structure, form, or colour, as are so correlated with the reproductive system as to create the physiological isolation that is required. To show how the principle of selective fertility may be combined with what apparently is the most improbable form of isolation for this purpose--the geographical--I quote the following suggestion made by Professor Lloyd Morgan in his _Animal Life and Intelligence_:-- Suppose two divergent local varieties were to arise in adjacent areas, and were subsequently (by stress of competition or by geographical changes) driven together into a single area.... If their unions be fertile, the isolation will be annulled by intercrossing--the two varieties will form one mean or average variety. But if the unions be infertile, the isolation will be preserved, and the two varieties will continue separate. Suppose now, and the supposition is by no means an improbable one, that this has taken place again and again in the evolution of species; then it is clear that those varietal forms which had continued to be fertile together would be swamped by intercrossing; while those varietal forms which had become infertile would remain isolated. Hence, in the long run, isolated forms occupying a common area would be infertile, (p. 107.) If then cross-sterility may thus arise even in association with geographical isolation, may it not also arise in its absence? And may it not thus give rise to the differentiation of varieties on account of this physiological isolation alone? Only two further points need be mentioned to make this statement of physiological selection as complete as the present _résumé_ of its main principles requires.