The Making of Species 



upon the importance of the principle of isolation, 

 rendered valuable service to biological science, 

 but, in common with most men having a new 

 theory, they have pushed their conclusions to 

 absurd lengths. 



As Romanes has pointed out, isolation may 

 be discriminate or indiscriminate. " If," he 

 writes, on p. 5 of vol. iii. of Darwin and after 

 Darwin, "a shepherd divides a flock of sheep 

 without regard to their characters, he is isolating 

 one section from the other indiscriminately ; but 

 if he places all the white sheep in one field, and 

 all the black sheep in another field, he is isolating 

 one section from the other discriminately. Or, if 

 geological subsidence divides a species into two 

 parts, the isolation will be indiscriminate ; but if 

 the separation be due to one of the sections 

 developing, for example, a change of instinct 

 determining migration to another area, or occu- 

 pation of a different habitat on the same area, 

 then the isolation will be discriminate, so far as 

 the resemblance of instinct is concerned." 



Other names for indiscriminate isolation are 

 separate breeding and apogamy. Discriminate 

 isolation is also called segregate breeding and 

 homogamy. The human breeder resorts to 

 discriminate isolation in that he separates all 

 those creatures from which he seeks to breed, 

 from those from which he does not wish to 

 breed. Natural selection itself is, therefore, a 



