RECORD: [Dallas, W. S.?]. 1868. [Review of] Variation of animals and plants under domestication. Westminster Review n.s. 35 (January): 207-27.
REVISION HISTORY: Transcribed (single key) by AEL Data, corrections by John van Wyhe 9.2010. RN1
ART. VII.—MR. DARWIN'S THEORIES.
The Variation of Animals and Plants under Domestication. By CHARLES DARWIN, M.A., F.R.S., &c. Two volumes 8vo. London: John Murray. 1868.
IT is now some eight years since the publication of Mr. Darwin's great work on the "Origin of Species," and most of our readers will bear in mind the storm of abuse which lighted upon the head of its unfortunate author, whose name, indeed, has ever since been a byeword of reproach among the upholders of orthodoxy both in science and religion. Nevertheless, notwithstanding all the obloquy which has been heaped upon him, Mr. Darwin's theory, or at all events views very similar to those put forward by him in his celebrated work, must be admitted to have made much progress in the minds of thinking men. Among the leading botanists and zoologists in this country, many of the very first are firm Darwinians, and on the continent, especially in Germany, the acceptance of the Darwinian theory is very general among zoologists. In the case of German authors indeed, it is curious to see how they are inclined to out-Darwin Darwin himself, by carrying out his results to consequences which he did not venture to enter upon in detail; and it would rather astonish some of those orthodox folks in our religious world, who look upon our author as a near relative of Antichrist, to find that in free-spoken Germany he is regarded as having been prevented by some lingering remnants of the prejudices of his English education, from writing upon certain questions which he regards as beyond the sphere of investigation. How far this may be true is a question which we shall not attempt to settle; so much is certain, however, that there is abundant evidence in his writings that he approaches these subjects in no irreverent frame of mind, whilst he most decidedly shows far more of the true Christian spirit in his treatment of his opponents, than they have ever done in their not over gentle handling of him.
But apart from the hard measure which the Darwinian theory has received from those who felt themselves bound to oppose it on supposed religious or theological grounds, its treatment by the champions of the old school of orthodox naturalists, from a socalled scientific point of view, has hardly been fair. Some of these writers have treated Mr. Darwin's opinions in a sneering tone, looking calmly down from the height of their own superiority, or remarking that Mr. Darwin is indeed an excellent zoologist, that his "Voyage of the Beagle" is a most interesting
work, his memoir on Coral-reefs an exceedingly important contribution to science, and his "Monograph of the Cirripedia" a perfect model of what a monograph ought to be, but that really this notion of his about the origin of species is merely a crotchet, and, (if they wish to be good natured at the end), they express their sorrow at seeing so eminent a man misled into supporting such views.
Others maintain that Darwin is merely Lamarck redivivus, but although there is at the first glance some show of truth in this statement, it will be found on investigation that the agreement between Lamarck and Darwin is almost limited to their holding in common the opinion that species were not independently created, but produced by evolution from pre-existing organisms. The means by which this result is supposed to have been brought about are quite different in the two theories, and whilst Darwin has the advantage in the precision and rationality of the processes which he endeavours to demonstrate as having taken place, Lamarck on the other hand places his theory on a higher standpoint than that occupied by Mr. Darwin in his first essay, by the recognition of a general law governing the whole of the changes assumed by him, a deficiency in his theory which is supplied by Mr. Darwin in the work now under consideration, by his hypothesis of "Pangenesis," to which we shall have to advert hereafter.
Another objection to Darwinism, and one which presses with great force on the minds of those who are not students of natural history, is embodied in the question,—where are the transitional forms? If these continual changes have been going on since the first appearance of life upon the earth, some traces of them ought to be preserved in the fossil remains of animals and plants with which the strata forming the crust of our globe abound. Palæontology, we are confidently told, furnishes no evidence of anything of the kind, and as a general rule we cannot hope to witness the production of a new species from one now existing. But as regards the palæontological evidence, it seems to the present writer to prove very little one way or the other; independently of the imperfection of the Geological Record to which Mr. Darwin himself calls attention, our knowledge of fossil organisms is still to a great extent very vague and imperfect, the determination of a vast number of so-called species is in the highest degree conventional and empirical, and finally, any one who will take up some well-worked group of fossils, such as the British Terebratulæ for example, with Mr. Davidson's Monograph, or even the Monograph alone, will speedily be convinced that no sweeping assertion as to the absence of transitional forms can be founded upon it.
Even among existing animals, moreover, we have some indications of the direction in which to look for evidence of recent changes. Mr. Wollaston in his most valuable researches upon the Beetles of the Atlantic islands (the Madeiras, Canaries, and Cape Verdes), which must be regarded as the remaining summits of a vast submerged continent, found abundant evidence of a community of fauna throughout these groups; but among the forms to which he felt himself obliged to accord specific rank, there are many (generally confined to one or a few of the islands) which differ so slightly from other known species, that he marks them as possibly derivative, and remarks that if they were inhabitants of a continuous region some hesitation would be experienced in regarding them as distinct species. In his last work on the Coleoptera of the Cape Verde islands,* Mr. Wollaston, although a strenuous supporter of the theory of independent creation of species, finds himself compelled to admit for these insects that it is difficult to conceive of the separate creation of organisms differing so slightly and limited so frequently to a single small spot of earth in the midst of the ocean. We have in these islands the conditions most favourable on the Darwinian hypothesis to the formation of new species—the breaking up of a widely extended fauna into small areas, and consequent segregation of the animal inhabitants, accompanied by a gradual change of conditions, and when in this very region we find a large series of doubtful specific forms, it is hard not to draw from this fact an inference favourable to the hypothesis which seemed to predict it.
