AGGREGATIONS OF HIGHER ANIMALS 13I 



species which has become broken up into many small 

 local races, perhaps as a result of restricted environ- 

 mental niches. Each of these local races breeds largely 

 within its own colony, but there is an occasional emi- 

 gration from one to another. Each tends, if it is small 

 in number, to give rise to different variations which 

 shift about in a non-adaptive manner. The total 

 number of relatively stable variations will be much 

 greater since the total number of individuals is so 

 much larger than in E. Under these conditions the 

 chances are good that some of the local colonies will 

 escape from the influence of Mount Minor Adapta- 

 tion and manage to cross the valley to Mount Major 

 Adaptation. Here the race will expand in numbers 

 and will send out more and more emigrants which 

 will interbreed with the stocks in the less adapted 

 colonies and tend to grade them all up toward a 

 higher adaptive level. 



The conclusion is as Professor Wright says: "A 

 subdivision of a large species into numerous small, 

 partially isolated races gives the most effective setting 

 for the operation of the trial and error mechanism 

 in the field of evolution that results from gene com- 

 binations." 



In the rate of evolution, therefore, population size 

 is as important as we have seen it to be in the growth 



