Now, to begin with, it is due to Mr. Catchpool to state that his only publication upon this subject is much too brief to justify Mr. Wallace's, inference, that he supposes variations in the way of cross-infertility always to arise "alone in an otherwise undifferentiated species." What Mr. Catchpool's opinion on this point may be, I have no knowledge; but, whatever it is, he was unquestionably the first writer who "clearly stated the leading principles" of physiological selection, and this fact I am very glad to have "acknowledged." In my correspondence with Mr. Wallace, however, I not only named Mr. Catchpool: I also named--and much more prominently--Mr. Gulick. For even if I were to grant (which I am far indeed from doing) that there was any want of clearness in my own paper touching the point in question, I have now repeatedly shown that it is simply impossible for any reader of Mr. Gulick's papers to misunderstand _his_ views with regard to it. Accordingly, I replied to Mr. Wallace in _Nature_ by saying:-- Not only have I thus from the first fully recognized the sundry other causes of specific change with which the physiological variations may be associated; but Mr. Gulick has gone into this side of our common theory much more fully, and elaborately calculated out the high ratio in which the differentiating 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 calculations above alluded to were communicated to the Linnaean Society by Mr. Wallace himself in 1887.