The great majority of Mr. Darwin's opponents have given sufficient evidence that they had never taken the trouble thoroughly to understand his theory, but none perhaps more than those who have maintained that the argument from animals and plants under domestication was inadmissible. They seem to have forgotten, or not to have noticed, that Mr. Darwin's use of this argument, and of the term "Natural Selection," which has been a stumblingblock to many, was purely analogical; and from this point of view we do not see why either the term or the argument should be objected to. The selecting power for organisms in a state of nature is to be found in the struggle for the means of existence to which all creatures must be subject, and we can only reject the argument derived from the variation of animals and plants under domestication by assuming that domestication of itself introduces some new quality or property into the organism, rendering it plastic under changed conditions of life. In fact, we must logically assume either that the potentiality to vary to the extent attained by our most improved domestic
* "Coleoptera Hesperidum." London, 1867.
[Vol. XCI. No. CLXXIX.]—NEW SERIES, Vol. XXXV. No. I. P
animals and plants existed in the wild progenitors of the species, or else that man has been able to act a creative part and to implant in the creatures which he has taken under his care a property altogether new and foreign to their original nature. But we know that animals and plants in a state of nature do vary under altered conditions of life. Local varieties and races are very common and well known to collectors, and their variations are the same in kind as those of domesticated species—the latter, in fact, merely presenting the phenomena in a condensed and intensified form, inasmuch as their conditions of existence have been more forcibly changed and subjected to the influence of an arbitrary selection. Thus it would be as unfair to deprive the student of the laws of variation of the data presented to him by domesticated organisms as to deny to the anatomist the use of his microscope, or to the astronomer that of his telescope.
In the work of which the title stands at the head of this article Mr. Darwin has in the first place greatly enlarged the data and arguments contained in the first chapter of his "Origin of Species," and afterwards discussed in great detail the general inferences to be drawn from the phenomena described. In approaching this great work for the purpose of giving some general idea of its contents, the writer must confess to a feeling of the greatest diffidence. The mass of most interesting and valuable material brought together is so great and so various that any attempt to analyse it is almost hopeless, whilst each fact is subsequently discussed so fully, and placed in so many different lights in connexion with the theory of selection, that the most conscientious efforts to trace out the general line of argument must almost inevitably leave many important points untouched.
In his description of the variation of animals and plants under domestication, Mr. Darwin brings together a vast mass of facts which cannot but be of the highest interest and value, even to those naturalists who do not believe in his theory of the origin of species. He traces the history of the domesticated breeds from the earliest periods, revealed to us by their remains preserved in the later geological deposits and in the lake-dwellings, turbaries, kitchen-middens, and other traces of pre-historic man, through, the early documentary references to such matters, to the present time, giving, finally, such a full account of the existing breeds, especially of pigeons and fowls, as renders his work a perfect mine of information upon these subjects. To this portion we must advert very briefly,—it furnishes the evidence upon which the theoretical considerations of the subsequent portions are built up, and some of the most important facts described in it will have to be cited in our consideration of the latter.
As regards the origin of the domestic breeds of dogs little can
now be ascertained. At a very early historical period, we learn from Egyptian monuments that several breeds of domestic dogs were already in existence,—figures of dogs resembling greyhounds, hounds, mastiffs, turnspits, Pariah dogs, and some other forms, are found upon sculptures belonging to the period between the fourth and the twelfth dynasties, the most ancient form being a sort of rough greyhound, with long pointed ears, and a short curled tail, a breed closely resembling which is still employed as a boar-hound in North Africa. In the monuments of pre-historic periods, the kitchen-middens and lake-dwellings, traces of dogs are met with, and these seem to have been very uniform all over Europe. In the early Danish kitchen-middens, and in the older lake-deposits belonging to the Neolithic or later stone period, a small breed occurs, which was succeeded in the bronze period by a larger, but still accordant form, both in Denmark and Switzerland, and this again in Denmark gave place to a still larger dog in the Iron age. The existence of a single uniform and widely-spread race during the whole Neolithic period might seem at first sight to be in favour of the origin of our European breeds from one primary form, but, as Mr. Darwin very truly remarks, the subsequent change in the character of the domestic dogs of Europe may be due to the importation of new breeds by the conquering tribes of the Bronze and Iron periods. This opinion, and indeed the general notion of the multiple origin of the extraordinarily varied breeds of dogs, receives strong support from the close resemblance of the domesticated breeds kept by savage and semi-barbarous tribes to the wild species of Canidæ in their immediate vicinity, and from the ease with which these domestic dogs will interbreed with the wild species, and the permanent fertility of the crossed offspring. This is particularly remarkable in North America, where the difference between the domestic dogs and the wolves of the country is very small, and in the case of the Esquimaux dog almost nil. So also with the Hare Indian dog, which presents no marked difference from the prairie wolf (Canis latrans), and is doubtless derived directly from it. Nevertheless, these two aboriginally distinct dogs cross freely with each other, with the wild wolves, and with European dogs. With regard to the latter also, and to Indian dogs, Mr. Darwin brings forward abundant evidence to prove that some of the breeds at least present the same close resemblance to, and facility of interbreeding with, the surrounding wild species (wolves and jackals).
The argument against the derivation of domestic dogs from wolves and jackals, derived from supposed differences in the periods of gestation, is clearly shown by Mr. Darwin to be without foundation; indeed, we may say that the extant
data are rather favourable to the above mentioned view. This period in domestic dogs is by no means fixed, varying between fifty-nine and sixty-seven days, and it appears from the testimony of several observers that it is usually longer in large dogs than in small ones. The average period in the dog is sixty-three days, which is exceeded by large dogs, whilst smaller breeds vary between sixty and sixty-three days. Now the latter is likewise the case with the jackal, from which the smaller dogs are probably descended, whilst the wolf, according to F. Cuvier, has a period of two months and a few days, which would seem to correspond with the longer period of our larger dogs. The whole evidence goes far to prove the multiplicity of origin of domestic dogs, but at the same time the amount of variation under domestication has been exceedingly great. Upon this subject Mr. Darwin gives a multitude of details, to which, however, we cannot advert.
