204 Animal Life and Intelligence. 



well known 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 

 the heath sheep of Scotland, if two flocks are mixed together, 

 each will breed with its own variety.' Mr. Darwin 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 Paraguay 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 will even 

 attack one another. In one of the Faroe Islands, not 

 more than half a mile in diameter, the half-wild 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 themselves from the rest of the flock when put 

 into enclosures with other sheep. The same rule applies 

 to birds, for Darwin was informed by the Eev. W. D. Fox 

 that his flocks of white 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 a fact of great importance in con- 

 sidering the origin of species by natural selection, since it 

 shows us that, so soon as a slight differentiation of form or 

 colour has been effected, isolation will at once arise by the 

 selective association of the animals themselves." 



Mr. Wallace thus allows, nay, he lays no little stress 



on, preferential mating, and his name is associated with 



the hypothesis of recognition-marks. But he denies that 



preferential mating, acting on recognition-marks, has had 



* See " Animals and Plants under Domestication," vol. ii. p. 80. 



