138 Darwin, and after Darwin. 



elaborately calculated out the high ratio in which the differ- 

 entiating agency of any of these other causes must be increased 

 when assisted by i. e. associated with -even a moderate degree 

 of the selective fertility, and vice versa. Therefore, it is simply 

 impossible for Mr. Wallace to show that "our theory" differs 

 from his hi this respect. Yet it is the only respect in which his 

 reply alleges any difference. (Vol. xliii. p. 127.) 



I think it is to be regretted that, in his answer to 

 this, Mr. Wallace alludes only to Mr. Catchpool, and 

 entirely ignores Mr. Gulick whose elaborate calcula- 

 tions above alluded to were communicated to the 

 Linnaean Society by Mr. Wallace himself in 1887. 



The time has now come to prove, by means of 

 quotations, that I have from the first represented 

 the " principle," or " essence," of physiological selec- 

 tion to consist in selective fertility furnishing a need- 

 ful condition to specific differentiation, in at least 

 a large proportional number of allied species which 

 afterwards present the reciprocal character of cross- 

 sterility ; that I have never represented variations 

 in the way of this selective fertility as necessarily 

 constituting the initial variations, or as always arising 

 "alone, in an otherwise undifferentiated species"; 

 and that, although I have uniformly given it as my 

 opinion that these variations do in some cases thus 

 arise (especially among plants and lower invertebrata), 

 I have as uniformly stated "that it makes no differ- 

 ence to the theory in what proportional number of 

 cases they have done so "or even if, as Mr. Wallace 

 supposes, they have never done so in any case at all l . 



1 This refers to what I understand Mr. Wallace to say in the Nature 

 correspondence is the supposition on which his own theory of the origin 

 of species by cross-infertility is founded. But in the original statement 

 of that theory itself, it is everywhere "supposed" that when species are 



