The Variation of Animals and Plants Under Domestication
REVIEWS AND LITERARY NOTICES.
By John Murray. 1868.. 2 vols. 8vo. London :
WHEN Mr. Darwin published his “ Origin of Species,” he stated it to be only the forerunner of a more complete work on the subject, in which he hoped to present the evidence on which his conclusions were founded with a much greater fulness of detail. The volumes now before us are, if we neglect a couple of essays on special subjects, the first instalment of the promised work. They are to be followed in due time by another treatise, on the variation of animals and plants in the wild state, and then by a review of the objections which have been made to the theory of Natural Selection.
The present work contains a great mass of facts drawn from a very wide range of original observation and a most extensive search through the material published by others. Whatever may be thought of his generalizations, no one can deny to the author the merit of painstaking and conscientious industry in the accumulation of facts. The first volume is devoted to a history of our most important domestic animals and plants. The pigeon is the most thoroughly treated, and is, as might have been expected, in a measure, the cheval de bataille of the author. In Volume II. all those general questions of reproduction which arise as soon as we begin to consider the subject of inheritance and the nature of species, such as reversion to ancestral forms, the effects of crossing, the causes of sterility and those of variation, are discussed, — with a spirit of candor, indeed, which no one can fail to be impressed by, and with which all who are acquainted with Mr. Darwin’s previous works are familiar, but at the same time with a degree of subtilty and ingenuity in places which we think may to many readers prove a poor substitute for that fulness and plainness in the evidence that alone can inspire perfect confidence.
Nevertheless, precipitate as some of the conclusions may seem to a cold judgment, some weight is to be allowed here, as everywhere else, to the instinctive guesses of men of genius and large practical experience ; and the book remains so important, both with regard to the general question of species transformation and the special ones of inheritance, that no one interested in the science of life can afford to leave it unread. It would be impossible, in the short space at our command, to convey the “gist” of it to the reader, nor would an abstract be of much value, apart from the special evidence. Still, as every one has heard more or less about “ Darwinism,” and many people have a most inaccurate notion of the contents of that mysterious expression, we will subjoin a brief account of a single factor in Mr. Darwin’s reasoning. It will give to the unlearned reader a slight idea of the kind of speculation indicated by the word, and at the same time give us an opportunity to notice a very curious fact or law which Mr. Darwin thinks he has discovered.
The factor we mean is that called atavism, or reversion. It is a matter of common knowledge that children frequently reproduce traits of their grandparents or still more remote ancestors, which nevertheless did not exist in their own immediate progenitors. Darwin gives a striking instance of a pointer bitch, who gave birth to some pups marked with blue. The color is so unusual in purely bred pointers, that it was considered the pups must be of base descent on the father’s side, and all but one were drowned. Two years afterwards it was accidentally discovered that this one was the great-great-grandson of an animal which had been marked in a similar manner,— so that the peculiarity had remained latent during three generations before appearing in this litter.
The most evident examples of this “law” are to be found in the reversions of crosses to one or the other original parent form. “In a litter of Essex pigs, two young ones appeared which were the image of the Berkshire boar that had been used twentyeight years before in giving size and constitution to the breed ”; and similar facts may almost be called notorious, but only more so in these particular cases, Mr. Darwin thinks, because the characteristic marks are too obvious to escape notice, which they must often do when the ancestors belong to the same breed. Now, to this fact or principle of reversion, the reality of which must needs be acknowledged, Mr. Darwin refers a number of apparently capricious variations in our domestic races, and then proceeds to draw many weighty conclusions, first as to the origin of the races in question, and then as to the extent of possible deviation from their origin of races in general. In pigeons, for instance, in all the fancy breeds, with their so greatly differing structure, there occasionally appear birds of a blue color, with certain other marks, of which the most important are double black bars on the wings and white or blue croups. “ Whenever a blue bird appears in any race, the wings almost invariably show the black bars.” Now the wild rock-pigeon, Columba livia, from whom, on many other accounts, Mr. Darwin thinks our domestic races are probably descended, is characterized by just these peculiarities of coloring ; and the coincidence of their appearance in all these separate tame kinds has its apparent strangeness much diminished if we are enabled to look at it as in each case owing to reversion to the original stock, or rock-pigeon. In our various domestic breeds of fowl, again, which widely differ in most respects, we meet “ black-breasted-red ” birds as occasional exceptions. Only in a very few pure breeds has Mr. Darwin not heard of their occurrence. Now, as this coloring is peculiar to the Gallus bankiva of Northern India, a bird which is almost certainly the parent of the game-fowl, and which, for many reasons, Darwin thinks likely to have been the parent form of all our other kinds, its sporadic appearance in our poultry-yards receives a plausible explanation. In the horse, to take a third example, individuals are everywhere to be met, but more frequently in some strains than in others, striped in a more or less complicated way down the back, over the shoulder, and across the legs. These marks are frequently associated with a dun color. The ass, as is well known, presents some of them normally, others occasionally, and so do the other wild members of the family. After the two former cases, the conclusion in this case will be obvious to the reader. But the imperfection of the reasoning throughout will also not escape him, — first, that these marks are facts of reversion to the wild form X, because many other circumstances make it likely that X is the common ancestor; and then X is all the more certainly the common ancestor, because these marks, being facts of reversion, are all found in X. It is a sort of circular reasoning, and at best helps to accumulate a probability.
