Hence, where the question is whether selective fertility has played any large or general part in the differentiation of specific types, the best criterion we can apply is to ascertain whether it is a general rule that closely allied species occur in intimate association, so that their individual members constitute, as it were, a single population, or, on the other hand, whether they occur rather on different sides of physical barriers. If they occur intimately associated, the form of homogamy to which their differentiation was due must have presumably been the physiological form; whereas, if they are proved to be correlated with physical barriers, the form of homogamy which was concerned in their differentiation must presumably have been the geographical form. Now, at first this consideration was a trouble to me, because Moritz Wagner had strenuously argued--and supported his argument by a considerable wealth of illustration--that allied species are always found correlated with physical barriers or discontinuous areas. Weismann's answer, indeed, had shown that Wagner's statement was much too general: nevertheless, I was disappointed to find that so much could be said in favour of the geographical (or topographical) form of isolation where closely allied species are concerned. Subsequently, however, I read the writings of Nägeli on this subject, and in them I find a very different state of matters represented. Seeing as clearly as Wagner that it is impossible under any circumstances for natural selection to cause specific _differentiation_ unless assisted by some other forms of homogamy, but committing the same oversight as Wagner and Weismann in supposing that the only other form of homogamy in nature is geographical isolation, Nägeli, with great force of reasoning, and by many examples, founded his argument against the theory of natural selection on the ground that in the vegetable kingdom closely allied species are most frequently found in intimate association with one another, not, that is to say, in any way isolated by means of physical barriers. This argument is everywhere logically intact; and, as he sustains it by a large knowledge of topographical botany, his indictment against natural selection as a cause of specific _differentiation_ appeared to be insurmountable. And, in point of fact, it _was_ insurmountable; so that the whole problem of the origin of species by _differentiation on common areas_ has hitherto been left in utter obscurity. Nor is there now any escape from this obscurity, unless we entertain the "supplementary factor" of selective fertility. And, apparently, the only reason why this has not been universally recognized, is because Darwinians have hitherto failed to perceive the greatness of the distinction between the _differentiation_ and the _transmutation_ of species; and hence have habitually met such overwhelming difficulties as Nägeli presented by an illogical confounding of these two totally distinct things.