called Independent Generation. But of course this does not hinder that under apogamy various other causes of homogamy are likely to arise—in particular natural selection.

That natural selection differs from most of the other forms of isolation in not being capable of causing divergent or polytypic evolution must at once become evident, if we remember that the only way in which isolation of any form can cause such evolution is by partitioning a given group of intergenerants into two or more groups, each of which is able to survive as thus separated from the other, and so to carry on the evolution in divergent lines. But the distinguishing peculiarity of natural selection, considered as a form of isolation, is that it effects the isolation by killing off all the individuals which it fails to isolate: consequently, this form of isolation differs from other forms in prohibiting the possibility of any ramification of a single group of intergenerants into two or more groups, for the purpose of carrying on the evolution in divergent lines. Therefore, under this form of isolation alone, evolution must proceed, palm-like, in a single line of growth. So to speak, the successive generations continuously ascend to higher things on the steps supplied by their own "dead selves"; but in doing so they must climb a single ladder, no rung of which can be allowed to bifurcate in the presence of the uniformity secured for that generation by the free intercrossing of the most fit. Even though beneficial variations may arise in two or more directions simultaneously, and all be simultaneously selected by survival of the fittest, the effect of free intercrossing (in the absence of any other form of isolation) will be to fuse all these beneficial variations into one common type, and so to end in monotypic evolution as before. In order to secure polytypic evolution, intercrossing between the different beneficial variants which may arise must be prevented; and there is nothing to prevent such intercrossing in the process of natural selection per se. In order that the original group of intergenerants should be divided and sub-divided into two or more groups of intergenerants, some additional form of isolation must necessarily supervene—when, of course, polytypic evolution will result. And, as Mr. Gulick has shown, the conclusion thus established by deductive reasoning is verified inductively by the facts of geographical distribution.

How, then, are we to account for the fact that Darwin attributed to natural selection the power to cause divergence of character? The answer is sufficiently simple. He does so by tacitly invoking the aid of some other form of homogamy in every case. If we carefully read pp. 86-97 of the Origin of Species, where this subject is under consideration, we shall find that in every one of the arguments and illustrations which are adduced to prove the power of natural selection to effect "divergence of character," he either pre-supposes or actually names some other form of homogamy as the originating cause of the diversity that is afterwards presented to natural selection for further intensification. To give only one example. At the starting-point of the whole discussion the priority of such other forms of homogamy is assumed in the following words:—

But how, it may be asked, can any analogous principle [to that of diversity caused by artificial selection] apply in nature? I believe it can and does apply most efficiently (though it was a long time before I saw how), from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.

Now, without question, so soon as segregate breeding in two or more lines of homogamy has been in any sufficient degree determined by some "change of structure, constitution, or habits," natural selection will forthwith proceed to increase the divergence in as many different lines as there are thus yielded discriminately isolated sections of the species. And this fact it must have been that Darwin really had before his mind when he argued that diversification of character is caused by natural selection, through the benefit gained by the diversified forms being thus "enabled to increase in number." Nevertheless he does not expressly state the essential point, that although diversification of character, when once begun, is thus promoted by natural selection, which forthwith proceeds to cultivate each of the resulting branches, yet diversification of character can never be originated by natural selection. The change of "structure," of "constitution," of "habits," of "station," of geographical area, of reciprocal fertility, and so on—this change, whatever it may have been, must clearly have been antecedent to any operation of natural selection through the benefit which arose from the change. Therefore the change must in all cases have been due, in the first instance, to some other form of isolation than the superadded form which afterwards arose from superior fitness in the possession of superior benefit—although, so long as the prior form of isolation endured, or continued to furnish the necessary condition to the co-operation of survival of the fittest, survival of the fittest would have continued to increase the divergence of character in as many ramifying lines as there were thus given to its action separate cases of isolation by other means.

In short, as divergence of character must in all cases be due to a prevention of intercrossing, and as in the process of natural selection there is, ex hypothesi, nothing to prevent the intercrossing until the divergence has already arisen, to suppose that natural selection alone can have caused the divergence, is to suppose that natural selection can have caused the conditions of its own activity, which is absurd.

Seeing, then, that even in cases where any "benefit" arises from divergence of character, such benefit can arise only after the divergence has already commenced, and seeing that on this as on other accounts previously mentioned it is plainly impossible to attribute the origin of such divergence to natural selection, we find that natural selection must be in all cases assisted by some other form of isolation, if it is to be concerned in polytypic as distinguished from monotypic evolution. But this does not hinder that, when it is so assisted, natural selection may become—and, I believe, does become—the most efficient of all the forms of isolation in promoting divergence of character. For, in the first place, of all the forms of isolation natural selection is probably the most energetic in promoting monotypic evolution; so that under the influence of such isolation monotypic evolution probably advances more rapidly than it does under any other form of isolation. In the second place, when polytypic evolution has been begun by any of these other forms of isolation, and natural selection then sets to work on each of the resulting branches, although natural selection is thus engaged in as many different acts of monotypic evolution as there are thus separate cases supplied to it by these other forms of isolation, the joint result of all these different acts is to hurry on the polytypic evolution which was originally started by the other forms of isolation. So to speak, natural selection is the forcing heat, acting simultaneously on each of the separate branches which has been induced to sprout by other means; and in thus rapidly advancing the growth of all the branches, it is still entitled to be regarded as the most