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closely allied species are almost invariably in some de
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: “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
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 uuable 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, 1969, 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. А “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
Origin of Species,” 5th edition, 1869, p. 314.
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 of 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 guineapig 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 of decidedly distinct species.
1 “Animals and Plants under Domestication,” vol. ii. p. 104.
Thus, then, it seems that a certain normal specific stability in species, accompanied by occasional sudden and considerable modifications, might be expected a priori from what we know of crystalline inorganic forms and from what we may suspect with regard to the lowest organic ones. This presumption is strengthened by the knowledge of the increasing difficulties which beset any attempt to indefinitely intensify any race characteristics. The obstacles to this indefinite intensification, as well as to certain lines of variation in certain cases, appear to be not only external, but to depend on internal causes or an internal cause. We have seen that Mr. Darwin himself implicitly admits the principle of specific stability in asserting the singular inflexibility of the organization of
We have also seen that it is not fair to conclude that all wild races can vary as much as the most variable domestic ones. It has been shown besides that there are grounds for believing in a tendency to reversion generally, as it is distinctly present in certain instances; further, that the doctrine of specific stability is confirmed by the fact that physiological obstacles oppose themselves to any considerable or continued intermixture of species, while no such barriers oppose themselves to the blending of varieties. All these considerations taken together may fairly be looked upon as strengthening the belief that specific manifestations are relatively stable. At the same time the view advocated in this book does not depend upon, and is not identified with, any such stability. All that the author contends for is that specific manifestation takes place along certain lines, and according to law; and not in an exceedingly minute, indefinite, and fortuitous
manner. Finally, he cannot but feel justified, from all that has been brought forward, in reiterating the opening assertion of this chapter, that something is still to be said for the view which maintains that species are stable, at least in the intervals of their comparatively rapid successive manifestations.