My dear Wallace,—­I am very much obliged to you for sending your article, which is very interesting and appears to me as clearly written as it can be.  You will not be surprised that I differ altogether from you about sexual colours.  That the tail of the peacock and his elaborate display of it should be due merely to the vigour, activity, and vitality of the male is to me as utterly incredible as my views are to you.  Mantegazza published a few years ago in Italy a somewhat similar view.  I cannot help doubting about recognition through colour; our horses, dogs, fowls, and pigeons seem to know their own species, however differently the individuals may be coloured.  I wonder whether you attribute the odoriferous and sound-producing organs, when confined to the males, to their greater vigour, etc.?  I could say a good deal in opposition to you, but my arguments would have no weight in your eyes, and I do not intend to write for the public anything on this or any other difficult subject.  By the way, I doubt whether the term voluntary in relation to sexual selection ought to be employed:  when a man is fascinated by a pretty girl it can hardly be called voluntary, and I suppose that female animals are charmed or excited in nearly the same manner by the gaudy males.

Three essays have been published lately in Germany which would interest you:  one by Weismann, who shows that the coloured stripes on the caterpillars of Sphinx are beautifully protective:  and birds were frightened away from their feeding-place by a caterpillar with large eye-like spots on the broad anterior segments of the body.  Fritz Mueller has well discussed the first steps of mimicry with butterflies, and comes to nearly or quite the same conclusion as you, but supports it by additional arguments.

Fritz Mueller also has lately shown that the males alone of certain butterflies have odoriferous glands on their wings (distinct from those which secrete matter disgusting to birds), and where these glands are placed the scales assume a different shape, making little tufts.

Farewell:  I hope that you find Dorking a pleasant place?  I was staying lately at Abinger Hall, and wished to come over to see you, but driving tires me so much that my courage failed.—­Yours very sincerely,

CHAS. DARWIN.

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Madeira Villa, Madeira Road, Ventnor, Isle of Wight.  September 3, 1877.

My dear Darwin,—­Many thanks for your letter.  Of course I did not expect my paper to have any effect on your opinions.  You have looked at all the facts so long from your special point of view that it would require conclusive arguments to influence you, and these, from the complex nature of the question, are probably not to be had.  We must, I think, leave the case in the hands of others, and I am in hopes that my paper may call sufficient attention to the subject to induce some of the great school