Protective Resemblance 279 
consent and that of Mr Francis Darwin, this letter, written four months 
before Darwin’s death on April 19, 1882, is reproduced here?: 
December 21, 1881. 
Dear Sir, 
I thank you much for having taken so much trouble in 
describing fully your interesting and curious case of mimickry. 
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 Giinther 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 Giinther’s case makes 
both of them the more interesting ; considering how far a fish and 
a mollusc stand apart. It w.’ 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, 
CHARLES DARWIN. 
Professor E. B. Wilson has kindly given the following account of 
the circumstances under which he had written to Darwin: “The case 
to which Darwin’s letter refers is that of the nudibranch mollusc 
Scyllaea, which lives on the floating Sargasswm and shows a really 
astonishing resemblance to the plant, having leaf-shaped processes 
very closely similar to the fronds of the sea-weed both in shape and 
in color. The concealment of the animal may be judged from the 
fact that we found the animal quite by accident on a piece of 
Sargassum that had been in a glass jar in the laboratory for some 
time and had been closely examined in the search for hydroids and 
the like without disclosing the presence upon it of two large specimens 
of the Scyllaea (the animal, as I recall it, is about two inches long). 
It was first detected by its movements alone, by someone (I think a 
casual visitor to the laboratory) who was looking closely at the 
Sargassum and exclaimed ‘Why, the sea-weed is moving its leaves’ ! 
1 The letter is addressed : 
“Edmund B. Wilson, Esq., Assistant in Biology, John Hopkins University, Baltimore 
Md., U. States.” 