The domestication of the cat, like that of the dog, extends to an ancient period, as evidenced by the mummied remains and monumental figures of cats found in Egypt. The mummies, according to de Blainville, belong to three species, two of which are still met with, both wild and domesticated, in Egypt. The cats of different parts of the world seem, indeed, to be descended in general from several wild species, and the common cat of Europe is probably the result of an intermixture of two or more. In all parts of the world wild and tame cats breed freely together. And it is supposed by Mr. Blyth that the resemblance presented by our common English cats to the wild cat (Felis sylvestris), a resemblance which is not so strongly marked elsewhere, is due "to frequent intermixture at a time when the tame cat was first introduced into Britain and continued rare, while the wild species was far more abundant than at present."
The domestic pigs, which probably present one of the most remarkable instances of variation under domestication, have likewise been subjected to the influence of man from a very early period. The Swiss lake dwellings contain the remains of pigs, and indeed of two forms,—namely, a pig evidently descended from the European wild swine, and a variety or species which has been denominated the "Torfschwein," the Sus scrofa palustris of Rütimeyer. It is curious that this peculiar form, which appears to have been domesticated in various parts of Europe during the Neolithic period, presents in some respects an approach towards the characters of the Sus indicus, the parent species of the well-known Chinese and other Eastern Asiatic domestic pigs, and may indicate either that the latter species formerly extended its range over the whole breadth of the Asiatic continent, from China to Europe, or that a third
allied species, now extinct, existed in Europe during the period when the Swiss lake-dwellings were inhabited.
However this may be, there is no question that the Eastern and European domesticated swine belong to two distinct types of structure,—the one that of Sus indicus, the other agreeing closely in all important particulars with the common European wild swine (Sus scrofa), and yet the animals belonging to these two types are perfectly fertile when crossed. A remarkable form from Japan, described by Dr. Gray as a distinct species under the name of Sus pliciceps, has also been found to be perfectly fertile when crossed with the Berkshire breed.
One particularly interesting point in connexion with the pig is the change which takes place in that animal when it becomes feral. It is generally believed, and Mr. Darwin attributes this belief chiefly to observations made upon pigs, that domestic animals, when they run wild, revert completely to the character of the parent stock, but as our author remarks, this is not even here "grounded on sufficient evidence; for the two main types of S. scrofa and indicus have never been distinguished in a feral state." The changes which occur are those which might be expected in such an animal returning to the habits of its original progenitors—namely, alterations in the general form of the body, and in the length of the limbs and muzzle. In general the colour approaches that of the European wild boar; but this is by no means always the case, and certain anomalous colours are supposed by Mr. Darwin to be produced by conditions of climate. The bristly covering which is so striking a peculiarity of the wild boar, but which is reduced to a very scanty coat in our improved domestic breeds, is reproduced in most feral races, but in different degrees, dependent apparently on climate; and the tusks of the male also resume their character of formidable weapons, in accordance with the rule recognised by Mr. Darwin, of a close correlation between the development of the hair and teeth. Another curious fact noticed here is that many of the feral pigs of Jamaica have the bristles of the end of the tail arranged in a double row, like the plumes of an arrow, a character which occurs in the Indian wild boar, but not in the European species, or in the domestic pigs of Europe, from which the feral pigs in question have been derived. In the young of feral pigs a character reappears which is common to the wild swine of Europe and India, but which has been eliminated in nearly all the domesticated breeds; these young animals are striped longitudinally.
Like the pigs, our domestic cattle have likewise undoubtedly a multiple origin, at least two distinct species generally recognised by naturalists being now domesticated and interbreeding
freely, whilst if we go back to pre-historic times, the origin even of our European cattle becomes evidently complex. Thus the Indian humped cattle or zebus (Bos indicus of authors) are certainly distinct from the European humpless breeds commonly denominated Bos taurus; and the researches especially of Rütimeyer and Nilsson show that the latter include the descendants of at least three distinct species. Bos primigenius was domesticated and had begun to undergo variation during the Neolithic period; it was wild in Europe in the time of Cæsar, and still survives in a semi-wild state in Chillingham Park. The larger breeds of cattle on the continent of Europe and the Pembroke breed are considered to be descendants of this fine species. Bos longifrons, the remains of which are so frequent in superficial deposits in this country and elsewhere, was also domesticated in Switzerland during the period of the early lake-dwellings, and seems to have been the common English form of cattle during the Roman occupation of Britain. It appears to be the original species of the Welsh and Highland cattle, and also of some small Swiss breeds. Nilsson's Bos frontosus, which is regarded by Mr. Boyd Dawkins as only a form of B. longifrons, occurs fossil with the latter in Scania and in Ireland; Nilsson believes it to be the parent form of the mountain cattle of Norway, which, like it, have a protuberance between the horns. But notwithstanding this undoubted multiplicity of origin, all our existing breeds of cattle are, as is well known, perfectly fertile amongst each other, and not only those of the European varieties among themselves, but also indiscriminately with the Indian humped species or zebus, and again with three other Indian species—namely, the yak, the gayal, and the arni.
As to the origin of the domestic sheep great diversity of opinion prevails, but there seems little reason to doubt that they are descended from several distinct species. Their variation, as is well known, is exceedingly great. The domestic goat, on the contrary, is generally regarded as descended with but little intermixture from a single species,—the Asiatic Capra œgagrus. Both the sheep and the goat were domesticated at a very early period, their bones being found among the remains of the Swiss pile-dwellings.
