important single cause of diversification in organic nature, although we must henceforth cease to regard it as in any instance the originating cause—or even so much as the sustaining cause.

So much by way of summary and recapitulation. I will now briefly consider the only objections which, so far as I can see, admit of being brought against the foregoing doctrine of Isolation as held by Mr. Gulick and myself. These possible objections are but two in number—although but one of them has been hitherto adduced. This, therefore, I will take first.

Mr. Wallace, with his customary desire to show that natural selection is everywhere of itself capable of causing organic evolution, seeks to minimize the swamping effects of free intercrossing, and the consequent importance of other forms of isolation. His argument is as follows.

Alluding to the researches of Mr. J. A. Allen, and others, on the amount of variation presented by individuals of a species in a state of nature, Mr. Wallace shows that, as regards any given part of the animal under consideration, there is always to be found a considerable range of individual variation round the average mean which goes to constitute the specific character of the type. Thus, for example, Mr. Allen says of American birds, "that a variation of from fifteen to twenty per cent. in general size, and an equal degree of variation in the relative size of different parts, may be ordinarily expected among specimens from the same species and sex, taken at the same locality, while in some cases the variation is even greater than this." Now, Mr. Wallace is under the impression that these facts obviate the difficulty which arises from the presence of free intercrossing—the difficulty, that is, against the theory of natural selection when natural selection is supposed to have been the exclusive means of modification. For, as he says, "if less size of body would be beneficial, then, as half the variations in size are above and half below the mean or existing standard of the species, there would be ample beneficial variations"; and similarly with regard to longer or shorter legs, wings, tails, &c., darker or lighter colour, and so on through all the parts of any given organism.

Well, although I have no wish at all to disparage the biological value of these actual measurements of the range of individual variation, I must point out that they are without any value at all in the connexion which Mr. Wallace adduces them. We did not require these measurements to tell us the broad and patent fact that "no being on this earthly ball is like another all in all"—or, in less Tennysonian words, that as regards every specific structure there is a certain amount of individual variability round an average mean. Indeed, in my own paper on Physiological Selection—against which Mr. Wallace is here specially arguing—I expressly said, as previously remarked, "that a specific type may be regarded as the average mean of all individual variations." The fact of such individual variability round a specific mean has always been well known to anatomists; it constitutes one of the basal pillars of the whole Darwinian theory; and is besides a matter of universal recognition as regards human stature, features, and so forth. The value of Mr. Allen's work consists in accurately measuring the amount or range of individual variation; but the question of its amount or range is without relevancy in the present connexion. For the desirability of isolation as an aid to natural selection even where monotypic evolution is concerned, does not arise with any reference to the amount or range of variation: it arises with reference to the number of variations which are—or are not—similar and simultaneous. If there be a sufficient number which are both similar and simultaneous, the desirability of any co-operating form of isolation is correspondingly removed, because natural selection may then have sufficient material wherewith to overcome the adverse influence of free intercrossing, and so of itself to produce monotypic evolution. Now, variations may be numerous, similar, and simultaneous, either on account of some common cause acting on many individuals at the same time, or on account of the structures in question being more or less variable round a specific mean. In the latter case—which is the only case that Mr. Allen's measurements have to do with—the law of averages will of course determine that half the whole number of variations in any given structure, in any given generation, will be above the mean line. But, equally of course, no one has ever denied that where, for either of these reasons, natural selection is provided with sufficient material, it is correspondingly capable of improving the specific type without the assistance of any other form of homogamy; so to speak, they protect themselves by their very numbers, and their superiority over others leads to their survival and accumulation. But what is the result? The result can only be monotypic evolution. No matter how great the number, or how great the range, of variations round an average specific mean, out of such material natural selection can never produce polytypic evolution: it may change the type to any extent during successive generations, and in a single line of change; but it cannot branch the type, unless some other form of homogamy intervenes. Therefore, when Mr. Wallace adduces the well-known fact that all structures vary more or less round a specific mean as proof that natural selection need not be incommoded by free intercrossing, but can of itself produce all the known phenomena of specific evolution, he fails to perceive that his argument refers only to one aspect of such evolution (viz. the transformation of species in time), and does not apply to the aspect with which alone my paper on Physiological Selection was concerned (viz. the multiplication of species in space).

The same thing may be shown in this way. It is perfectly obvious that where the improvement of type in a linear series is concerned (monotypic evolution), free intercrossing, far from being a hindrance to the process, is the very means by which the process is accomplished. Improvement here ascends by successive steps, in successive generations, simply because of the general intercrossing of the generally most fit with the result that the species, as a whole, gradually becomes transformed into another species, as a whole. Therefore, it would be mere fatuity in any one to adduce free intercrossing as a "difficulty" against natural selection alone being competent to produce evolution of this kind. But where the kind of evolution is that whereby the species is differentiated—where it is required, for instance, to produce different structures in different portions of the species, such as the commencement of a fighting spur on the wing of a duck, or novel characters of any sort in different groups of the species—free intercrossing is no longer a condition to, but an absolute preventive of, the process; and, therefore, unless checked as between each portion of the species by some form of homogamy other than natural selection, it must effectually inhibit any segregation of specific types, or divergence of character.

Hence it is that, while no Darwinian has ever questioned the power of unaided selection to cause improvement of character in successive generations, in common now with not a few other Darwinians I have emphatically denied so much as the abstract possibility of selection alone causing a divergence of character in two or more simultaneous lines of change.

And, although these