5. In no case can natural selection have been the cause of mutual infertility between allied, or any other, species—i.e. of the most general of all "specific characters."
6. Without Isolation, or the prevention of free intercrossing, organic evolution is in no case possible. Therefore, it is isolation that has been "the exclusive means of modification," or, more correctly, the universal condition to it. Therefore, also, Heredity and Variability being given, the whole theory of organic evolution becomes a theory of the causes and conditions which lead to Isolation.
7. Isolation may be either discriminate or indiscriminate. When discriminate, it has reference to resemblances between individuals constituting the isolated colony or group; when indiscriminate, it has no such reference. In the former case there arises Homogamy, and in the latter case there arises Apogamy.
8. Except where very large populations are concerned, indiscriminate isolation always tends to become increasingly discriminate; and, in the measure that it does so, apogamy passes into homogamy, by virtue of Independent Variability.
9. Natural Selection is one among many other forms of discriminate isolation, and presents in this relation the following peculiarities:—(a) The isolation is with reference to superiority of fitness; (b) is effected by death of the excluded individuals; and (c) unless assisted by some other form of isolation, can only effect monotypic as distinguished from polytypic evolution.
10. It is a general law of organic evolution that the number of possible directions in which divergence may occur can never be more than equal to the number of cases of efficient isolation; but, excepting natural selection, any one form of isolation need not necessarily require the co-operation of another form in order to create an additional case of isolation, or to cause polytypic as distinguished from monotypic evolution.
11. Where common areas and polytypic evolution are concerned, the most general and most efficient form of isolation has been the physiological, and this whether the mutual infertility has been the antecedent or the consequent of morphological changes on the part of the organisms concerned, and whether or not these changes are of an adaptive character.
12. This form of isolation—which, in regard to incipient species, I have called Physiological Selection—may act either alone or in conjunction with other forms of isolation on common areas: in the former case its agency is of most importance among plants and the lower classes of animals; in the latter case its importance consists in its greatly intensifying the segregative power of whatever other form of isolation it may be with which it is associated.