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consent and that of Mr Francis Darwin, this letter, written four months 

 before Darwin's death on April 19, 1882, is reproduced here 1 : 



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. y ) 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 Gunther's case makes 

 both of them the more interesting ; considering how far a fish and 

 a mollusc stand apart. It w. d 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 Sargassum 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." 



