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 in 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 



