AGGREGATIONS OF HIGHER ANIMALS 123 



species will become divided into local races. This 

 will happen although at the time of separation the 

 populations were all homogeneous and the environ- 

 ment of all remains essentially similar. 



If the environment does remain steady the larger 



SELECTION 



4 



H UTATIO rt MUTATIOM 



CHANCE 



0.5 100 



Fig. 20. In medium populations, gene frequencies 

 drift at random about an intermediate point but not so 

 much so that complete fixation or loss is likely to occur. 

 (After Wright.) 



colonies will tend to keep the same hereditary consti- 

 tution as that which the whole species formerly had. 

 (Figure 21.) Small breeding colonies will, how- 

 ever, become pure cultures for different characters, 

 and it is impossible to predict the course of the 

 hereditary drift in any of these populations. As illus- 

 trated in Figure 20, the fixation will be a matter of 

 chance, and local races will result without any neces- 

 sary reference to adaptation. 



The snails in the different mountain valleys of 

 Hawaii afford the classical illustration of this point. 



