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Show 172 DARWINISM CHAP. very change of conditions, which is the initial power in starting such new forms, leads also to a local or stational sccrrcgation of the forms acted upon. But there is also a very po~vcrful cause of isolation in the mental nature-the likes aml dislikes-of animals; and to this is probably due the fact of the comparative rarity of hybrids in a state of nature. The differently coloured herds of cattle in the Falkland Islands, ettch of which keeps separate, have been already mentioned; and it ma.y be added, that the white variety seem to have already developed a physiological peculiarity in breeding tht·cc months earlier than the others. Similar facts occur, however, among our domestic animals :uul arc well known to breeders. Professor Low, one of the greatest authorities on our domestica,tcd animals, says : "The female of the dog, when not under restraint, makes selection of her ma.te, the mastifl' selecting the mastiff, the terrier the terrier, and so on." Aml again : "The Merino sheep and Heath sheep of Scotland, if two flocks are mixed together, each will breed with its own variety." Mr. Darwin has collected many facts illustmting this point. One of the chief pigeon-fanciers in England informed him that, if free to choose, each breed would prefer pairing with its own kinll. Among the wild hor. cs in Pamguay those of the sa.mc colour u.nd size associu.te together; while in Circassia there arc three races of horses which h<wc recci ved speci::tl names, and which, when living a free life, almost always refuse to mingle and cross, and will even attack one another. On one of the Faroe Islands, not more than half a mile in dia.mctcr, the hu.lf-wild nu.tivc black sheep do not readily mix with imported white sheep. In the' Forest of Dean, and in the New Forest, the du.rk and pak coloured herds of fallow deer have never been known to mingle; and even the curious Ancon sheep of 1uitc modern origin h~we been observed to keep together, scpu.rati11g th cn1 scl ve · from the rest of the flock when put into enclosnreH with other sheep. The ·amc rule applies to birds, for Darwin was informed by the Rev. \V. D. Fox that his flocks of whiLe and Chinese geese kept distinct.1 This constant preference of animals for their like, even in the case of slightly difl'crent va.rictiesof the same ·pccics, is eviden tly 1 Anirnals and Plants ttnder Domesticcttion, vol. ii. pp. 102, 103. VII ON THE INFERTILITY OF CROS~ES 173 <t fact of grca.t importance in considering the origin of .'pccics by 1mtuml selection, since it shows us th~tt, so soon as a slight differentiation of form or colour has been efi'cctcd, isolation will at once arise by the selective association of the animals themselves; a.nd thus the great stumbling-block of "the swamping effects of intercrossing," which has been so prominently brought forward by many naturalists, will be completely obviated. If now we combine with this fact the correlation of colour with import:Lnt constitutional peculiarities, and, in some cases, with infertility; and consider, further, the curious pamllclism that has been shown to exist between the effect of changed conditions and the intcrcrossing of varieties in producing either an increase or a decrca. ·e of fertility, we shn1l have obtained, at a.ll events, a starting-point for the production of that infertility which is so cha.ractcri tic a feature of distinct species when intercrossed. All we need, now, i .. omc means of increasing or accumulating this initia.l tendency; and to a discussion of this problem we will therefore address ourselves. The Injluf'nce nj Natnml Selection upon Sterility and Fertility. It wi 11 occnr to many persons that, as the in fcrtili ty or sterility of incipient species would be useful to them when occupying the same or adjacent areas, by neutrn1ising the effects of intercrossing, this infertility might have been increased by the action of natural selection ; and this will be thought the more probable if .we admit, as we have seen reason to do, that variations in fertility occur, perhaps as frequently as other variations. Mr. Darwin tells us that, at one time, this appeared to him probable, but he found the problem to be one of extreme complexity; and he was al o influenced against the view by many considemtions which seemed to render such an origin of the sterility or infertility of species when intercrosscd very improbable. The fact that species which occupy distinct areas, and which nowhere come in contact with each other, a.re often sterile when crossed, is one of the difficulties; but this may perhaps he overcome by the consideration that, though now isolated, they may, and often must, have been in contact at their origimttion. More important is the objection that natnml selection conl<l not |
