within a given period few favourable variations; consequently, the process of giving birth to new specific forms will thus be retarded. When any species becomes very rare, close interbreeding will help in exterminating it; at least authors have thought that this comes into play in accounting for the deterioration of Aurochs in Lithuania, of Red Deer in Scotland, and of Bears in Norway, &C. As far as animals are concerned, some species are closely adapted to prey on some one other being; but if this other being had been rare, it would not have been any advantage to the animal to have been produced in close relation to its prey: therefore, it would not have been produced by natural selection. Lastly, and this I am inclined to think is the most important element, a dominant species, which has already beaten many competitors in its own home, will tend to spread and supplant many others. Alph. de Candolle has shown that those species which spread widely tend generally to spread very widely; and, consequently, they will tend to exterminate several species in several areas, and thus check the inordinate increase of specific forms throughout the world. Dr. Hooker has recently shown that in the S. E. corner of Australia, where, apparently, there are many invaders from different quarters of the world, the endemic Australian species have been greatly reduced in number. How much weight to attribute to these several considerations I do not pretend to assign; but conjointly they must limit in each country the tendency to an indefinite augmentation of specific forms.

Summary of Chapter.

If under changing conditions of life organic beings

group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. Small and broken groups and sub-groups will finally tend to disappear. Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which as yet have suffered least extinction, will for a long period continue to increase. But which groups will ultimately prevail, no man can predict; for we well know that many groups, formerly most extensively developed, have now become extinct. Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that of the species living at any one period, extremely few will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on Classification, but I may add that on this view of extremely few of the more ancient species having transmitted descendants, and on the view of all the descendants of the same species making a class, we can understand how it is that there exist but very few classes in each main division of the animal and vegetable kingdoms. Although extremely few of the most ancient species may now have living and modified descendants, yet at the most remote geological period, the earth may have been as well peopled with many species of many genera, families, orders, and classes, as at the present day. ↑ ↑ ↑ ↑

Summary of Chapter .—

If during the long course of ages and under varying conditions of life, organic beings