EVOLUTION IN NATURE 151 



heritable diversity of the originally uniform herit- 

 age and may pave the way for the establishment 

 of various pure lines and for the operation of the 

 principles of hybridization and selection. We have 

 granted the possibility that some of these muta- 

 tions may be of use to the individual, while noting 

 that experimental evidence fails to support this 

 possibility. Certainly while the environment re- 

 mains fairly constant the modification of species 

 by selection among these mutations must be at a 

 minimum. 



It is inconceivable, however, that the environ- 

 ment would remain even approximately constant 

 for all individuals. Even though it may be so within 

 the area occupied by the original stock, future 

 generations must be forced out of this area through 

 the natural expansion of the group or the natural 

 means of dispersal. In various regions they must 

 encounter changed conditions. These conditions 

 must be met by some reorganization of their vital 

 processes. They are able to respond to the condi- 

 tions governing their existence in various degrees, 

 and so the individuals which encounter more and 

 brighter sunlight at lower latitudes, or low pressure 

 at high altitudes, or a different food supply in a 

 new region, will develop and behave differently 

 from their ancestors. It probably makes little 

 difference to a coyote whether it eats rabbit or 

 gopher, but the technique of hunting the two ani- 



