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 indefinite variability, 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 parts of the world.1 Nevertheless, a Porto 1 "Animals and Plants under Domestication,” vol. i. p. 115.

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."1 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.

2

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.

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 5

1

3 Ibid. vol. ii. p. 361.

"Animals and Plants under Domestication," vol. i. p. 114. 2 Ibid. vol. i. p. 243. 4 Ibid. vol. ii. p. 16.

"Genesis of Species,"

Since the publication of the first edition of the

Mr. Darwin has admitted his mistake in this matter, as also in the attribution by him of supernumerary mammæ to reversion. See "Descent of Man," vol. i. p. 125.

5 Animals and Plants under Domestication," vol. ii. p. 57.

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.1

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 based upon the 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 remarks2 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

1 This has been shown by my late friend, Mr. H. N. Turner, jun., in an excellent paper by him in the "Proceedings of the Zoological Society for 1849," p. 147. The untimely death, through a dissecting wound, of this promising young naturalist, was a great loss to zoological science.

2 "Animals and Plants under Domestication," vol. ii. p. 189.

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 of the evidence that the pollen of a distinct variety or race is prepotent over a flower's own pollen, Mr. Darwin adds the very significant remark:1 "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 says: 2 "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."

1 "Origin of Species," 5th edition, 1869, p. 115.

2 Ibid. p. 323.

Mr. Darwin objects to the notion that there is any peculiar sterility imposed to check specific intermixture. and change, saying,1 “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 from a 1 "Origin of Species," 5th edition, 1869, p. 314.