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_