114 THE PROBLEMS OF EVOLUTION 



jections to the theory has been pointed out enough 

 times in detail to free us from the necessity for 

 additional critical comment. Most of the ob- 

 jections are readily met with no greater strain on 

 our credulity than the theory itself involves, but 

 still we have no explanation for the cumulative 

 modification of characters nor for the origin of 

 variations in the beginning. The result is that the 

 theory becomes an explanation of the adaptation 

 of species through the selective action of the en- 

 vironment on the characters already present, and 

 not an unlimited explanation of evolution. As a 

 theory of species-adaptation, and to that extent 

 of evolution, few scientists will deny that it is 

 logical and therefore worthy of a place in our 

 explanation of organic evolution. 



The idea of natural selection has been responsible 

 for several other expressed selection theories. Dar- 

 win himself recognized the futility of ascribing 

 definite usefulness to all organic characters, or even 

 of classifying all characters as either useful or 

 harmful. In partial explanation of development 

 beyond the reach of natural selection he advanced 

 the theory of sexual selection, in which the deliber- 

 ate choice of mates was regarded as the active 

 force in determining what degree of development 

 of a variable character should survive. Granting 

 that a choice occurs and that it is based upon a 

 fairly uniform standard of excellence, such a proc- 



