Appendix II. 329 



specially concerned with the question of the proportion in 

 which " individuals of the same species have been similarly 

 modified without the aid of any form of selection" the oversight 

 with respect to " the important word ' all ' " would still have 

 remained an oversight of a recurrent character, as the fol- 

 lowing additional quotations from other parts of Darwin's 

 writings may perhaps render apparent. 



" There must be some efficient cause for each slight individual 

 difference, as well as for more strongly marked variations 

 which occasionally arise ; and if the unknown cause were to 

 act persistently, it is almost certain that all the individuals 

 of the species would be similarly modified 1 ." 



" The acquisition of a useless part can hardly be said to 

 raise an organism in the natural scale We are so igno- 

 rant of the exciting cause of the above specified modifications ; 

 but if the unknown cause were to act almost uniformly for a 

 length of time, we may infer that the result would be almost 

 uniform ; and in this case all the individuals of the species 

 would be modified in the same manner 2 ." 



Moreover, when dealing even with such comparatively 

 slight changes as occur between our domesticated varieties 

 and which, a fortiori, are less likely to become " stable " 

 through the uniform operation of causes other than selec- 

 tion, seeing that they are not only smaller in amount than 

 occurs among natural species, but also have had but a 

 comparatively short time in which to accumulate Darwin 

 is emphatic in his assertion of the same principles. For 

 instance, in the twenty-third chapter of the Variation of 

 Plants and Animals under Domestication, he repeatedly 

 uses the term " definite action of external conditions," and 

 begins the chapter by explaining his use of the term 

 thus : 



" By the term definite action, as used in this chapter, I mean 

 an action of such a nature that, when many individuals of 



1 Origin of Species, p. 171. ' Ibid. p. 175. 