The horse, which was likewise a possession of the ancient inhabitants of Europe, the discovery of the long-concealed traces of whose life and manners has opened up such a vast field of interesting research in our days, has generally been regarded as descended from a single species; but as we know that several species of horses existed in Europe in the later Tertiary periods, and as, according to Professor Rütimeyer, the earliest domesticated horses showed differences in the form of their skulls, it
seems very probable that the origin of the numerous varieties of this most valuable animal may be more complex than is commonly supposed. There is indeed the difficulty that in this case we must assume several species to have become entirely extinct in the wild state, for we cannot indicate any wild species to which a share in the parentage of our domestic horses can be ascribed,—the so-called wild horses of the East being in all probability feral animals,—whilst, on the other hand, the amount of variation which horses undoubtedly descended from domesticated animals are known to have undergone, under changes of conditions, shows very clearly that all the varieties with which we are familiar may very well have originated from a single primary form. The latter is Mr. Darwin's opinion.
We have dwelt at some length upon these examples of well-known domesticated animals, not for the purpose of indicating the amount of variation to which they are subject, but in order to show how many of them may trace their pedigree to a multiple origin, thus greatly invalidating the objection so commonly raised against the Darwinian theory from the supposed sterility of all hybrids. If it be true that, as implied in the Pallasian doctrine, the domestication of two allied species gradually eliminates the tendency to sterility of their descendants when crossed, and of this fact there seems to be no doubt, what becomes the real value of the supposed divinely ordained law that hybrids shall always be sterile in order to maintain the purity of that mysterious entity the species?
The fourth, fifth, sixth, and seventh chapters of Mr. Darwin's book are occupied by a discussion of the natural history and variations of the domesticated breeds of rabbits, pigeons, and fowls, upon which, especially the latter two, he enters into great details. To the zoologist these chapters are invaluable; it is impossible to conceive a more exhaustive treatment of any subjects than the history of the domestic fowl and pigeon has received at our author's hand. But this very copiousness of detail renders it impossible for us to give any idea of the contents of these chapters without greatly exceeding the space which can be devoted to the present article. For the same reason we must pass over the chapters in which the variations of several other domesticated animals and cultivated plants are described in more or less detail, and proceed at once to those matters bearing more directly upon theoretical points.
There are certain peculiar modes of variation which sometimes produce a marked effect upon the progeny of domesticated animals and cultivated plants, and some of these phenomena are so curious as to deserve at least a passing notice. Under the general name of "Bud-variation," Mr. Darwin includes all those
cases of variation in plants in which changes are introduced during the development of certain flower or leaf buds. Mr. Darwin cites a great number of examples in which changes of greater or less importance, amounting sometimes to the production of a new variety, have thus been introduced among cultivated trees. Thus, a late and early variety of the grosse mignonne peach have been produced by bud-variation; the large tawny nectarine originated in the same way from a tree bearing smaller fruit of the same character; and instances of a similar kind, although not taken advantage of in the same way, are very numerous. The occurrence of single branches bearing fruit of peculiar character, or of several branches or buds on the same tree each producing a different kind of fruit, are recorded in the cases of the peach, plum, cherry, vine, gooseberry, currant, pear, apple, and banana. In flowering plants the same phenomenon is of constant occurrence, and is particularly remarkable in the sweet William now so much cultivated, in which the flowers seen in the same truss often vary greatly. In like manner the forms of leaves and shoots may often vary greatly from those upon other parts of the same tree, and, as might be expected, those stems which proceed from underground buds, suckers, bulbs, and tubers are liable to similar changes. All the anomalous forms thus produced by bud-variation may be propagated by cuttings and grafts, when they seem to become more fixed, and Mr. Salter has even applied the principle of selection to such abnormal growths by propagating only from the buds developed at the base of variegated leaves.
A very singular case, intimately connected with this phenomenon of bud-variation, is presented by the laburnum known as Cytisus Adami, which appears to be a hybrid between the common laburnum (Cytisus laburnum) and Cytisus purpureus. Its dull red flowers are always sterile, even when it is growing in the midst of examples of its supposed parent species; but not unfrequently individual branches revert to one or other of these parents, and the pure yellow or purple flowers produced on these modified branches furnish fertile seed. With regard to the origin of Cytisus Adami, some difference of opinion prevails, but the evidence appears to be against the notion that it is an ordinary hybrid between the two species which it so remarkably reproduces. According to M. Adam, after whom it is named, he "inserted in the usual manner a shield of the bark of C. purpureus into a stock of C. laburnum, and the bud lay dormant, as often happens, for a year; the shield then produced many buds and shoots, one of which grew more upright and vigorous, with larger leaves, than the shoots of C. purpureus, and was consequently propagated."Therefore, as Mr. Darwin
says, "If we admit as true M. Adam's account, we must admit the extraordinary fact that two distinct species can unite by their cellular tissue, and subsequently produce a plant bearing leaves and sterile flowers intermediate in character between the scion and the stock, and producing buds liable to reversion,—in short, resembling in every important respect a hybrid formed in the ordinary way by seminal reproduction."
A case almost still more extraordinary than this is presented by the Bizzarria orange, which was first raised in Florence in 1644. This tree produces simultaneously the leaves, flowers, and fruit of the bitter orange and citron, and also fruit compounded of that of the two species; and the trifacial orange of Alexandria and Smyrna presents a similar anomaly in its fruit-bearing, but in this case the sweet orange and the citron are blended together. The gardener who raised the Bizzarria orange declared that it originated from a grafted seedling, in which the graft had died away; the stock subsequently shot out and produced the singularly composite form which has since been propagated by cuttings.
