Darwin and Modern Science Part 23

I have preferred to show the influence of the older teleology upon Natural History by quotations from a single great and insufficiently appreciated naturalist. It might have been seen equally well in the pages of Kirby and Spence and those of many other writers. If the older naturalists who thought and spoke with Burch.e.l.l of "the intention of Nature" and the adaptation of beings "to each other, and to the situations in which they are found," could have conceived the possibility of evolution, they must have been led, as Darwin was, by the same considerations to Natural Selection. This was impossible for them, because the philosophy which they followed contemplated the phenomena of adaptation as part of a static immutable system. Darwin, convinced that the system is dynamic and mutable, was prevented by these very phenomena from accepting anything short of the crowning interpretation offered by Natural Selection. ("I had always been much struck by such adaptations (e.g. woodp.e.c.k.e.r and tree-frog for climbing, seeds for dispersal), and until these could be explained it seemed to me almost useless to endeavour to prove by indirect evidence that species have been modified." "Autobiography" in "Life and Letters of Charles Darwin", Vol.

I. page 82. The same thought is repeated again and again in Darwin's letters to his friends. It is forcibly urged in the Introduction to the "Origin" (1859), page 3.) And the birth of Darwin's unalterable conviction that adaptation is of dominant importance in the organic world,--a conviction confirmed and ever again confirmed by his experience as a naturalist--may probably be traced to the influence of the great theologian. Thus Darwin, speaking of his Undergraduate days, tells us in his "Autobiography" that the logic of Paley's "Evidences of Christianity" and "Moral Philosophy" gave him as much delight as did Euclid.

"The careful study of these works, without attempting to learn any part by rote, was the only part of the academical course which, as I then felt and as I still believe, was of the least use to me in the education of my mind. I did not at that time trouble myself about Paley's premises; and taking these on trust, I was charmed and convinced by the long line of argumentation." ("Life and Letters", I. page 47.)

When Darwin came to write the "Origin" he quoted in relation to Natural Selection one of Paley's conclusions. "No organ will be formed, as Paley has remarked, for the purpose of causing pain or for doing an injury to its possessor." ("Origin of Species" (1st edition) 1859, page 201.)

The study of adaptation always had for Darwin, as it has for many, a peculiar charm. His words, written Nov. 28, 1880, to Sir W.

Thiselton-Dyer, are by no means inapplicable to-day: "Many of the Germans are very contemptuous about making out use of organs; but they may sneer the souls out of their bodies, and I for one shall think it the most interesting part of natural history." ("More Letters" II. page 428.)

PROTECTIVE AND AGGRESSIVE RESEMBLANCE: PROCRYPTIC AND ANTICRYPTIC COLOURING.

Colouring for the purpose of concealment is sometimes included under the head Mimicry, a cla.s.sification adopted by H.W. Bates in his cla.s.sical paper. Such an arrangement is inconvenient, and I have followed Wallace in keeping the two categories distinct.

The visible colours of animals are far more commonly adapted for Protective Resemblance than for any other purpose. The concealment of animals by their colours, shapes and att.i.tudes, must have been well known from the period at which human beings first began to take an intelligent interest in Nature. An interesting early record is that of Samuel Felton, who (Dec. 2, 1763) figured and gave some account of an Acridian (Phyllotettix) from Jamaica. Of this insect he says "THE THORAX is like a leaf that is raised perpendicularly from the body." ("Phil.

Trans. Roy. Soc." Vol. LIV. Tab. VI. page 55.)

Both Protective and Aggressive Resemblances were appreciated and clearly explained by Erasmus Darwin in 1794: "The colours of many animals seem adapted to their purposes of concealing themselves either to avoid danger, or to spring upon their prey." ("Zoonomia", Vol. I. page 509, London, 1794.)

Protective Resemblance of a very marked and beautiful kind is found in certain plants, inhabitants of desert areas. Examples observed by Burch.e.l.l almost exactly a hundred years ago have already been mentioned.

In addition to the resemblance to stones Burch.e.l.l 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 ma.n.u.script journals kept by the great explorer during his journey. I owe the opportunity of studying it to the kindness of Mr Francis A. Burch.e.l.l of the Rhodes University College, Grahamstown. The following account is given under the date July 5, 1812, when Burch.e.l.l 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 Cra.s.sula (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 pa.s.serine 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 a.s.signed by Darwin to s.e.xual 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 s.e.xual 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 s.p.a.ce 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 pa.s.sage 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 acc.u.mulated 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 cla.s.ses, 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 sh.e.l.ls 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 ch.o.r.edon, 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 coc.o.o.n (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., a.s.sistant 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 circ.u.mstances 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 Sarga.s.sum and shows a really astonis.h.i.+ng 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 Sarga.s.sum that had been in a gla.s.s 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 Sarga.s.sum 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 princ.i.p.al 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 pa.s.sed 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 b.u.t.terflies--those of Mrs M.E.

Barber upon Papilio nireus--was 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 s.e.xual Selection, was puzzled by the splendid colours of s.e.xually 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 we are told that he had taken the advice given by Bates: "You had better ask Wallace." After some consideration Wallace replied that he believed the colours of conspicuous caterpillars and perfect insects were a warning of distastefulness and that such forms would be refused by birds.

Darwin's reply ("Life and Letters", III. pages 94, 95.) is extremely interesting both for its enthusiasm at the brilliancy of the hypothesis and its caution in acceptance without full confirmation:

"Bates was quite right; you are the man to apply to in a difficulty. I never heard anything more ingenious than your suggestion, and I hope you may be able to prove it true. That is a splendid fact about the white moths (A single white moth which was rejected by young turkeys, while other moths were greedily devoured: "Natural Selection", 1875, page 78.); it warms one's very blood to see a theory thus almost proved to be true."

Two years later the hypothesis was proved to hold for caterpillars of many kinds by J. Jenner Weir and A.G. Butler, whose observations have since been abundantly confirmed by many naturalists. Darwin wrote to Weir, May 13, 1869: "Your verification of Wallace's suggestion seems to me to amount to quite a discovery." ("More Letters", II. page 71 (footnote).)

RECOGNITION OR EPISEMATIC CHARACTERS.

This principle does not appear to have been in any way foreseen by Darwin, although he draws special attention to several elements of pattern which would now be interpreted by many naturalists as epismes.

He believed that the markings in question interfered with the cryptic effect, and came to the conclusion that, even when common to both s.e.xes, they "are the result of s.e.xual selection primarily applied to the male."

("Descent of Man", page 544.) The most familiar of all recognition characters was carefully explained by him, although here too explained as an ornamental feature now equally transmitted to both s.e.xes: "The hare on her form is a familiar instance of concealment through colour; yet this principle partly fails in a closely-allied species, the rabbit, for when running to its burrow, it is made conspicuous to the sportsman, and no doubt to all beasts of prey, by its upturned white tail."

("Descent of Man", page 542.)

The a.n.a.logous episematic use of the bright colours of flowers to attract insects for effecting cross-fertilisation and of fruits to attract vertebrates for effecting dispersal is very clearly explained in the "Origin". (Edition 1872, page 161. For a good example of Darwin's caution in dealing with exceptions see the allusion to brightly coloured fruit in "More Letters", II. page 348.)

It is not, at this point, necessary to treat sematic characters at any greater length. They will form the subject of a large part of the following section, where the models of Batesian (Pseudaposematic) mimicry are considered as well as the Mullerian (Synaposematic) combinations of Warning Colours.