NATURAL SELECTION 43 



mals and plants, and among those that are restricted to a 

 limited locality, breeding in and in, or breeding between near 

 relatives, must be frequent or even usual. An occasional 

 cross with some individual less closely related will be suffi- 

 cient to avoid deleterious effects from the close inbreeding. 



The influence of locality will sometimes serve to hinder 

 swamping of variations by free intercrossing. The environ- 

 mental conditions are frequently not uniform throughout the 

 whole range of a species. Take as an example a species of 

 plant which spreads over a wide area, part of which is moist 

 bottom-land, and part drier upland. If the individuals of the 

 species vary in their adaptability to conditions of moisture 

 and drouth, as they almost surely would do, some being 

 better fitted for life where moisture is abundant, others for 

 life in drier soil, then natural selection would, in each gener- 

 ation, eliminate from the bottom-lands a large proportion of 

 the plants best fitted for dry soil, and, conversely, would 

 destroy on the dry hills a large proportion of the individuals 

 adapted to wet soil. Thus in each locality, in each genera- 

 tion, the chances would be greater of like breeding with like 

 than with unlike. Natural selection, acting on each genera- 

 tion separately, would in this way raise a bar to free inter- 

 crossing of all variants in the species and would create a 

 probability of like breeding with like that would materially 

 increase the cumulative effect of natural selection from gen- 

 eration to generation. 



Variations in the time of breeding act as a direct bar 

 to free intercrossing between the members of a species, 

 those which mature their reproductive products at differ- 

 ent times being, of course, by this fact, prevented from 

 interbreeding. In this way differences in breeding season 