Other facts, bearing more or less upon the production of grafthybrids, and illustrating the influence which this mode of union may exert both upon the graft and upon the stock, are cited by Mr. Darwin, and lead him to the discussion of another very singular phenomenon, namely, the immediate action of the male element upon the mother form. In plants, the colour of the outer coats of the seed (in peas and stocks), and even the colour and texture of the pods (in peas), the nature of the fruit (in Chamœrops humilis, the apple and the orange), may be affected by fertilization with the pollen of another species or variety. This direct effect of the male element upon the female reproductive organs, exterior to and independent of its influence upon the germ contained within the seed, seems to be accounted for by the observations of Gärtner, more or less confirmed by those of other botanists, that in all cases, besides the pollen grains employed in the fertilization of the ovule, a certain number must first be expended in acting upon, or "satiating" as he terms it, the pistil and ovarium.
Mr. Darwin applies these facts to the explanation of the analogous phenomena known to breeders to occur also among domesticated animals. He says:—"If we could imagine the same flower to yield seeds during successive years, then it would not be very surprising that a flower, of which the ovarium had been modified by foreign pollen, should next year produce, when self-fertilized, offspring modified by the previous male influence." And he indicates that this is closely analogous to what actually takes place in animals. Thus, a nearly pure-bred Arabian
chestnut mare bore a hybrid to a quagga, and afterwards two colts to a black Arabian horse.
"These colts were partially dun-coloured, and were striped on the legs more plainly than the real hybrid, or even than the quagga. One of these two colts had its neck and some other parts of its body plainly marked with stripes. … But what makes the case still more striking, is that the hair of the mane in these colts resembled that of the quagga, being short, stiff, and upright. Hence there can be no doubt that the quagga affected the character of the offspring subsequently begot by the black Arabian horse." This is only one of many similar examples which might be cited.
Passing from these curious but apparently somewhat isolated examples and causes of individual variation to the general arguments deduced by Mr. Darwin from the consideration of the whole phenomena of variation under domestication, we find the first place naturally given to the primary law of Inheritance, the recognition of which is in fact essential to the comprehension of all the rest. Without this law of inheritance it is perfectly evident that no permanent species or varieties could exist, for the very idea of a species or variety implies the transmission of the characters of the parent to the offspring; and this will be admitted even by those who maintain the absolute immutability of species, although they will hardly give the law so extended an application as Mr. Darwin's. Nevertheless, although even in common parlance we are in the habit of saying "that like begets like," and so forth,
"The whole subject of inheritance," as Mr. Darwin remarks, "is wonderful. When a new character arises, whatever its nature may be, it generally tends to be inherited, at least in a temporary, and sometimes in a most persistent manner. What can be more wonderful than that some trifling peculiarity, not primordially attached to the species, should be transmitted through the male or female sexual cells, which are so minute as not to be visible to the naked eye, and afterwards, through the incessant changes of a long course of development, undergone either in the womb or in the egg, and ultimately appear in the offspring when mature, or even when quite old, as in the case of certain diseases? Or again, what can be more wonderful than the well-ascertained fact, that the minute ovule of a good milking cow will produce a male, from whom a cell, in union with an ovule, will produce a female; and she, when mature, will have large mammary glands, yielding an abundant supply of milk, and even milk of a particular quality."—Vol. ii. p. 2.
Some writers cited by Mr. Darwin have expressed doubts as to the prevalence of this law of inheritance, maintaining that the statistics of the subject (in connexion with man) are hardly
sufficient to establish the fact of the transmission of individual peculiarities from the parents to their descendants. There may, doubtless, be a deficiency of accurate statistical information upon this question—and in the case of the human subject some other slight difficulties may come in the way—but so many examples of the heredity of family peculiarities must have occurred to every one who has passed through the world with his eyes open, that the fact of the hereditary transmission of even slight peculiarities must be evident to every unprejudiced mind. When we come to domestic animals, there can be no longer the least doubt on the subject. "In fact," as Mr. Darwin says, "the whole art of breeding, from which such great results have been attained during the present century, depends on the inheritance of each small detail of structure." And this is the main point of Mr. Darwin's argument, that not merely great alterations in the organism (if such can take place suddenly), but that the smallest changes, even down to morbid peculiarities and mutilations, have a tendency to become impressed upon the offspring, and to be handed down through them to subsequent generations. We may cite a few examples. Mr. Darwin tells us of a race of pigs in which the hind legs were quite deficient, and which continued for three generations. A single young rabbit with one ear, produced in a litter, gave origin to a kind of one-eared rabbits. In 1781, a one-horned stag was observed in a German forest; in 1788, two such stags were noticed; and for several years afterwards the same forest contained many stags with a single horn on the right side of the head. In man, polydactylism, or the presence of supernumerary fingers and toes, is a very great anomaly, for no existing mammal, bird, or reptile, possesses more than five digits, and yet this peculiarity has been transmitted through five generations—a fact the more remarkable from its being well known that the persons exhibiting this monstrosity have married, in most cases, persons not similarly affected.
"In such cases," says Mr. Darwin, "a child of the fifth generation would have only 1-32nd part of the blood of his first sedigitated ancestor. Other cases are rendered remarkable by the affection gathering force—as Dr. Struthers has shown—in each generation, though in each the affected person had married one not affected. Moreover, such additional digits are often amputated soon after birth, and can seldom have been strengthened by use. Dr. Struthers gives the following instance:—In the first generation an additional digit appeared on one hand; in the second, on both hands; in the third, three brothers had both hands, and one of the brothers a foot affected; and in the fourth generation, all four limbs were affected."—Vol. ii. p. 13.
This peculiarity of polydactylism is especially remarkable from the circumstance that the supernumerary digits have a tendency to grow again when amputated.
Peculiarities of vision and morbid affections of the eyes seem to furnish particularly striking examples of hereditary transmission. Thus long and short sight are certainly transmitted from father to son, as are also squinting and even cataract.
