But this leads us to another general fact, also mentioned by Darwin, and well recognized by all naturalists, namely, that closely allied species, or species differing from one another in trivial details, usually occupy contiguous areas; or, conversely stated, that contiguity of geographical position is favourable to the appearance of species closely allied to one another. Now, the large body of facts to which I here allude, but need not at present specify, appear to me to constitute one of the strongest of all my arguments in favour of physiological selection. Take, for instance, a large continental area, and follow across it a chain of species, each link of which differs from those on either side of it by the minute and trivial distinctions of a secondary kind, but all the links of which differ from one another in respect of the primary distinction, so that no one member of the series is perfectly fertile with any other member. Can it be supposed that in every case this constant primary distinction has been superinduced by the secondary distinctions, distributed as they are over different parts of all these kindred organisms, and yet nowhere presenting any but a trifling amount of morphological change? For my own part, I cannot believe--any more than Darwin could believe--that all these numerous, diverse, and trivial changes have always had the accidental effect of inducing the same peculiar change in the reproductive system, and so producing it without any reference to the process of specific divergence. Nor can I believe, as Darwin incidentally and provisionally suggested, that prolonged exposure to uniform conditions of life have so generally induced an equally meaningless result. I can only believe that all the closely allied species inhabiting our supposed continent, and differing from one another in so many and such divers points of small detail, are merely so many records of the fact that selective fertility has arisen among their ancestry, and has thus given as many opportunities for the occurrence of morphological differentiations as it has furnished cases of efficient isolation. Of course, I do not deny that many, or probably most, of these trivial morphological differentiations have been produced by natural selection on account of their utility: I merely deny that they could have been so produced on this common area, but for the sexual isolation with which every distinct set of them is now found to be associated. _Evidence from Topographical Distribution of Species._ By topographical distribution I mean the distribution of organisms with reference to comparatively small areas, as distinguished from larger regions with reference to which the term geographical distribution is appropriate.