Genesis of Species
As to the peacock it is excused on the pleas (1), that the individuals maintained are so few in number, and (2) that its beauty is so great it can hardly be improved. But the individuals maintained have not been too few for the independent origin of the black-shouldered form, or for the supplanting of the commoner one by it. As to any neglect in selection, it can hardly be imagined that with regard to this bird (kept as it is all but exclusively for its beauty), any spontaneous beautiful variation in colour or form would have been neglected. On the contrary, it would have been seized upon with avidity and preserved with anxious care. Yet apart from the black-shouldered and white varieties, no tendency to change has been known to show itself. As to its being too beautiful for improvement, that is a proposition which can hardly be maintained. Many consider the Javan bird as much handsomer than the common peacock, and it would be easy to suggest a score of improvements as regards either species.
The guinea-fowl is excused, as being “no general favourite, and scarcely more common than the peacock;” but Mr. Darwin himself shows and admits that it is a noteworthy instance of constancy under very varied conditions.
These instances alone (and there are yet others) seem sufficient to establish the assertion, that degree of change is different in different domestic animals. It is, then, somewhat unwarrantable in any Darwinian to assume that all wild animals have a capacity for change similar to that existing in some of the domestic ones. It seems more reasonable to assert the opposite, namely, that if, as Mr. Darwin says, the capacity for change is different in different domestic animals, it must surely be limited in those which have it least, and a fortiori limited in wild animals.
Indeed, it cannot be reasonably maintained that wild species certainly vary as much as do domestic races; it is possible that they may do so, but at least this has not been yet shown. Indeed, the much greater degree of variation amongst domestic animals than amongst wild ones is asserted over and over again by Mr. Darwin, and his assertions are supported by an overwhelming mass of facts and instances.
Of course, it may be asserted that a tendency to indefinite change exists in all cases, and that it is only the circumstances and conditions of life which modify the effects of this tendency to change so as to produce such different results in different cases. But assertion is not proof, and this assertion has not been proved. Indeed, it may be equally asserted (and the statement is more consonant with some of the facts given), that domestication in certain animals induces and occasions a capacity for change which is wanting in wild animals—the introduction of new causes occasioning new effects. For, though a certain degree of variability (normally, in all probability, only oscillation) exists in all organisms, yet domestic ones are exposed to new and different causes of variability, resulting in such striking divergencies as have been observed. Not even in this latter case, however, is it necessary to believe that the variability is indefinite, but only that the small oscillations become in certain instances intensified into large and conspicuous ones. Moreover, it is possible that some of our domestic animals have been in part chosen and domesticated through possessing variability in an eminent degree.
That each species exhibits certain oscillations of structure is admitted on all hands. Mr. Darwin asserts that this is the exhibition of a tendency to vary which is absolutely indefinite. If this indefinite variability does exist, of course no more need be said. But we have seen that there are arguments a priori and a posteriori against it, while the occurrence of variations in certain domestic animals greater in degree than the differences between many wild species, is no argument in favour of its existence, until it can be shown that the causes of variability in the one case are the same as in the other. An argument against it, however, may be drawn from the fact, that certain animals, though placed under the influence of those exceptional causes of variation to which domestic animals are subject, have yet never been known to vary, even in a degree equal to that in which certain wild kinds have been ascertained to vary.
In addition to this immutability of character in some animals, it is undeniable, that domestic varieties have little stability, and much tendency to reversion, whatever be the true explanation of such phenomena.
In controverting the generally received opinion as to “reversion,” Mr. Darwin has shown that it is not all breeds which in a few years revert to the original form; but he has shown no more. Thus, the feral rabbits of Porto Santo, Jamaica, and the Falkland Islands, have not yet so reverted in those several localities. Nevertheless, a Porto Santo rabbit brought to England reverted in a manner the most striking, recovering the proper colour of its fur “in rather less than four years.” Again, the white silk fowl, in our climate, “reverts to the ordinary colour of the common fowl in its skin and bones, due care having been taken to prevent any cross.” This reversion taking place in spite of careful selection, is very remarkable.
Numerous other instances of reversion are given by Mr. Darwin, both as regards plants and animals; amongst others, the singular fact of bud reversion. The curiously recurring development of black sheep, in spite of the most careful breeding, may also be mentioned, though, perhaps, reversion has no part in the phenomenon.
These facts seem certainly to tell in favour of limited variability, while the cases of non-reversion do not contradict it, as it is not contended that all species have the same tendency to revert, but rather that their capacities in this respect, as well as for change, are different in different kinds, so that often reversion may only show itself at the end of very long periods indeed.
