EVOLUTIONARY THEORIES SINCE DARWIN 235 



duce their own fit kind. Many biologists since his 

 time have doubted whether unaided Natural Selec- 

 tion will account for the constant advance in orean- 

 isms. This is the part of the work which is often 

 seriously questioned. 



Weissman and his co-workers have contended that 

 this unaided principle will serve. Most biologists have 

 asked for some more efficient cause, and assert that se- 

 lection does not account for the appearance of varia- . 

 tions, but only for their preservation, and that any 

 valid theory of evolution must show how variations 

 originate. It is chiefly in this respect that Darwin's 

 work has failed to satisfy many later biologists. '' 

 When we hear a scientist speak of Darwinism as be- 

 ing dead, this is what he means. He does not think 

 evolution false, but believes that Natural Selection is 

 not sufficient to account for evolution. There are 

 three main difficulties involved in Darwin's theory.. 

 The chief defect lies in the fact that selection cannot * 

 originate varieties. In all his earlier works Darwin -" 

 simply accepted variations as he found them. He was 

 content to say that all species varied constantly, and 

 in every direction. He gave no theory to account for 

 variation. Whenever he took measurements of the 

 dimensions of any large series of objects of the same 

 kind he found these measurements to vary, apparently, 

 in all directions. Upon the facts of these variations, 



