are mainly congenital, and so through these it selects the 

 hereditary factors that determine favorable variations. 



In one fundamental respect the doctrine is incomplete, 

 as it fails to explain the causes for the variations with 

 which selection deals. It accounts for the perpetuation of 

 favoring variations, but it does not account for their incep- 

 tion. Because of this defect, investigators reacted from 

 the academic discussion of Darwin's original doctrine, and 

 returned to deeper and wider study of heredity and varia- 

 tion with brilliant success. Some neo-Darwinians have en- 

 deavored to make the selective process an originative 

 influence notably Roux, and Weismann in his theory of 

 germinal selection. Darwin himself added the subsidiary 

 process of sexual selection, which regards the preference 

 by one sex of characteristics of the opposite sex as a con- 

 serving influence. But while such attempts have failed, 

 zoologists believe, to explain the whole method of evolu- 

 tion, much of the process has been demonstrated more and 

 more clearly with further study. The laws of fluctuating 

 variations have now been formulated with mathematical 

 accuracy, through the employment of the statistical 

 methods used earlier by anthropologists like Quetelet. The 

 studies of Galton, and Pearson, Boas, Weldon, and Da- 

 venport have demonstrated that structural and physio- 

 logical characters of men, of other animals, and of plants 

 as well, vary according to the formulas of chance or error, 

 a result they say that follows from the combined influ- 

 ence of innumerable and independent factors. Variation 

 is a natural phenomenon of chance. Furthermore, the 

 reality of the selective process has also been proved by 

 statistical methods. Bumpus' English sparrows, Weldon's 

 snails and crabs, and many other cases show that the indi- 

 viduals which depart widely from an average condition, or 

 that are uncorrelated in their organization, are marked for 

 destruction. 



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