The second difficulty which I have to mention as at first sight attaching to the views of Mr. Gulick and myself on the subject of Isolation is, that in an isolated section of a species Mr. Francis Galton's law of regression in the average character of offspring to the typical character of the group through reversion or atavism (_Natural Inheritance_, p. 97) must have the effect of neutralizing the segregative influence of mere apogamy. That such, however, cannot be the case has been well shown by Mr. Gulick in his paper on _Intensive Segregation_. Without at all disputing the validity of Mr. Galton's law, he proves that "it can hold in full force only where there is free crossing, otherwise no divergent race could ever be formed by any amount of selection and independent breeding[14]." This is so self-evident that I need not quote his demonstration of the point.
[14] _Zool. Journal Lin. Soc._, vol. xxiii. p. 313.
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.