Now for the curious law we spoke of as having been discovered by Darwin. It is that in crossing itself we have a direct cause of reversion to characters long extinct ; or, in other words, when two individuals which have diverged from a common parent stock are mated, there is a tendency in their offspring to take on features of that stock that may have been absent for great numbers of generations. Some crosses made in France first called his attention to the subject in pigeons, and he then made experiments himself, both with them and with fowls. Many of the pigeons which he crossed belonged to breeds in which blue birds are of excessive rarity, and many of these crosses were most complicated; yet there appeared among the mongrels a surprising number colored (in many instances almost exactly) like the Columba livia. With fowls of long-established breeds, in which, when kept pure, there is no record of a red feather ever having appeared, he continually got mongrels exhibiting a tendency to approach the plumage of the Gallus bankiva. One of these was a gorgeous cock, whose plumage was almost identical with that of the wild bird. Its father being a Black Spanish, and its mother a Silk fowl, both of which are notorious for breeding true, and the race of the mother being in many respects so peculiar as to have been considered by some authors a separate species. The crossing of the several equine species, in its turn “tends in a marked manner to cause stripes to appear on various parts of the body, especially on the legs,” This, of course, “can be only hypothetically attributed to reversion. But most persons, after considering ” the case of pigeons, fowls, &c., “ will come to the same conclusion in respect to the horse genus, and admit that the progenitor of the group was striped on the legs, shoulders, face, and probably over the whole body, like a zebra.”
The interest and importance of these facts, if Mr. Darwin’s interpretation of them be correct, is evident. But unfortunately the interpretation has just so much of the hypothetical element in it, in all the cases, that a sceptic who should refuse to accept it would have no trouble in presenting a legal and logical justification for his conduct. The author adds to them some other facts concerning instincts which are curious. Thus, the aboriginal species of hen must, of course, have been a good incubator ; but so many cases are on record of the crossed offspring from two races of non-sitting hens “ becoming firstrate incubators, that, the reappearance of this instinct must be attributed to reversion from crossing.” One author says: “ A cross between two non-sitting varieties almost invariably produces a mongrel that becomes broody, and sits with remarkable steadiness.” Again : “ The parents of all our domesticated animals were, of course, originally wild in disposition ; and when a domesticated species is crossed with a distinct species, whether this be a domesticated or only a tamed animal, the hybrids are often wild to such a degree that the fact is intelligible only on the principle that the cross has caused a partial return to the primitive disposition.” He gives instances from cattle, swine, and various birds, and finally asks whether the degraded and savage disposition which many travellers have reported to exist in certain half-caste races of men may not have a similar cause amcly, reversion to the condition of a savage ancestor.
From all this the nature of the reasoning on which Darwin’s hypothesis is based will be seen. It is nowhere of strictly logical cogency, for the conclusions drawn from certain premises are assumed in their turn as true, in order to make those same premises seem more probable. Perhaps from the very nature of the case, and the enormous spaces of time in question, it may never be any more possible to give a physically strict proof of it, complete in every link, than it now is to give a logically binding disproof of it. This may or may not be a misfortune ; at any rate it removes the matter from the jurisdiction of critics who are not zoölogists, but mere reasoners (and who have already written nonsense enough about it), and leaves it to the learned tact of experts, which alone is able to weigh delicate facts against each other, and to decide how many possibilities make a probability, and how many small probabilities make an almost certainty. Among those experts Mr. Darwin’s own name stands high, and this work will probably not lower its place. The “ general reader,” anxious only for results, will find it much drier and less interesting than the “ Origin of Species ” ; but the student, as we have already said, must read it, and, whichever way his conclusions may tend, cannot fail to learn a great deal from it.