128 EVOLUTION AND ANIMAL LIFE 



selection also tends to intensify and to preserve its nonadaptive 

 characteristic marks. The more pride the breeders take in 

 their stock, the more certain is the preservation of the breed's 

 useless peculiarities. Very few of the characters which usually 

 distinguish a breed of domestic animals have the slightest phys- 

 iological value to the species. Each of them would disappear 

 in a few generations of crossing, and in each race prized by the 

 breeder the actual virtues exist wholly independently of these 

 race marks. 



Analogous to the race peculiarities of domestic animals 

 are the minor traits among the men of different regions. Cer- 

 tain gradual changes in speech are due to adaptation, the fitness 

 of the word for its purpose, analogous to natural selection. 

 The nonadaptive matters of dialect find their origin in the 

 exigencies of isolation, while languages in general are ex- 

 plainable by the combined facts of migration, isolation, and 

 the adaptation of words for the direct uses of speech. 



In the animal kingdom generally we may say therefore: 

 Whenever a barrier is to some extent traversable, the forms 

 separated by it are likely to cross from one side to the other, 

 thus producing intergradations, or forms more or less inter- 

 mediate between the one and the other. For every subspecies, 

 where the nature of the variation has been carefully studied, 

 there is always a geographical basis. This basis is defined 

 by the presence of some sort of physical barrier. It is ex- 

 tremely rare to find two subspecies inhabiting or breeding in 

 exactly the same region. When such appears to be the case, 

 there is really some difference in habit or in habitat: the one 

 form lives on the hills, the other in the valleys; the one feeds 

 on one plant, the other on another; the one lives in deep water, 

 the other along the shore. There can be no possible doubt that 

 subspecies are nascent species, and that the accident of inter- 

 gradation in the one case and not in the other implies no real 

 difference in origins. 



For a final example, we may compare the species of Ameri- 

 can orioles constituting the genus Icterus. We may omit from 

 consideration the various subspecies, set off by the mountain 

 chains, and the usual assemblage of insular forms, one in 

 each of the West Indies, and confine our attention to the 

 leading species as represented in the United States. (See 

 frontispiece.) 



