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THE AMERICAN NATURALIST [Vol. XLIII 



accord with the consensus of opinion at present. What 

 the current opinion is can only be a matter of conjecture, 

 but notwithstanding the proofs of multiple origin af- 

 forded by adaptation and mutation, it would seem that the 

 majority of botanists and nearly all zoologists still adhere 

 to the doctrine of single origin. 



Migration 



Darwin 's treatment of migration is limited to evidence 

 of the possibility of distant and occasional migration as 

 the explanation of the many puzzles of distribution. It 

 is evident that his position in regard to single origin 

 caused him to turn to migration as the necessary solution 

 of all the problems of distribution. His attitude upon 

 both questions is shown by the following statement: 



Whenever it is fully admitted, as it will some day be, that each 

 species has proceeded from a single birthplace, and when in the course 

 of time we know something definite about the means of distribution, 

 we shall be enabled to speculate with security on the former extension 

 of the land. 4 



His experiments upon the carriage of seeds and fruits 

 by ocean currents and by birds, though necessarily crude 

 and simple, are classic, They still serve to indicate one 

 of the really fundamental points of attack in the detailed 

 quantitative study of migration. 



Variation 



A brief summary of Darwin's views upon variation is 

 an impossibility. Under the headings 1 1 Causes of Varia- 

 tion," < 'Habitat as Cause," "Use and Disuse," I have 

 assembled his own statements to make clear his position. 

 These seem to differ in some essentials from the views 

 often ascribed to Darwin, or at least held by many who 

 regard themselves as his followers. They appear to be 

 more in accord with the views of the ecologist who looks 

 to the habitat for the final explanation of all changes, 

 111™ ° + ph 1 lions llelcl b >~ biol °gists in general. They 



*~ have been much more in sympathy with 



