I am most glad that in your last letter you deal with what I consider the real 'question'--viz. not whether degrees of sterility obtain among a large proportional number of species, but whether there is any such correlation between them and absence of isolation of other kinds as my theory would expect. And, in dealing with this question you hit upon precisely the two greatest difficulties which I have myself concluded lie against the theory. The first is about areas now discontinuous having been once continuous, and our being so often unable to say whether or not such has been the case. But this difficulty is one that lies against _verification_ of the theory, not against the theory itself. It was in view of this difficulty that I mentioned oceanic islands as furnishing the best flora for trying experiments upon; but since I published the paper, I have not been able to hear of any botanists visiting islands. Should you ever hear of any you might let me know. The second difficulty is one that lies against the theory itself, and has always seemed to me most formidable. But as nobody else has ever mentioned it, I have not hitherto done so, as I want to work it out quietly. I allude to your remark about the extraordinary differences that obtain among different genera with regard to the capability of intercrossing exhibited by their constituent species. This, I confess, has from the first appeared a tremendous objection to my theory. On the other hand, I have taken comfort from the consideration that besides being a tremendous objection, it is also a tremendous mystery. For, as it must admit of some explanation, and as this explanation must almost certainly have to do with the sexual system, it becomes not improbable that when found the explanation may square with p.s. That the difference in question is functional and not structural (or physiological as distinguished from morphological) seems to be proved by the fact that in some cases it obtains as between the most closely allied genera, being, _e.g._, most strongly pronounced of all between Geranium and Pelargonium. Even quite apart from my own theory, it seems to me that this is a subject of the highest importance to investigate.