102 Animal Life and Intelligence. 



range freely during the winter throughout the Northern 

 hemisphere ; but their breeding area is restricted to 

 the north polar region. The interbreeding within this 

 area keeps the species one and homogeneous, notwith- 

 standing its wide range, and, at the same time, prevents 

 intercrossing with allied species with different breeding- 

 grounds. 



Another most important mode of segregation among 

 animals arises out of habitual or instinctive preferences. 

 "Where varieties are formed there is a tendency for like to 

 breed with like. In the Falkland Islands the differently 

 coloured herds of cattle, all descended from the same stock, 

 keep separate, and interbreed with each other, but not with 

 individuals outside their own colour-caste. If two flocks 

 of merino sheep and heath sheep be mixed together, they 

 do not interbreed. In the Forest of Dean and in the New 

 Forest, the dark and pale coloured herds of fallow deer 

 have never been known to intermingle.* Here we have a 

 case of selective segregation through preferential mating, and 

 may find therein the basis of sexual selection in its higher 

 ranges as advocated by Darwin. 



The question of sexual selection will, however, be briefly 

 considered in the chapter on " Organic Evolution." At 

 present what we have to notice is that, through preferential 

 mating, segregation is effected. The forms that interbreed 

 have a distinguishing colour. From this it is but a step 

 to the possession, not merely of a distinguishing colour, 

 but of distinguishing colour-markings. Hence, through 

 preferential mating, may arise those special markings 

 which so frequently distinguish allied species. They not 

 only enable us to recognize species as distinct, but enable 

 the species which possess them to recognize the members 

 of their own kind. Mr. Wallace calls these diacritical 

 marks recognition-marks, and gives many illustrative 

 examples. f They are especially noticeable in gregarious 

 animals and in birds which congregate in flocks or which 



* Wallace, " Darwinism, " p. 172, where other examples are cited, 

 f Ibid. pp. 217, et seq. 



