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,[[124]] "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.
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 anticipate 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 the goose. 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
also been shown that there are grounds for believing in a tendency to reversion generally, as it is distinctly present in certain instances. Also that specific stability is confirmed by the physiological obstacles which 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 considered 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.
CHAPTER VI.
SPECIES AND TIME.