In conclusion, then, and having regard to the principle of isolation as a whole, or in all the many and varied forms in which this principle obtains, I trust that I have redeemed the promise with which I set out--viz. to show that in relation to the theory of descent this principle is of an importance second to no other, not even excepting heredity, variability, and the struggle for existence. This has now been fully shown, inasmuch as we have clearly seen that the importance of the struggle for existence, and consequent survival of the fittest, arises just because survival of the fittest is a form, and a very stringent form, of isolation; while, as regards both heredity and variability, we are now in a position to see that the more fully we recognize their supreme importance as principles concerned in organic evolution, the more must we also recognize that any rational theory of such evolution becomes, in the last resort, a theory of the different modes in which efficient isolation can be secured. For, in whatever degree the process of organic evolution has been dependent upon heredity with variability, in that degree must it also have been dependent upon the means of securing homogamy, whereby alone the force of heredity can be made to expend itself in the innumerable directions of progressive change, instead of continually neutralizing the force of variability by promiscuous intercrossing. CHAPTER III. PHYSIOLOGICAL SELECTION. So far we have been concerned with the principle of Isolation in general. We have now to consider that form of isolation which arises in consequence of mutual infertility between the members of any group of organisms and those of all other similarly isolated groups occupying simultaneously the same area. * * * * * Against the view that natural selection is a sufficient explanation of the origin of species, there are two fatal difficulties: one, the contrast between natural species and domesticated varieties in respect of cross-sterility; the other, the fact that natural selection cannot possibly give rise to polytypic as distinguished from monotypic evolution. Now it is my belief that the theory of physiological selection fully meets both these difficulties. Indeed I hold this to be undeniable in a formal or logical sense: the only question is as to the evidence which can be adduced for the theory in a practical or biological sense. Therefore in this chapter, where the theory has first of all to be stated, I shall restrict the exposition as much as possible to the former, leaving for subsequent consideration the biological side. The following is a brief outline sketch of this theory[15]. [15] _See Nineteenth Century_, January, 1887, pp. 61, 62.