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Soon after I returned home, in the summer of 1862, Mr. Darwin invited me to come to Down for a night, where I had the great pleasure of seeing him in his quiet home, and in the midst of his family. A year or two later I spent a week-end with him in company with Bates, Jenner Weir, and a few other naturalists; but my most frequent interviews with him were when he spent a few weeks with his brother, Dr. Erasmus Darwin, in Queen Anne Street, which he usually did every year when he was well enough, in order to see his friends and collect information for his various works. On these occasions I usually lunched with him and his brother, and sometimes one other visitor, and had a little talk on some of the matters specially interesting him. He also sometimes called on me in St. Mark's Crescent for a quiet talk or to see some of my collections.

My first letter from him dealing with scientific matters was in August, 1862, and our correspondence was very extensive during the period occupied in writing or correcting his earlier books on evolution, down to the publication of “The Expression of the Emotions in Man and Animals,” in 1872, and afterwards, at longer intervals, to within less than a year of his death. A considerable selection of our correspondence has been published in the “ Life and Letters ” (1887), and especially in "More Letters ” (1903); while several of the more interesting of these were contained in the one-volume life, entitled, “ Charles Darwin,” which appeared in 1892. As many of my readers, however, may not have these works to refer to, I will here give a few of his letters to myself which have not yet been published, together with some of my own, and also occasional extracts from some of Darwin's that have already appeared, in order to make clear the nature of our discussions, and also, perhaps, to throw a little light upon our respective characters.

In a letter entirely without date, but which was evidently written in 1863, he gives me some information for which I had asked about reviews of the “ Origin of Species.”

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“Down, Bromley, Kent (1863). “ MY DEAR MR. WALLACE,

"I write one line to thank you for your note, and to say that the B. of Oxford wrote the Quarterly R. (paid £60), aided by Owen. In the Edinburgh, Owen no doubt praised himself. Mr. Maw's review in Zoologist is one of the best, and staggered me in parts, for I did not see the sophistry of (those) parts. I could lend you any which you might wish to see, but you would soon be tired. Hopkins in Fraser and Pictet are two of the best.

“I am glad you like the little orchid book; but it has not been worth the ten months it has cost me; it was a hobby horse, and so beguiled me.

“How puzzled you must be to know what to begin at! You will do grand work, I do not doubt. My health is, and always will be, very poor; I am that miserable animal, a regular valetudinarian.

"Yours very sincerely,


In March, 1864, he wrote me from Malvern Wells that he had been very ill at home, having fits of vomiting every day for two months, and been able to do nothing. These attacks were brought on by the least mental excitement, which often

rendered it impossible for him to see his friends, and which appear to have lasted at intervals throughout his life. This must always be remembered when we consider the enormous amount of work he was able to do; but, unfortunately, the quiet interest of carrying out observations or experiments lasting for months, and often for years, seem to have been beneficial. On the other hand, writing his books and correcting the MSS. and the proofs in the very careful manner he always practised was most wearying and distasteful to him.

On February 23, 1867, he wrote to me asking if I could solve a difficulty for him. He says: “On Monday evening I called on Bates, and put a difficulty before him which he could not answer, and, as on some similar occasion, his first suggestion was, 'You had better ask Wallace.' My difficulty is, Why are caterpillars sometimes so beautifully and artistically coloured? Seeing that many are coloured to escape dangers, I can hardly attribute their bright colour in other cases to mere physical conditions. Bates says the most gaudy caterpillar he ever saw in Amazonia was conspicuous at the distance of yards, from its black and red colours, whilst feeding on large, green leaves. If anyone objected to male butterflies having been made beautiful by sexual selection, and asked why they should not have been made beautiful as well as their caterpillars, what would you answer? I could not answer, but should maintain my ground. Will you think over this, and some time, either by letter or when we meet, tell me what you think?"


On reading this letter, I almost at once saw what seemed to be a very easy and probable explanation of the facts. I had then just been preparing for publication (in the Westminster Review) my rather elaborate paper on “Mimicry and Protective Colouring,” and the numerous cases in which specially showy and slow-flying butterflies were known to have a peculiar odour and taste which protected them from the attacks of insect-eating birds and other animals, led me at once to suppose that the gaudily-coloured caterpillars must have a similar protection. I had just ascertained from Mr. Jenner Weir that one of our common white moths (Spilosoma menthrastri) would not be eaten by most of the small birds in his aviary, nor by young turkeys. Now, as a white moth is as conspicuous in the dusk as a coloured caterpillar in the daylight, this case seemed to me so much on a par with the other that I felt almost sure my explanation would turn out correct. I at once wrote to Mr. Darwin to this effect, and his reply, dated February 26, is as follows:


“ Bates was quite right; you are the man to apply to in a difficulty. I never heard anything more ingenious than your suggestion, and I hope you may be able to prove it true. That is a splendid fact about the white moths; it warms one's very blood to see a theory thus almost proved to be true.”

The following week I brought the subject to the notice of the Fellows of the Entomological Society at their evening meeting (March 4), requesting that any of them who had the opportunity would make observations or experiments during the summer in accordance with Mr. Darwin's suggestion. I also wrote a letter to The Field newspaper, which, as it explains my hypothesis in simple language, I here give entire :



“May I be permitted to ask the co-operation of your readers in making some observations during the coming spring and summer which are of great interest to Mr. Darwin and myself? I will first state what observations are wanted and then explain briefly why they are wanted. A number of our smaller birds devour quantities of caterpillars, but there is reason to suspect that they do not eat all alike. Now we want direct evidence as to which species they eat and which they reject. This may be obtained in two ways. Those who keep insectivorous birds, such as thrushes, robins, or any of the warblers (or any other that will eat caterpillars), may

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offer them all the kinds they can obtain, and carefully note (1) which they eat, (2) which they refuse to touch, and (3) which they seize but reject. If the name of the caterpillar cannot be ascertained, a short description of its more prominent characters will do very well, such as whether it is hairy or smooth, and what are its chief colours, especially distinguishing such as are green or brown from such as are of bright and conspicuous colours, as yellow, red, or black. The food plant of the caterpillar should also be stated when known. Those who do not keep birds, but have a garden much frequented by birds, may put all the caterpillars they can find in a soup plate or other vessel, which must be placed in a larger vessel of water, so that the creatures cannot escape, and then after a few hours note which have been taken and which left. If the vessel could be placed where it might be watched from a window, so that the kind of birds which took them could also be noted, the experiment would be still more complete. A third set of observations might be made on young fowls, turkeys, guinea-fowls, pheasants, etc., in exactly the same manner.

Now the purport of these observations is to ascertain the law which had determined the coloration of caterpillars. The analogy of many other insects leads us to believe that all those which are green or brown, or of such speckled or mottled tints as to resemble closely the leaf or bark of the plant on which they feed, or the substance on which they usually repose, are thus to some degree protected from the attacks of birds and other enemies. We should expect, therefore, that all which are thus protected would be greedily eaten by birds whenever they can find them. But there are other caterpillars which seem coloured on purpose to be conspicuous, and it is very important to know whether they have another kind of protection, altogether independent of disguise, such as a disagreeable odour and taste. If they are thus protected, so that the majority of birds will never eat them, we can understand that to get the full benefit of this protection they should be easily recognized, should have some outward character by which birds would soon learn to know them and thus let them

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