SOURCES OF CHANGE 71 



with optimum development will be more likely to 

 succeed than those less favored and more likely to 

 perpetuate their characters in posterity. This con- 

 stitutes an isolation through usefulness; the species 

 consists more and more, as generations of pass, 

 that group of individuals which possess the useful 

 character. Instead of embracing the original wide 

 range of variation it embraces a restricted portion 

 of that range. 



Sexual selection has rather less evident impor- 

 tance. It has not been demonstrated by human 

 standards that any particular kind of excellence is 

 chosen by females in their acceptance of mates, 

 although the possibility seems admissible in the 

 case of higher animals. Certainly the display 

 made by birds during courtship implies that there 

 may be a basis for choice. When males habitually 

 fight for mates, it seems probable that the aggres- 

 siveness and jBghting ability of the species must be 

 improved, but since the male takes the active part 

 in this process it is rather a "survival of the fittest" 

 than a selection by the female. 



In addition to these processes whereby certain 

 parts of the original hereditary complex are pre- 

 served and certain others eliminated, or the species 

 is split up into races by the simultaneous preserva- 

 tion of various characters in various regions, we 

 must not overlook the effects of external environ- 

 ment in producing changes in entire species. In 



