284 EVOLUTION OF TO-DAY. 



pletely hidden to the naturalist ; and, second, that 

 they are primarily unrelated to any advantage re- 

 sulting from them. They arise indefinitely, and 

 some of them chance to be useful. Darwin spent 

 many years in trying to prove to himself and others, 

 that it was the natural selection of such chance 

 favorable variations and their transmission to suc- 

 ceeding generations that forms the fundamental 

 explanation of the gradual modification of species 

 and the consequent production of new ones. He 

 admitted that other factors had a certain though 

 subordinate part, such as use and disuse, or the 

 direct effect of physical conditions. He thought 

 that ornamental structures and sexual differences 

 were due to the interaction of the sexes. To make 

 his theory complete, he forms an hypothesis to ex- 

 plain heredity, which, however, really forms no part 

 of his general theory, and is of little importance. 

 But aside from this, natural selection has been found 

 inadequate in many other points, so that even its 

 author, in his later writings, was induced to think it 

 of less importance than he at first believed. Wag- 

 ner suggested that species were formed by isolation 

 of a few individuals, and this theory eventually led 

 him to abandon natural selection. Isolation avoids 

 many difficulties met by the theory of Darwin, and 

 probably has had its influence. But while isolation 

 may have been a factor in many cases favoring the 

 modification of species, it is impossible to believe 

 that all species have arisen in this way. The theory 

 is too small a conception for the facts. 



Since the simple selection of chance variation 



