Alfred Russel Wallace : Alfred Wallace : A. R. Wallace :
Russel Wallace : Alfred Russell Wallace (sic)

 
 
The Difficulties of Natural Selection
(S176: 1870)

 
Editor Charles H. Smith's Note: A letter to the Editor printed in the Nature issue of 1 December 1870, responding to comments made about S175. Original pagination indicated within double brackets. To link directly to this page, connect with: http://people.wku.edu/charles.smith/wallace/S176.htm


    [[p. 85]] As Mr. Bennett complains that I have charged him with errors he has not committed (which I should much regret to have done), I must ask permission to justify my statements by a reference to his own words.

    1. Mr. Bennett says that he is unable to discover where he has led his readers to understand that there is only one completely mimicking species of Leptalis. I will therefore show him where he has done so. In the third column of his article (p. 31) he says: "Another South American genus of Lepidoptera, the Leptalis, belongs structurally to an entirely different class, the Pieridæ, and the majority of its species differ correspondingly from the Heliconidæ in their size, shape, colour, and manner of flying, being nearly pure white. There is, however, one particular species of Leptalis, which departs widely in external facies from all its allies, and so closely resembles a species of Ithomia as to deceive," &c. &c. Then comes the argument and the mathematical calculations always referring to "the Leptalis," and it is at the end of this, at the bottom of the next column, that we have the following passage (of which Mr. Bennett in his reply has only quoted a line and a half): "For supposing the chance is reduced from one in ten million to one in ten thousand, and it is said that the world has existed quite long enough to give a fair chance of this having occurred once, it is not a solitary instance that we have. Mr. Bates states that in a comparatively small area several distinct instances of such perfect mimicry occur, Mr. Wallace has a store in the Malay Archipelago, Mr. Trimen records several of wonderful completeness in South Africa," &c. Now, as there is not a word here about other species of Leptalis, but only about other cases of mimicry, as Leptalis is unknown in Africa or the East, as mimicry occurs in other genera and families of Lepidoptera, and other orders of insects, and as Mr. Bennett has himself stated, that the "one particular species of Leptalis departs widely in external facies from all its allies," I think it will be admitted that I was justified in asserting that Mr. Bennett's readers would be "led to understand," that there was only one species of completely mimicking Leptalis. If I was not so justified I confess my ignorance of the English language, and beg Mr. Bennett's pardon.

    2. I leave your readers to judge for themselves whether the fact of a Leptalis having twenty offspring does or does not affect the mathematical argument as set forth by Mr. Bennett; but when, in answer to my statement, that the right variation has, by the hypothesis, a greater chance of surviving than the rest, he asks: "By what hypothesis? The hypothesis that these small variations are useful to the individual, the very hypothesis against which I am contending as unproved,"--I must protest against his denying his own words. For, at p. 31, col. 1, he says: "The next step in my argument is, that the smallest change in the direction of the Ithomia which we can conceive, on any hypothesis, to be beneficial to the Leptalis is, at the very lowest, one-fiftieth of the change required to produce perfect resemblance;" and six lines farther on, "For the sake of argument, however, I will suppose that a change to the extent of one-fiftieth is beneficial," and then comes the calculation. Again, I must acknowledge my ignorance of the meaning of words if Mr. Bennett does not here directly contradict himself. I never said the hypothesis was proved, but only that Mr. Bennett's argument, founded on it, was unsound, and for the sake of the argument he had admitted the hypothesis.

    Mr. Bennett goes on to say: "The new factor, of which I take no account, is, again, entirely dependent on the admission of the natural selectionist premiss." This new factor is the principle of heredity. As he acknowledges that he takes no account of it, we must presume that he denies its existence; and as the whole of Mr. Darwin's theories and my own fall to the ground without it, he might have spared himself the trouble of his "mathematical demonstration."

    3. I do not consider, as Mr. Bennett seems to do, that the distinction between "protective resemblance" and "mimicry" is a subtle one. Anyone who reads his paragraph on this subject (p. 32, col. 2) will, I think, be under the impression, as I was, that he alluded to mimicry, or mimetism, properly so called, as being strongly developed in birds. It seems, however, that he means only protective resemblance; but this, I believe, to be equally common among the very lowest forms of life. Transparency, for example, is a great protection to aquatic animals, and it is very prevalent in low organisms. Fishes are all, or almost all, protectively coloured, by the back being dark and the belly light, so that, whether looked at from above on the dark background, or from below on the light one, they are equally difficult to see. In many fishes, too, we have a specific protective resemblance as perfect as in any birds (see "Contributions to the Theory of Natural Selection," p. 55), and this is as much opposed to Mr. Bennett's theory as the absence of true mimicry in birds and mammals.

    4. Mr. Bennett says, I have "brought no evidence to show that extremely small variations afford any immunity from the attacks of enemies,"--but this was quite unnecessary, because I show that the variations which continually occur in insects are by no means "extremely small." He also says that I "give no explanation of the tendency of the Leptalis, referred to by Mr. Bates, to produce naturally varieties of a nature to resemble Ithomiæ." But Mr. Bates introduces this remark with--"It would seem as if;" and though I think that the fact may be so, [[p. 86]] and that it is not difficult to explain, yet I do not feel bound to explain every supposed fact as if it were a well-established one. As to the "parallelism of the development of protective resemblance and of instinct in the animal world," which I am also asked to explain, I deny that it has been proved to exist.

    In conclusion, I will observe that the theory of Natural Selection, and its subordinate theory, Mimicry--have now been so fully developed by Mr. Darwin, Mr. Bates, Mr. Trimen, and myself, that I conceive it to be a full and sufficient answer to any opponent if we can show that his particular objections are unsound. This, I believe, I have done in the case of Mr. Bennett, although I am sorry to find that he cannot see it, and it is therefore unnecessary to go fully into the collateral points on which he has touched, and which have already been sufficiently explained by Mr. Darwin or myself.


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