1909.] CONKLIN— THE WORLD'S DEBT TO DARWIN. xlvii 



causes of variation are in the main these: (i) The influence of the 

 environment and of changed conditions of life (2), the effects of the 

 use and disuse of parts, (3) the organic correlation of one varia- 

 tion with another so that the two necessarily arise together. Again 

 and again he asserts as one of his principal conclusions, which he 

 makes especially emphatic by placing it at the head of certain 

 chapters, that " variability of every kind is due to changed condi- 

 tions of life." He considered the value of sports, or what De Vries 

 calls " mutations," in the production of new races, and he decided 

 that their value was not usually very great. He considered the 

 question as to whether variations occur in every direction, or prin- 

 cipally in one, whether they are multifarous or unifarous, and he 

 concluded on the whole that the evidence was chiefly favorable to 

 the former view. 



It is in these three directions that our knowledge of the origin 

 of variations has made the greatest advance within recent years, 

 viz., (i) The effects of the conditions of life in producing new races, 

 (2) the value of sudden sports or mutations, (3) the question 

 whether variations are fluctuating or definitely directed. All of 

 these factors were considered by Darwin and to the first he assigned 

 great importance; and if the evidences now to be had show that the 

 second and third factors named are more important than he sup- 

 posed, they do not fundamentally nor seriously change his theory. 

 In some quarters there is a tendency to hail the mutation theory of 

 De Vries and the orthogenesis theory of Eimer and Whitman as 

 antagonistic to the Darwinian theory, but there is absolutely no 

 reason why all of these factors may not coexist harmoniously. Both 

 De Vries and Whitman hold that natural selection is a factor, and 

 an important one, in the evolution of organisms, and if the theories 

 of mutations or orthogenesis shall prove to be well founded, the 

 whole problem of evolution will be immensely simplified and the 

 greatest objections to the Darwinian theory will disappear, viz., 

 (i) The lack of sufficient time for evolution, (2) the paleonto- 

 logical evidence that evolution has been in directed lines, (3) the 

 inutility of many specific characters, (4) the complete disappear- 

 ance of many rudimentary organs, (5) the harmonious coadapta- 

 tion of parts. 



