X OBITUARY 291 



resulting from the operation of the former. Nor 

 is it essential that one should take up any 

 particular position in regard to the mode of 

 variation, whether, for example, it takes place per 

 saltum or gradually ; whether it is definite in 

 character or indefinite. Still less are those who 

 accept the theory bound to any particular views as 

 to the causes of heredity or of variation. 



That Darwin held strong opinions on some or all 

 of these points may be quite true ; but, so far as 

 the theory is concerned, they must be regarded as 

 obiter dicta. With respect to the causes of vari- 

 ation, Darwin's opinions are, from first to last, 

 put forward altogether tentatively. In the first 

 edition of the " Origin/' he attributes the strongest 

 influence to changes in the conditions of life of 

 parental organisms, which he appears to think act 

 on the germ through the intermediation of the 

 sexual organs. He points out, over and over again, 

 that habit, use, disuse, and the direct influence of 

 conditions have some effect, but he does not think 

 it great, and he draws attention to the difficulty 

 of distinguishing between effects of these agencies 

 and those of selection. There is, however, one 

 class of variations which he withdraws from the 

 direct influence of selection, namely, the variations 

 in the fertility of the sexual union of more or less 

 closely allied forms. He regards less fertility, or 

 more or less complete sterility, as "incidental to 

 other acquired differences." (Ibid., p. 245.) 



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