172 DARWINISM 



very change of conditions, which is the initial power in 

 starting such new forms, leads also to a local or stational 

 segregation of the forms acted upon. But there is also a very 

 powerful cause of isolation in the mental nature — the likes 

 and 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, each of which keeps separate, have been already 

 mentioned ; and it may be added, that the white variety seem 

 to have already developed a physiological peculiarity in breed- 

 ing three months earlier than the others. Similar facts occur, 

 however, among our domestic animals and are well kno^vn to 

 breeders. Professor Low, one of the greatest authorities on 

 our domesticated animals, says : " The female of the dog, when 

 not under restraint, makes selection of her mate, the mastiff 

 selecting the mastiff, the terrier the terrier, and so on." And 

 again : " The Merino sheep and Heath sheep of Scotland, if 

 two flocks are mixed together, each Avill breed with its own 

 variety." Mr. Darmn has collected many facts illustrating 

 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 kind. Among the wild horses in Para- 

 guay those of the same colour and size associate together; 

 while in Circassia there are three races of horses which have 

 received special names, and which, when living a free life, 

 almost always refuse to mingle and cross, and mil even 

 attack one another. On one of the Faroe Islands, not more 

 than half a mile in diameter, the half-^\dld native black sheep 

 do not readily mix with imported white sheep. In the 

 Forest of Dean, and in the New Forest, the dark and pale 

 coloured herds of fallow deer have never been known to 

 mingle ; and even the curious Ancon sheep of quite modern 

 origin have been observed to keep together, separating them- 

 selves from the rest of the flock Avhen put into enclosures 

 with other sheep. The same rule applies to birds, for Dar^vin 

 was informed by the Eev. W. I). Fox that his flocks of Avhite 

 and Chinese geese kept distinct.^ 



This constant preference of animals for their like, even in the 

 case of slightly different varieties of the same species, is evidently 

 ^ Animals and Plants under Domestication, vol. ii. pp. 102, 103. 



