Lastly, about species not being able to exist as species without the physiological isolation of physiological selection (p. 403), the statement of course only applies to nearly allied species occupying common areas (see p. 404). If this statement is wrong, no one has yet shown me wherein it is so. I fancy you do not quite appreciate that by 'sterility' I always mean (unless otherwise expressly stated) sterility _in some degree_, and this not only with regard to the fertile hybrids. It is by no means enough to point to natural and fertile hybrids as cases opposed to physiological selection unless it has been shown by experiment through a generation or two that these hybrids are _fully_ fertile--_i.e._ as fertile as their parent species. Now, experiments of this kind have rarely been carried through. If you assume that the result of carrying them through would be destructive of physiological selection by proving that fertile hybrids are, as a rule, fully fertile, and also (which is very important) that in any cases where experiment may show them to be so, further experiment would fail to show that isolation has not been effected in any other way (as by pre-potency, differences of insect fertilisation, &c.)--in short, if you assume that fertility is as complete between the two associated species as it is within each species, how is it conceivable that they should continue to be distinct? In this connection it is well to consult Gulick's paper already referred to (especially p. 259, paragraph 1st) on the theoretical side, and Jordan's papers and books on the practical side. I have repeated the latter's observations on poppies, and find that where any considerable number of individuals are concerned, natural selection is not nearly so great a power in this respect. (Even in cases where it happens that in-breeding is necessarily confined to single hermaphrodite individuals for numberless generations, the handicapping is not fatal: witness flowers which habitually fertilise themselves before opening--especially some species of orchids, which never seem to do otherwise, notwithstanding the elaborate provisions for cross-fertilisation in other species.) Now, I believe most of all in what I have called 'collective variation' of the reproductive system in the way of physiological selection, whereby, owing to some common influence acting on a large number of individuals similarly and simultaneously, they all become sexually co-adapted _inter se_ while physiologically isolated from the rest. This essential feature of the theory seems to me entirely to remove the difficulty about in-breeding, as well as that which Wallace urged about the chances against a suitable meeting of 'physiological complements.'