4. That I suppose physiological selection always to act alone. This I have never supposed. The essential point is, not that the physiological isolation is unassociated with other forms of isolation, but that unless associated with some degree of physiological isolation, no one of the other forms is capable of originating species on common areas with any approach to frequency. This proposition is the essence of the new theory, and I take it to be proved, not only by general deductive reasoning which shows that it _must_ be so, but also by the fact of an otherwise inexplicable association between specific divergence on common areas and some more or less considerable degree of mutual infertility. CHAPTER IV. EVIDENCES OF PHYSIOLOGICAL SELECTION. I will now give an outline sketch of the evidences in favour of the theory which has been set forth in the preceding chapter, stating first what is the nature of the verification which it requires. The theory is deduced from a highly general association between distinctive specific characters of _any_ kind and a relatively constant specific character of a _particular_ kind--namely, sexual exclusiveness. For it is from this highly general association that the theory infers that this relatively constant specific character has been at least one of the needful conditions to the development of the other specific characters with which it is found associated. Hence the necessary verification must begin by showing the strength of the theory on these merely deductive, or antecedent, grounds. It may then proceed to show how far the facts of organic nature corroborate the theory in other and independent ways. First, let it be carefully observed that here we have to do only with the _fact_ of selective fertility, and with its _consequences_ as supposed by the theory: we have nothing to do either with its _causes_ or its _degrees_. Not with its causes, because in this respect the theory of physiological selection is in just the same position as that of natural selection: it is enough for both if the needful variations are provided, without its being incumbent on either to explain the causes which produce them. Not with its degrees, because, in the first place, it can only be those degrees of variation which in particular cases are supposed adequate to induce specific divergence, that fall within the scope of the theory; and because, in the second place, degrees which are adequate only to induce--or to assist in inducing, _varietal_ divergence, must always tend to increase, or pass into higher degrees. _Antecedent Standing of the Theory._ The antecedent standing or logical basis of the theory has already been in large measure displayed in the preceding chapter; for it was impossible to state the theory without thereby showing in how considerable a degree it is self-evident. A brief recapitulation is therefore all that is here necessary.