I can make no answer with respect to the elephants.  With respect to the female reindeer, I have hitherto looked at the horns simply as the consequence of inheritance not having been limited by sex.

Your idea about colour being concentrated in the smaller males seems good, and I presume that you will not object to my giving it as your suggestion.—­Believe me, my dear Wallace, with many thanks, yours very sincerely,

CH.  DARWIN.

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Wallace’s more recent views on the question of Natural Selection and Sterility may be found in a note written by him in 1899:  “When writing my ‘Darwinism’ and coming again to the consideration of the problem of the effect of Natural Selection in accumulating variations in the amount of sterility between varieties or incipient species, twenty years later, I became more convinced than I was when discussing with Darwin, of the substantial accuracy of my argument.  Recently a correspondent who is both a naturalist and a mathematician has pointed out to me a slight error in my calculation at p, 183 (which does not, however, materially affect the result) disproving the physiological selection of the late Dr. Romanes, but he can see no fallacy in my argument as to the power of Natural Selection to increase sterility between incipient species, nor, so far as I am aware, has anyone shown such fallacy to exist.

“On the other points on which I differed from Mr. Darwin in the foregoing discussion—­the effect of high fertility on population of a species, etc.—­I still hold the views I then expressed, but it would be out of place to attempt to justify them here.”—­A.R.W.

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9 St. Mark’s Crescent, N.W.  August 16, [1868?].

Dear Darwin,—­I ought to have written before to thank you for the copies of your paper on “Primula” and on “Cross Unions of Dimorphic Plants, etc.”  The latter is particularly interesting, and the conclusion most important; but I think it makes the difficulty of how these forms, with their varying degrees of sterility, originated, greater than ever.  If Natural Selection could not accumulate varying degrees of sterility for the plant’s benefit, then how did sterility ever come to be associated with one cross of a trimorphic plant rather than another?  The difficulty seems to be increased by the consideration that the advantage of a cross with a distinct individual is gained just as well by illegitimate as by legitimate unions.  By what means, then, did illegitimate unions ever become sterile?  It would seem a far simpler way for each plant’s pollen to have acquired a prepotency on another individual’s stigma over that of the same individual, without the extraordinary complication of three differences of structure and eighteen different unions with varying degrees of sterility!

However, the fact remains an excellent answer to the statement that sterility of hybrids proves the absolute distinctness of the parents.