126 



THE SOCIAL LIFE OF ANIMALS 



When a species is broken up into different breed- 

 ing colonies, as it is with the snails in the Hawaiian 



fN5=80 



Fig. 22. As intensity of selection increases it becomes 

 more and more dominant in determining the end result, 

 and the degree of variation is lessened; 4Ns gives selec- 

 tion pressure. (From Wright.) 



valleys, (57) it can be similarly shown that the effects 

 produced depend on the rate of emigration between 

 colonies, as well as selection pressure, mutation pres- 

 sure, and population size, other factors being con- 

 stant. Cross-breeding introduces genes into a popula- 



