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d B。 Wilson； Esq。； Assistant in Biology； John Hopkins University； Baltimore Md； U。 States。〃：
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 Gunther in 〃Proc。 Zoolog。 Socy。〃） 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 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；
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 colour。 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'！ We found the example in the summer of 1880 or 1881 at Beaufort； N。C。； where the Johns Hopkins laboratory was located for the time being。 It must have been seen by many others； before or since。
〃I wrote and sent to Darwin a short description of the case at the suggestion of Brooks； with whom I was at the time a student。 I was； of course； entirely unknown to Darwin （or to anyone else） and to me the principal interest of Darwin's letter is the evidence that it gives of his extraordinary kindness and friendliness towards an obscure youngster who had of course absolutely no claim upon his time or attention。 The little incident made an indelible impression upon my memory and taught me a lesson that was worth learning。〃
VARIABLE PROTECTIVE RESEMBLANCE。
The wonderful power of rapid colour adjustment possessed by the cuttle…fish was observed by Darwin in 1832 at St Jago； Cape de Verd Islands； the first place visited during the voyage of the 〃Beagle〃。 From Rio he wrote to Henslow； giving the following account of his observations； May 18； 1832： 〃I took several specimens of an Octopus which possessed a most marvellous power of changing its colours； equalling any chameleon； and evidently accommodating the changes to the colour of the ground which it passed over。 Yellowish green； dark brown； and red； were the prevailing colours； this fact appears to be new； as far as I can find out。〃 （〃Life and Letters〃； I。 pages 235； 236。 See also Darwin's 〃Journal of Researches〃； 1876； pages 6… 8； where a far more detailed account is given together with a reference to 〃Encycl。 of Anat。 and Physiol。〃）
Darwin was well aware of the power of individual colour adjustment； now known to be possessed by large numbers of lepidopterous pupae and larvae。 An excellent example was brought to his notice by C。V。 Riley （〃More Letters〃 II； pages 385； 386。）； while the most striking of the early results obtained with the pupae of butterfliesthose of Mrs M。E。 Barber upon Papilio nireuswas communicated by him to the Entomological Society of London。 （〃Trans。 Ent。 Soc。 Lond。〃 1874； page 519。 See also 〃More Letters〃； II。 page 403。）
It is also necessary to direct attention to C。W。 Beebe's （〃Zoologica： N。Y。 Zool。 Soc。〃 Vol。 I。 No。 1； Sept。 25； 1907： 〃Geographic variation in birds with especial reference to the effects of humidity〃。） recent discovery that the pigmentation of the plumage of certain birds is increased by confinement in a superhumid atmosphere。 In Scardafella inca； on which the most complete series of experiments was made； the changes took place only at the moults； whether normal and annual or artificially induced at shorter periods。 There was a corresponding increase in the choroidal pigment of the eye。 At a certain advanced stage of feather pigmentation a brilliant iridescent bronze or green tint made its appearance on those areas where iridescence most often occurs in allied genera。 Thus in birds no less than in insects； characters previously regarded as of taxonomic value； can be evoked or withheld by the forces of the environment。
WARNING OR APOSEMATIC COLOURS。
From Darwin's description of the colours and habits it is evident that he observed； in 1833； an excellent example of warning colouring in a little South American toad （Phryniscus nigricans）。 He described it in a letter to Henslow； written from Monte Video； Nov。 24； 1832： 〃As for one little toad； I hope it may be new； that it may be christened 'diabolicus。' Milton must allude to this very individual when he talks of 'squat like a toad'； its colours are by Werner （〃Nomenclature of Colours〃； 1821） ink black； vermilion red and buff orange。〃 （〃More Letters〃； I。 page 12。） In the 〃Journal of Researches〃 （1876； page 97。） its colours are described as follows： 〃If we imagine； first； that it had been steeped in the blackest ink； and then； when dry； allowed to crawl over a board； freshly painted with the brightest vermilion； so as to colour the soles of its feet and parts of its stomach； a good idea of its appearance will be gained。〃 〃Instead of being nocturnal in its habits； as other toads are； and living in damp obscure recesses； it crawls during the heat of the day about the dry sand…hillocks and arid plains；。。。〃 The appearance and habits recall T。 Belt's well…known description of the conspicuous little Nicaraguan frog which he found to be distasteful to a duck。 （〃The Naturalist in Nicaragua〃 （2nd edition） London； 1888； page 321。）
The recognition of the Warning Colours of caterpillars is due in the first instance to Darwin； who； reflecting on Sexual Selection； was puzzled by the splendid colours of sexually immature organisms。 He applied to Wallace 〃who has an innate genius for solving difficulties。〃 （〃Descent of Man〃； page 325。 On this and the following page an excellent account of the discovery will be found； as well as in Wallace's 〃Natural Selection〃； London； 1875； pages 117…122。） Darwin's original letter exists （〃Life and Letters〃； III。 pages 93； 94。）； and in it