SE GRE GA TION 6 5 



tion are so well known, usually breed year after year in the 

 same localities, the same individuals returning each spring to 

 the same spot and often occupying the same nest that was 

 left the year before. Of course, as those individuals of the 

 species which occupy the intermediate area will breed freely 

 with those nearer the two extremes, the segregation of the 

 latter is but partial, yet it must be sufficient to affect 

 evolution. 



Natural selection, sexual selection, and segregation all 

 mutually interact, as we can readily see. Sexual selection, 

 the exercise of choice in mating, causes reproductive segrega- 

 tion, and this, in turn, may affect natural selection. Let me 

 again quote Lloyd Morgan : " 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. 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. . . . This 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 considering 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 action 