Absence of the iris was transmitted for three generations, and a cleft iris for four generations,—in the latter case only the male side of the family was affected. In one very remarkable instance, a father and two sons became blind whenever the head was bent downwards, apparently owing to the crystalline lens with its capsule slipping through a very large pupil into the anterior chamber of the eye. Night-blindness, or incapacity to see except under a strong light, is also inherited; one instance is recorded in which this peculiarity affected eighty-five members of the same family during six generations. Colour-blindness or Daltonism is hereditary, and has been traced through five generations, in which, singularly enough, it was confined to the female side. Peculiarities in the colour of the iris are also transmitted.
From all the facts brought forward by Mr. Darwin, and especially from the experience of breeders, we may accept it as almost a certainty, that the peculiar characters of parents will be transmitted to their progeny, and probably intensified in those cases in which both parents are similarly modified, and as a very common result even where only one parent exhibits some more or less striking peculiarity. But in this latter case the results are very variable,—sometimes the peculiarity of the modified parent may be transmitted to the offspring unimpaired and even intensified, as in the above-cited case of polydactylism, or a portion of the offspring may be affected,—or the peculiarity may be wholly lost. In the present state of our knowledge of the subject, it is evident that we have no means of accounting for this variability of the phenomenon, and Mr. Darwin's discussion of the causes of non-inheritance shows this very clearly. It seems probable indeed that in addition to the causes, such as counteracting changes in conditions of life, adduced by him, the law of prepotency of transmission of character recognised by him in connexion with crossed breeds, (Chap. xiv.) may come into play, for as an animal or plant showing peculiarity of character is the potential representative of a new breed, which if subsequently crossed with its parent stock might be absorbed in consequence of the prepotency of the latter, there seems to be no cause whatever why the same law (which indeed is purely empirical) should not come into play in the first instance, and thus prevent the transmission of its
peculiarities. In fact, the analogy of the two cases appears to be very strong, for in both, whilst the peculiar characters of the individual may apparently be lost in its immediate progeny, they may, and frequently do reappear in its later descendants.
To this reappearance of apparently lost characters the term reversion or atavism has been applied; and it is manifested as a variation, imitative of the characters of distant ancestors in the products both of sexual and bud reproduction. In some cases, even in pure breeds, some of the characters proper to the original stock make their appearance suddenly, as in the well-known instance of the pigeon, in the various breeds of which individuals reproducing the colouring of the parent species (Columba livia) are frequently produced. Similar examples occur in the common fowl. In the ass, notwithstanding the length of time that it has been domesticated, the legs frequently exhibit the transverse markings characteristic of its wild progenitor, the Abyssinian Asinus tœniopus. The earliest domesticated sheep are believed to have been "brown or dingy black," but at a very early historical period we see clearly enough that the majority must have been white, and in the time of David certain flocks are spoken of as "white as snow." It is a remarkable fact of reversion that even at the present day black and parti-coloured lambs are often dropped by ewes of our most improved breeds. A fact of the same kind is the reappearance of rudimentary horns on the young of hornless breeds of sheep and cattle. In the case of crossed breeds of animals, the tendency to reversion becomes so exceedingly strong that Mr. Darwin regards crossing as a direct cause of reversion towards an earlier parent than those by means of which the cross was made. Mr. Darwin gives a curious instance of a reversion in the case of an important instinct due to crossing. Certain breeds of fowls are commonly known as "everlasting layers," because, having lost the instinct that prompts other fowls to incubate, the process of egg-laying goes on in them almost constantly. Now it appears that when fowls belonging to two of these peculiar breeds are crossed, the mixed progeny usually prove to be first-rate sitters, thus going back, in consequence of the cross, to the original condition of the species, in which the incubatory instinct is certainly very strong.
As to the causes of this singular phenomenon we are still entirely in the dark.
"When animals run wild" (says Mr. Darwin), "the tendency to reversion, which, though it has been greatly exaggerated, no doubt exists, is sometimes to a certain extent intelligible. Thus with feral pigs, exposure to the weather will probably favour the growth of the bristles, as is known to be the case with the hair of other domesticated
animals, and through correlation the tusks will tend to be redeveloped. But the reappearance of coloured longitudinal stripes on young feral pigs cannot be attributed to the direct action of external conditions."—Vol. ii. p. 47.
To account for the more remarkable instances of reversion, those, namely, in which, whether by means of crossing or otherwise, the peculiarities of distant ancestors, not possessed by the immediate parents, make their appearance, Mr. Darwin assumes that the characters were latent during the intermediate generations. This of course is a pure assumption, and may be said to be only another way of stating the case; but Mr. Darwin illustrates it by reference to the peculiarities often presented in the development of the secondary sexual characters. These are the external peculiarities of form, clothing, and so forth, which in many cases serve to distinguish the sexes of animals, quite independently of differences in the essential organs of reproduction.
"Now, it is well known that a large number of female birds, such as fowls, various pheasants, partridges, peahens, ducks, &c., when old or diseased, or when operated on, partly assume the secondary male characters of their species. … A duck ten years old has been known to assume both the perfect winter and summer plumage of the drake. Waterton gives a curious case of a hen which had ceased laying, and had assumed the plumage, voice, spurs, and warlike disposition of the cock; when opposed to an enemy she would erect her hackles and show fight. Thus every character, even to the instinct and manner of fighting, must have lain dormant in this hen as long as her ovaria continued to act."—Vol ii. p. 57.
A similar suppression of the male reproductive functions leads to the same results in male animals; not only do they lose male characters, but to a greater or less extent these are supplanted by female peculiarities. Mr. Darwin cites various examples of this curious fact, and sums up the results as follows:—
"We thus see that in many, probably in all cases, the secondary characters of each sex lie dormant or latent in the opposite sex, ready to be evolved under peculiar circumstances. We can thus understand how, for instance, it is possible for a good milking cow to transmit her good qualities through her male offspring to future generations; for we may confidently believe that these qualities are present, though latent, in the males of each generation. So it is with the game cock, who can transmit his superiority in courage and vigour through his female to his male offspring; and with man it is known that diseases, such as hydrocele, necessarily confined to the male sex, can be transmitted through the female to the grandson."—Vol. ii. p. 52.
