The only point which I might add to my short discussion on this subject, is that I think it probable that the want of adaptation to uniform conditions of life in our domestic varieties has played an important part in preventing their acquiring sterility when crossed. For the want of uniformity, and changes in the conditions of life, seem the only cause of the elimination of sterility (when crossed) under domestication. (92/3. The meaning which we attach to this obscure sentence is as follows: Species in a state of nature are closely adapted to definite conditions of life, so that the sexual constitution of species A is attuned, as it were, to a condition different from that to which B is attuned, and this leads to sterility. But domestic varieties are not strictly adapted by Natural Selection to definite conditions, and thus have less specialised sexual constitutions.) This elimination, though admitted by many authors, rests on very slight evidence, yet I think is very probably true, as may be inferred from the case of dogs. Under nature it seems improbable that the differences in the reproductive constitution, on which the sterility of any two species when crossed depends, can be acquired directly by Natural Selection; for it is of no advantage to the species. Such differences in reproductive constitution must stand in correlation with some other differences; but how impossible to conjecture what these are! Reflect on the case of the variations of Verbascum, which differ in no other respect whatever besides the fluctuating element of the colour of the flower, and yet it is impossible to resist Gartner's evidence, that this difference in the colour does affect the mutual fertility of the varieties. The whole case seems to me far too mysterious to rest (92/4. The word "rest" seems to be used in place of "to serve as a foundation for.") a valid attack on the theory of modification of species, though, as you say, it offers excellent ground for a mere advocate.