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 Burchell 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. woodpecker 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 classification adopted by H.W. Bates in his classical
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 attitudes, 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
Burchell almost exactly a hundred years ago have already been mentioned.
In addition to the resemblance 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.