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.