20 THE THEORY OF EVOLUTION 



of very many animals, especially of butterflies and 

 birds. Inasmuch, however, as the effects of use 

 and disuse as an evolutionary factor involve the 

 assumption that characters acquired during the 

 lifetime of the parent are or may be transmitted to 

 the offspring, an assumption which is now vigorously 

 denied, modern disciples of Darwin reject this factor 

 and attribute little importance to sexual selection. 

 They accept Weismann's dictum that natural se- 

 lection is the sole and all-powerful agent of evolu- 

 tion. 



Darwin's book, reenforced by his subsequent 

 works, brought about the astonishing revolution 

 in scientific opinion which has already been de- 

 scribed, but, while the evolutionary theory thus 

 gained a complete and almost universal victory, 

 Darwin's particular theory, that of natural selection, 

 was not so fortunate. True, it was and still is very 

 widely accepted, but there has always been a large 

 body of opinion which rejected it as vague, inade- 

 quate and unsatisfactory and there have been many 

 attempts to supplement it, or to substitute some 

 more convincing explanation for it. Thus, the 

 eminent German botanist Carl von Nageli (1817- 

 1891) propounded an elaborate theory of develop- 

 ment, in which he attempted to show that natural 

 selection was an insufficient explanation of evolution 

 and that a "principle of perfection" must be as- 

 sumed, which drives organisms to take on higher 

 and higher forms. He compares the living world 