We shall not attempt to follow Mr. Darwin through the remainder of his discussion of inheritance in its various bearings
upon the variation of organisms; but we may quote the concluding paragraph of his fourteenth chapter, in which he sums up the results arrived at from the consideration of the phenomena. He says:—
"Finally, though much remains obscure with respect to inheritance, we may look at the following laws as fairly well established. Firstly, a tendency in any character, new and old, to be transmitted by seminal and bad generation, though often counteracted by various known and unknown causes. Secondly, reversion or atavism, which depends on transmission and development being distinct powers; it acts in various degrees and manners through both seminal and bud generation. Thirdly, prepotency of transmission, which may be confined to one sex, or be common to both sexes of the prepotent form. Fourthly, transmission, limited by sex, generally to the same sex in which the inherited character first appeared. Fifthly, inheritance at corresponding periods of life, with some tendency to the earlier development of the inherited character. In these laws of inheritance, as displayed under domestication, we see an ample provision for the production, through variability and natural selection, of new specific forms."—Vol. ii. p. 84
Space warns us that we must hasten to a conclusion. One effect of the crossing of distinct races or varieties has already been indicated—namely, the production of a strong tendency to reversion in the progeny, not only to one or other of the immediate progenitors, but towards a still earlier and probably common ancestor. Free intercrossing seems generally to produce, sooner or later, uniformity of character ; hence the necessity, in order to preserve or improve a domesticated breed, of keeping it strictly separate from other breeds Nevertheless, when the individuals of one breed are much less numerous than those of another, and the two breeds are allowed to intercross, the less numerous one will speedily be absorbed by the other; and by the judicious adoption of this mode of crossing, with subsequent segregation of the crossed animals, many of the improvements in our domestic animals have been effected. Close interbreeding, in fact, as Mr. Darwin carefully points out, tends eventually to produce deterioration; and thus, in order to preserve a breed in some degree of vigour, a certain amount of crossing seems to be necessary. From the consideration of the effects of crossing and close interbreeding in domesticated productions is deduced the general law, that organic beings are not destined to "fertilize themselves for perpetuity." In support of this view, many observations of botanists may be adduced, showing in what numerous instances in plants with hermaphrodite flowers arrangements exist for preventing the pistils from being fertilized by the pollen produced in the same flower. To this quality, which Mr. Darwin denominates "self-impotence," are
due the various dimorphic and trimorphic flowers, upon some of which our author has elsewhere published most valuable observations (Primula, Lythrum). The orchids also require generally to be fertilized by the pollen from a different flower; and this seems in a fair way to be proved in a great many other plants. In most cases insects appear to be the agents employed in the transportation of the fertilizing material from one flower to another. In attempting to account for the good effects produced by crossing, and the evil results of too close interbreeding, Mr. Darwin remarks that "it is a widely prevalent and ancient belief that animals and plants profit from slight changes in their conditions of life; and it would appear that the germ, in a somewhat analogous manner, is more effectually stimulated by the male elements, when taken from a distinct individual, and therefore slightly modified in nature, than when taken from a male having the same identical constitution." On the other hand, wild animals, when first subjected to captivity, are often sterile, so that in their case the reproductive system must have been impaired by the change in the conditions of life.
"It is impossible not to be struck with the double parallelism between the two classes of facts just alluded to. On the one hand, slight changes in the conditions of life, and crosses between slightly modified forms or varieties, are beneficial as far as prolificness and constitutional vigour are concerned. On the other hand, changes in the conditions greater in degree, or of a different nature, and crosses between forms which have been slowly and greatly modified by natural means—in other words between species—are highly injurious, as far as the reproductive system is concerned, and in some few instances as far as constitutional vigour is concerned. Can this parallelism be accidental? Does it not rather indicate some real bond of connection? As a fire goes out unless it be stirred up, so the vital forces are always tending, according to Mr. Herbert Spencer, to a state of equilibrium, unless disturbed and renovated through the action of other forces."— Vol. ii. p. 177.
Mr. Darwin discusses at some length the nature of this apparently very great difference between natural species and domestic varieties with regard to their facility of crossing, and comes to the conclusion that the distinction is not so great as it at first appears. He dwells particularly upon his observations on dimorphic and trimorphic flowers, and on the infertility of the seeds produced by the illegitimate union of their sexual elements; but the subject is treated in so condensed a form, that except by the transfer of the whole section to our pages it would be impossible to give a clear idea of the line of argument adopted. In like manner we must hurry over the chapters on selection, and on the laws and causes of variation; the former, indeed, need little notice, as probably no one in the present day doubts either
the fact or the efficacy of selection in the modification of domesticated animals.
The primary cause of variability seems to be a change in the conditions of life, and this need not be very great to set up that state of instability in the organism which may lead to a good deal of individual variation. In some cases the action of the changed conditions seems to be direct or definite, causing all, or nearly all, the individuals of a species or variety subjected to them to vary in a particular direction; but more commonly the tendency to variability is general, placing the organism in an unstable state, in which various influences come into play. Thus the increased use or the disuse of parts often impress themselves strikingly upon the organism by an increased or diminished development of these parts in the progeny; although, probably from the fact that animals have not been long enough under domestication, the latter seems never to be carried so far as to have merely a rudiment "When rudiments are formed or left under domestication, they are the result of a sudden arrest of development, and not of long-continued disuse with the absorption of all superfluous parts. Nevertheless they are of interest, as showing that rudiments are the relics of organs once perfectly developed." By the correlation of variability further changes are introduced; for in accordance with this, when one part varies other parts vary, producing often very singular results. Hence it seems probable that many modifications are of no direct service, having arisen in correlation with other and useful changes."
