blance to stones Burchell observed； although he did not publish the fact； a South African plant concealed by its likeness to the dung of birds。 （Sir William Thiselton…Dyer has suggested the same method of concealment （〃Annals of Botany〃； Vol。 XX。 page 123）。 Referring to Anacampseros papyracea； figured on plate IX。； the author says of its adaptive resemblance： 〃At the risk of suggesting one perhaps somewhat far… fetched； I must confess that the aspect of the plant always calls to my mind the dejecta of some bird； and the more so owing to the whitening of the branches towards the tips〃 （loc。 cit。 page 126）。 The student of insects； who is so familiar with this very form of protective resemblance in larvae； and even perfect insects； will not be inclined to consider the suggestion far…fetched。） The observation is recorded in one of the manuscript journals kept by the great explorer during his journey。 I owe the opportunity of studying it to the kindness of Mr Francis A。 Burchell of the Rhodes University College； Grahamstown。 The following account is given under the date July 5； 1812； when Burchell was at the Makkwarin River； about half…way between the Kuruman River and Litakun the old capital of the Bachapins （Bechuanas）： 〃I found a curious little Crassula （not in flower） so snow white； that I should never has （have） distinguished it from the white limestones。。。It was an inch high and a little branchy；。。。and was at first mistaken for the dung of birds of the passerine order。 I have often had occasion to remark that in stony place（s） there grow many small succulent plants and abound insects （chiefly Grylli） which have exactly the same colour as the ground and must for ever escape observation unless a person sit on the ground and observe very attentively。〃
The cryptic resemblances of animals impressed Darwin and Wallace in very different degrees； probably in part due to the fact that Wallace's tropical experiences were so largely derived from the insect world； in part to the importance assigned by Darwin to Sexual Selection 〃a subject which had always greatly interested me；〃 as he says in his 〃Autobiography〃； （〃Life and Letters〃； Vol。 I。 page 94。） There is no reference to Cryptic Resemblance in Darwin's section of the Joint Essay； although he gives an excellent short account of Sexual Selection （see page 295）。 Wallace's section on the other hand contains the following statement： 〃Even the peculiar colours of many animals； especially insects； so closely resembling the soil or the leaves or the trunks on which they habitually reside； are explained on the same principle； for though in the course of ages varieties of many tints may have occurred； YET THOSE RACES HAVING COLOURS BEST ADAPTED TO CONCEALMENT FROM THEIR ENEMIES WOULD INEVITABLY SURVIVE THE LONGEST。〃 （〃Journ。 Proc。 Linn。 Soc。〃 Vol。 III。 1859； page 61。 The italics are Wallace's。）
It would occupy too much space to attempt any discussion of the difference between the views of these two naturalists； but it is clear that Darwin； although fully believing in the efficiency of protective resemblance and replying to St George Mivart's contention that Natural Selection was incompetent to produce it （〃Origin〃 （6th edition） London； 1872； pages 181； 182； see also page 66。）； never entirely agreed with Wallace's estimate of its importance。 Thus the following extract from a letter to Sir Joseph Hooker； May 21； 1868； refers to Wallace： 〃I find I must （and I always distrust myself when I differ from him） separate rather widely from him all about birds' nests and protection； he is riding that hobby to death。〃 （〃More Letters〃； I。 page 304。） It is clear from the account given in 〃The Descent of Man〃； （London； 1874； pages 452…458。 See also 〃Life and Letters〃； III。 pages 123…125； and 〃More Letters〃； II。 pages 59…63； 72…74； 76…78； 84…90； 92； 93。）； that the divergence was due to the fact that Darwin ascribed more importance to Sexual Selection than did Wallace； and Wallace more importance to Protective Resemblance than Darwin。 Thus Darwin wrote to Wallace； Oct。 12 and 13； 1867： 〃By the way； I cannot but think that you push protection too far in some cases； as with the stripes on the tiger。〃 （〃More Letters〃； I。 page 283。） Here too Darwin was preferring the explanation offered by Sexual Selection （〃Descent of Man〃 （2nd edition） 1874； pages 545； 546。）； a preference which； considering the relation of the colouring of the lion and tiger to their respective environments； few naturalists will be found to share。 It is also shown that Darwin contemplated the possibility of cryptic colours such as those of Patagonian animals being due to sexual selection influenced by the aspect of surrounding nature。
Nearly a year later Darwin in his letter of May 5； 1868？； expressed his agreement with Wallace's views： 〃Expect that I should put sexual selection as an equal； or perhaps as even a more important agent in giving colour than Natural Selection for protection。〃 （〃More Letters〃； II。 pages 77； 78。） The conclusion expressed in the above quoted passage is opposed by the extraordinary development of Protective Resemblance in the immature stages of animals； especially insects。
It must not be supposed； however； that Darwin ascribed an unimportant role to Cryptic Resemblances； and as observations accumulated he came to recognise their efficiency in fresh groups of the animal kingdom。 Thus he wrote to Wallace； May 5； 1867： 〃Haeckel has recently well shown that the transparency and absence of colour in the lower oceanic animals； belonging to the most different classes； may be well accounted for on the principle of protection。〃 （〃More Letters〃； II。 page 62。 See also 〃Descent of Man〃； page 261。） Darwin also admitted the justice of Professor E。S。 Morse's contention that the shells of molluscs are often adaptively coloured。 （〃More Letters〃； II。 page 95。） But he looked upon cryptic colouring and also mimicry as more especially Wallace's departments； and sent to him and to Professor Meldola observations and notes bearing upon these subjects。 Thus the following letter given to me by Dr A。R。 Wallace and now； by kind permission； published for the first time； accompanied a photograph of the chrysalis of Papilio sarpedon choredon； Feld。； suspended from a leaf of its food…plant：
July 9th； Down； Beckenham； Kent。
My Dear Wallace；
Dr G。 Krefft has sent me the enclosed from Sydney。 A nurseryman saw a caterpillar feeding on a plant and covered the whole up； but when he searched for the cocoon （pupa）； was long before he could find it； so good was its imitation in colour and form to the leaf to which it was attached。 I hope that the world goes well with you。 Do not trouble yourself by acknowledging this。
Ever yours
Ch。 Darwin。
Another deeply interesting letter of Darwin's bearing upon protective resemblance； has only recently been shown to me by my friend Professor E。B。 Wilson； the great American Cytologist。 With his kind consent and that of Mr Francis Darwin； this letter； written four months before Darwin's death on April 19； 1882； is reproduced here （The letter is addressed： 〃Edmund 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 vi