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regarding the nature of the causes of evolution. It is desired 

 therefore, to distinguish clearly at this point between the facts 

 of variation first adequately recognized by Darwin and the 

 theory of environmental causes of evolution often called Dar- 

 winism. Naturalists do not all believe in environmentally 

 caused evolution, but nearly all are now agreed in thinking of 

 species, not as single morphological points, but as large groups 

 of similar individuals. 



Since the time of Darwin it has been believed that evolution 

 has been accomplished by means of variations, but there is still 

 the widest divergence of scientific opinion regarding the kinds 

 of variations which cause or contribute to developmental changes. 

 Some theories depend upon one or another of the different kinds 

 of variations and ignore the others, and some hold that all varia- 

 tions are caused by the environment and that evolution itself is 

 merely a summary of environmental influences. 



Many writers have approached the subject from the stand- 

 point of formal definitions and narrowly technical distinctions, 

 but the practical divergences between the different views become 

 most apparent from the types of variation — the kinds of intra- 

 specific differences — upon which they depend as showing the 

 nature of evolutionary motion. To correctly fix upon the kind 

 or kinds of variations which contribute to evolution, is the first 

 step of progress toward knowledge of the true evolutionary 

 factors, and brings us by the most direct route to the determina- 

 tion of the primary question, whether the true, efficient causes 

 of evolution lie in the environment or in the organisms them- 

 selves. Are the variations which are induced by the environ- 

 ment those by which evolutionary progress is accomplished? 



In Darwin's original suggestion environment was held to 

 bring about evolution, first by inducing variations and then by 

 selecting those which proved to be advantageous. The environ- 

 ment was considered as at once the cause of variations and of 

 evolution. This view is still generally accepted as the teaching 

 of science regarding organic evolution, although many modi- 

 fications and collateral suggestions have appeared necessary to 

 Darwin himself and to many of his successors. Some have 

 approached the Lamarckian idea of direct adaptation, in ascrib- 



