reproductive system as to bring about the primary distinction, and thus to protect themselves against the destructive power of free intercrossing." [3] The evidence, so far as yet published, may be read by any one who cares to purchase the original paper, which can be obtained from the Linnean Society in a separate form. Now for Mr. Wallace's criticism of this theory, as presented in his recently published work on "Darwinism." Briefly put, he furnishes a numerical calculation, showing that when "the physiological peculiarity is not correlated with any external differences of form or color, or with inherent peculiarities of likes or dislikes leading to any choice as to pairing," even when so large a proportion as ten per cent. of the exceptional variety arises every year in the midst of the species, "it is unable to increase its numbers much above its starting-point, and remains wholly dependent on the continued renewal of the variety for its existence beyond a few years." This, it must be observed, is a reproduction of the criticism which I answered in 1888; but, as Mr. Wallace ignores that answer, I must now repeat it. The criticism does not dispute the fact that the required variation in the way of "selective sterility" occurs. Indeed, Mr. Wallace allows that it certainly must be of very general occurrence as between incipient species (or pronounced varieties in a state of nature), seeing that it is of such general occurrence as between allied species when fully differentiated as such. In other words, this variation in the way of selective sterility must be recognised as a very general _fact_, even if it be not regarded as a _condition_, or a _cause_, of specific differentiation. Which is merely another way of saying that the particular variation which is required by the theory in question is admittedly a variation which does occur; and occurs, moreover, in very frequent association with the origin of a new species. But Mr. Wallace's objection to regarding this variation as itself a cause of (or condition to) the origin of a new species is, as we have seen, that the changes must always be greatly against the similar variations of the opposite sexes meeting—i. e., of the "physiological complements" happening to pair. Now, I have already shown, in the _Nineteenth Century_ of three years ago, that this criticism can only apply to species the sexes of which unite for every birth; but as Mr. Wallace continues to ignore this important consideration, I will now present it in somewhat more detail.