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 moditied'." 



" 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"." 



Moreover, when dealing even with such comparatively 

 slight changes as occur between our domesticated varieties — 

 and which, a fortiori, are less hkely 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 



' Origin of Species, p. 171. ' Ibid. p. i^j. 



