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.
The first is, that, as Mr. Wallace remarks, "every species has come into existence coincident both in space and time with a pre-existing and closely allied species." I regard this as important evidence that physiological selection is one of the natural causes concerned. For the general fact implied is that every species has come into existence on an area occupied by its parent type, and therefore under circumstances which render it imperative that intercrossing with that type should be prevented. In the case of monotypic evolution by natural selection alone, intercrossing with the parent type is prevented through the gradual extinction of that type by successive generations of the developing type. But in the case of polytypic evolution, intercrossing with the parent type can only be prevented by some form of isolation other than natural selection; and here it is evident that cross-infertility with the parent type must be as efficient to that end as any other form of isolation that can be imagined. Consequently we might almost have expected beforehand that in a large proportional number of cases cross-infertility should have been the means employed. And the fact that this is actually the case so far corroborates the only theory which is able to explain it.