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CHARLES DARWIN ON ANIMAL MIMICRY.

THE death of Charles Darwin has awakened so much interest in his personal character, as well as in his scientific attainments, that the following letter which he recently addressed to one of the workers in zoology in this university,¹ is here printed as an excellent illustration of the qualities which were characteristic of the departed naturalist.

The letter was written in reply to a description, sent Mr. Darwin, of a mollusk which closely resembled in colour and form the seaweed upon which it habitually lives.² This sea-weed is the well known Sargassum, or Gulf-weed, which is found floating at the surface of the sea, sometimes in immense quantity. It has long, narrow, leaflike fronds of a peculiar form, with serrated margins, and is of an olive or reddish brown color, with darker blotches and spots.

The mollusk, which is somewhat like a common garden slug, is provided with three pairs of appendages which exactly resemble in form and color the fronds of the Sargassum, The body is also colored like the sea-weed, and the whole animal is so similar in appearance to the plant on which it lives as almost to defy detection.

This resemblance is an instance of what is called "protective mimicry," many cases of which have been noted in other groups of animals. The mimicry is "protective" because the organism which "mimics" the other is concealed from view, and thus protected from the attacks of enemies. Thus in the present case the mollusk is not distinguished from the plant by fish or other predatory animals, is therefore passed by, and escapes destruction. Some idea of the perfection of the resemblance may be gained from the fact that the mollusks escaped discovery for several hours during which the Sargassum was kept in a large, clear glass jar in the laboratory, and examined several times in the presence of other animals.

The phenomena of mimicry are of interest on account of their clear illustration of the action of natural selection. Obviously there must be a tendency towards the survival of those individuals which are most effectually hidden—that is, those which most nearly resemble the objects with which they are constantly associated—while other individuals will tend to be weeded out. From this tendency has very commonly resulted general resemblances between organisms and their environments, such as the prevailing white color of arctic animals or the tawny colors of desert animals; and now and then have arisen resemblances so special and detailed as to merit the name of mimickry.

Down, Beckenham, Kent,

Dec. 21, 1881.

Dear Sir,—I thank you much for having taken so much trouble in describing fully your interesting and curious case of mimicry. I am in the habit of looking through many scientific journals, and though my memory is now not nearly so good as it was, I feel pretty sure that no such case as yours has been described amongst the nudibranch molluscs.

You perhaps know the case of a fish allied to Hippocampus (described some years ago by Dr. Günther in Proc. Zoolog. Soc.) which clings by its tail to sea-weeds, and is covered with waving filaments so as itself to look like a piece of the same sea-weed. The parallelism between your and Dr. Günther's case makes both of them the more interesting, considering how far a fish and mollusc stand apart. It would be difficult for anyone to explain such cases by the direct action of the environment.

I am glad that you intend to make further observations on this mollusc, and I hope that you will give a figure, and if possible a coloured figure.

With all good wishes from an old brother naturalist, I remain, dear sir, yours faithfully, (Signed), CHARLES DARWIN.

To E. B. Wilson.

John Hopkins University Circulars, August, 1882.

¹ Edmund Beecher Wilson (1856-1939), American embryologist and cytologist. This letter appeared, with additional contextual information in Seward 1909 and Poulton 1909.

² See Calendar 13530 and Correspondence vol. 29.