DARWIN AND CRYPTIC COLOURS 107 



December 21, 1881. DOWN, 



BECKENHAM, KENT. 

 (Railway Station, 

 Orpington, S.E.R.) 



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. Gttnther in Proc. Zooilog. 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. Gunther's case makes 

 both of them the more interesting ; considering how far 

 a fish and a mollusc stand apart. It w 1 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 Sci/llaca, which lives on the floating 

 Sargassum and shows a really astonishing resemblance to 

 the plant, having leaf-shaped processes very closely similar 



