IN ORGANIC EVOLUTION 75 



THE WAY ISOLATION WORKS 



Isolation, as a factor in evolution, represents 

 the failure of a species to unify itself or to main- 

 tain a homogeneous character among its mem- 

 bers. Within a unified species, each member will 

 be fertile with any other of the opposite sex. In 

 time, the descendants of any one may cross with 

 descendants of all the others, thus bringing all 

 individuals to that degree of common relation- 

 ship implied by membership in a common species. 

 Wherever inter-crossing is checked along any 

 line, a part of the individuals will be set off from 

 the mass, and here divergence at once begins. 

 One cause of divergence may lie in the fact that 

 in each isolated group there is some original de- 

 viation from the average of the common stock, 

 thus giving at the start some slight difference in 

 heredity. But this is purely hypothetical and it 

 is not probable in any special case. Other and 

 apparently more potent causes of divergence he 

 in the difference of experiences to which each 

 group is exposed. The stress of the struggle for 

 existence is never quite the same in different lo- 

 calities, and the nature of selection must vary 

 accordingly. 



That notable differences obtain in time, even 

 in pure stocks, and when there is no visible reason 

 for change, is clearly shown in the experience of 

 stock breeders. Of this, a typical example will 

 suffice. Darwin tells us that the two flocks of 



