172 
DARWINISM 
CHAP. 
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 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 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 laced 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 will even 
attack one another. On 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 slice}). 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 when put into enclosures 
with other sheep. The same rule applies to birds, for Darwin 
was informed by the Uev. W. I). Fox that his flocks of white 
and Chinese geese kept distinct. 1 
This constant preference of animals for their like, even in the 
case of slightly different varietiesof the same species, is evidently 
1 Animals and Plants under Domestication, vol. ii. pp. 102, 103. 