We have now to advert, as briefly as possible, to the remarkable hypothesis which Mr. Darwin has proposed for the purpose of bringing to one focus, as it were, all the scattered facts and inferences which he has in this work brought together with such astonishing labour. He points out that between the repair of an injury done to any part of the organism and the production of a new individual by fissiparous generation, there is so complete an analogy and so perfect a gradation of intermediate steps, that we may regard them as essentially identical processes. Sexual reproduction, although generally regarded as a totally distinct phenomenon from gemmation and fission, is really connected therewith so gradually through the phenomena of Parthenogenesis and various forms of alternate generations, that it is not easy to draw a strict line of demarcation between the two processes. Again, the production of graft hybrids in plants, such as the Cytisus Adami, is a most important phenomenon in showing that sexual and asexual reproduction are essentially the same; and consequently we may infer that one general fundamental cause presides over all the functions of the animal and vegetable organism.
[Vol. XCI. No. CLXXIX.]—NEW SERIES, Vol. XXXV. No. I. Q
Now, it is very generally admitted that the whole organism, whether simple or complex, consists of a great number of elemental parts—whether we call them cells or not does not much matter in the present case—which differ in their nature in the different organs of the body, have the faculty of self-reproduction, and to a certain extent may be regarded as leading an independent existence. Some writers have gone so far as virtually to affirm the individuality of each cell or histological element in every organism, arranging these microscopic entities as a class of Protozoa, and distributing them under orders and general Mr. Darwin, as might be expected, stops short of this view, but he accepts in its fullest extent the doctrine that the "cells, or the units of the body, propagate themselves by self-division or proliferation, retaining the same nature, and ultimately becoming converted into the various tissues and substances of the body." But he adds to this the assumption "that cells, before their conversion into completely passive or 'formed material,' throw off minute granules or atoms, which circulate freely throughout the system, and when supplied with proper nutriment multiply by self-division, subsequently becoming developed into cells like those from which they were derived." This assumption of the development of "cell-gemmules" thrown off from every cell of every organ of the body, circulating through every part, constituting a portion of every organ, and consequently transmitted by the parents to their offspring, forms the basis of Mr. Darwin's hypothesis of Pangenesis. He supposes the development of these gemmules to "depend on their union with other partially developed cells or gemmules which precede them in the regular course of growth,"—that they may lie dormant in the organism during one or more generations, and that in their dormant state they have a mutual affinity which leads to their aggregation in the form of buds, or of the sexual elements. "Hence, speaking strictly, it is not the reproductive elements, nor the buds which generate new organisms, but the cells themselves throughout the body,"—in other words, as all the elements of the body are represented by their gemmules in every part of it, and especially in the buds or sexual elements, the elementary constitution of the parent must be in a very distinct manner reflected in its progeny.
"Ce n'est que le premier pas qui coûte" is a proverb applicable to many things in this world of ours, and as Mr. Darwin remarks we have only to accept this simple assumption and the rest will follow easily. In fact, it is so clear that if the constitution of the organism be as laid down by Mr. Darwin in this hypothesis, nearly all the varied phenomena of animal and vegetable life are at once accounted for, that we shall not follow
him through his illustrative development of his new doctrine, especially as the present article has already extended to a very great length. That it is a pure hypothesis, and that it is impossible that we should ever see a germinule, are evident; but how many other statements do we not accept as truths upon similarly hypothetical grounds? The principal objection to his hypothesis discussed by Mr. Darwin, is that arising from the excessive minuteness which we must of necessity ascribe to his gemmules. He remarks that—
"A codfish has been found to produce 6,867,840 eggs, a single ascaris about 64,000,000 eggs, and a single orchidaceous plant probably as many million seeds. In these several cases, the spermatozoa and pollen-grains must exist in considerably larger numbers. Now when we have to deal with numbers such as these, which the human intellect cannot grasp, there is no good reason for rejecting our present hypothesis on account of the assumed existence of cellgemmules a few thousand times more numerous."
The mind certainly recoils from the attempt to realize numbers so great and minuteness so excessive as are implied in this hypothesis of the gemmular constitution of the organism, but as matter must be assumed to be infinitely divisible, and as we know that practically in the case of odoriferous bodies and infectious diseases, the particles given off must be inconceivably minute, whilst in the latter they possess a power of self-reproduction within the body almost precisely analogous to that claimed by Mr. Darwin for his gemmules, we must agree with him in thinking that the difficulty of conceiving the existence of gemmules so numerous and so small has really little weight as an argument against his hypothesis.
It is evident that, unless we accept the notion of supernatural interference in every case, some such hypothesis as this must be adopted to account for the transmission of constitutional peculiarities from parents to their offspring, across a bridge narrower even than that which according to Mahometan tradition conducts the Faithful into Paradise. Of the defects and shortcomings of this "provisional hypothesis" no one, probably, is more conscious than its gifted author himself,—his whole theory, fruitful as it has been in results and brilliant as is the light it throws upon many of the most secret operations of Nature, brings us at last face to face with questions which the human intellect will, perhaps, never be able to answer, and we can fully sympathize with Mr. Darwin in the mournful feeling, leading almost to a wail of despair, with which he finally, as it were, lets fall the partially raised veil as the inscrutable Fate-phantom advances upon him from its ambush behind the revealed portion of the majestic Temple of Nature.
JANUARY AND APRIL,
"Truth can never be confirm'd enough,
Though doubts did ever sleep."
Wahrheitsliebe zeigt sich darwin, dass man ueberall das Gute zu finden und zu schaetzen weiss.
TRÜBNER & CO., 60, PATERNOSTER-ROW.
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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)
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