Yet some of the instances given as probable or possible causes of reversion by Mr. Darwin, can hardly be such. He cites, for example, the occasional presence of supernumerary digits in man. For this notion, however, he is not responsible, as he rests his remark on the authority of a passage published by Professor Owen. Again, he refers to “the greater frequency of a monster proboscis in the pig than in any other animal.” But with the exception of the peculiar muzzle of the Saiga (or European antelope), the only known proboscidian Ungulates are the elephants and tapirs, and to neither of these has the pig any close affinity. It is rather in the horse than in the pig that we might look for the appearance of a reversionary proboscis, as both the elephants and the tapirs have the toes of the hind foot of an odd number. It is true that the elephants are generally considered to form a group apart from both the odd and the even-toed Ungulata. But of the two, their affinities with the odd-toed division are more marked.
Another argument in favour of the, at least intermitting, constancy of specific forms and of sudden modification, may be drawn from the absence of minute transitional forms, but this will be considered in the next chapter.
It remains now to notice in favour of specific stability, that the objection drawn from physiological difference between “species” and “races” still exists unrefuted.
Mr. Darwin freely admits difficulties regarding the sterility of different species when crossed, and shows satisfactorily that it could never have arisen from the action of “Natural Selection.” He remarks also: “With some few exceptions, in the case of plants, domesticated varieties, such as those of the dog, fowl, pigeon, several fruit trees, and culinary vegetables, which differ from each other in external characters more than many species, are perfectly fertile when crossed, or even fertile in excess, whilst closely allied species are almost invariably in some degree sterile.”
Again, after speaking of “the general law of good being, derived from the intercrossing of distinct individuals of the same species,” and the evidence that the pollen of a distinct variety or race is prepotent over a flower’s own pollen, adds the very significant remark, “When distinct species are crossed, the case is directly the reverse, for a plant’s own pollen is almost always prepotent over foreign pollen.”
Again he adds: “I believe from observations communicated to me by Mr. Hewitt, who has had great experience in hybridizing pheasants and fowls, that the early death of the embryo is a very frequent cause of sterility in first crosses. Mr. Salter has recently given the results of an examination of about 500 eggs produced from various crosses between three species of Gallus and their hybrids. The majority of these eggs had been fertilized, and in the majority of the fertilized eggs the embryos either had been partially developed and had then aborted, or had become nearly mature, but the young chickens had been unable to break through the shell. Of the chickens which were born, more than four-fifths died within the first few days, or at latest weeks, ‘without any obvious cause, apparently from mere inability to live,’ so that from 500 eggs only twelve chickens were reared. The early death of hybrid embryos probably occurs in like manner with plants, at least it is known that hybrids raised from very distinct species are sometimes weak and dwarfed, and perish at an early age, of which fact Max Wichura has recently given some striking cases with hybrid willows.”
Mr. Darwin objects to the notion that there is any special sterility imposed to check specific intermixture and change, saying, “To grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems a strange arrangement.”
But this only amounts to saying that the author himself would not have so acted had he been the Creator. A “strange arrangement” must be admitted anyhow, and all who acknowledge teleology at all, must admit that the strange arrangement was designed. Mr. Darwin says, as to the sterility of species, that the cause lies exclusively in their sexual constitution; but all that need be affirmed is that sterility is brought about somehow, and it is undeniable that “crossing” is checked. All that is contended for is that there is a bar to the intermixture of species, but not of breeds; and if the conditions of the generative products are that bar, it is enough for the argument, no special kind of barring action being contended for.
He, however, attempts to account for the modification of the sexual products of species as compared with those of varieties, by the exposure of the former to more uniform conditions during longer periods of time than those to which varieties are exposed, and that as wild animals, when captured, are often rendered sterile by captivity, so the influence of union with another species may produce a similar effect. It seems to the author an unwarrantable assumption that a cross with what, on the Darwinian theory, can only be a slightly diverging descendant of a common parent, should produce an effect equal to that of captivity, and consequent change of habit, as well as considerable modification of food.
No clear case has been given by Mr. Darwin in which mongrel animals, descended from the same undoubted species, have been persistently infertile inter se; nor any clear case in which hybrids between animals, generally admitted to be distinct species, have been continuously fertile inter se.
It is true that facts are brought forward tending to establish the probability of the doctrine of Pallas, that species may sometimes be rendered fertile by domestication. But even if this were true, it would be no approximation towards proving the converse, i.e. that races and varieties may become sterile when wild. And whatever may be the preference occasionally shown by certain breeds to mate with their own variety, no sterility is recorded as resulting from unions with other varieties. Indeed, Mr. Darwin remarks, “With respect to sterility from the crossing of domestic races, I know of no well-ascertained case with animals. This fact (seeing the great difference in structure between some breeds of pigeons, fowls, pigs, dogs, &c.) is extraordinary when contrasted with the sterility of many closely-allied natural species when crossed.”
It has been alleged that the domestic and wild guinea-pig do not breed together, but the specific identity of these forms is very problematical. Mr. A. D. Bartlett, superintendent of the Zoological Gardens, whose experience is so great, and observation so quick, believes them to be decidedly distinct species.