Darwin and Modern Science
by
A.C. Seward
Part 7 out of 14
the eggs are fertilised in the body and begin to develop in the uterus.
Since there is room only for a few larvae in the uterus, a large number of
eggs perish and this number is the greater the longer the period of
gestation. It thus happens that when the animals retain their eggs a long
time, very few young ones are born; and these are in a rather advanced
stage of development, owing to the long time which elapsed since they were
fertilised. When the animal lays its eggs comparatively soon after
copulation, many eggs (from 12 to 72) are produced and the larvae are of
course in an early stage of development. In the early stage the larvae
possess gills and can therefore live in water, while in later stages they
have no gills and breathe through their lungs. Kammerer showed that both
forms of Salamandra can be induced to lay their eggs early or late,
according to the physical conditions surrounding them. If they are kept in
water or in proximity to water and in a moist atmosphere they have a
tendency to lay their eggs earlier and a comparatively high temperature
enhances the tendency to shorten the period of gestation. If the
salamanders are kept in comparative dryness they show a tendency to lay
their eggs rather late and a low temperature enhances this tendency.
Since Salamandra atra is found in rather dry alpine regions with a
relatively low temperature and Salamandra maculosa in lower regions with
plenty of water and a higher temperature, the fact that S. atra bears young
which are already developed and beyond the stage of aquatic life, while S.
maculosa bears young ones in an earlier stage, has been termed adaptation.
Kammerer's experiments, however, show that we are dealing with the direct
effects of definite outside forces. While we may speak of adaptation when
all or some of the variables which determine a reaction are unknown, it is
obviously in the interest of further scientific progress to connect cause
and effect directly whenever our knowledge allows us to do so.
VII. CONCLUDING REMARKS.
The discovery of De Vries, that new species may arise by mutation and the
wide if not universal applicability of Mendel's Law to phenomena of
heredity, as shown especially by Bateson and his pupils, must, for the time
being, if not permanently, serve as a basis for theories of evolution.
These discoveries place before the experimental biologist the definite task
of producing mutations by physico-chemical means. It is true that certain
authors claim to have succeeded in this, but the writer wishes to apologise
to these authors for his inability to convince himself of the validity of
their claims at the present moment. He thinks that only continued breeding
of these apparent mutants through several generations can afford convincing
evidence that we are here dealing with mutants rather than with merely
pathological variations.
What was said in regard to the production of new species by physico-
chemical means may be repeated with still more justification in regard to
the second problem of transformation, namely the making of living from
inanimate matter. The purely morphological imitations of bacteria or cells
which physicists have now and then proclaimed as artificially produced
living beings; or the plays on words by which, e.g. the regeneration of
broken crystals and the regeneration of lost limbs by a crustacean were
declared identical, will not appeal to the biologist. We know that growth
and development in animals and plants are determined by definite although
complicated series of catenary chemical reactions, which result in the
synthesis of a DEFINITE compound or group of compounds, namely, NUCLEINS.
The nucleins have the peculiarity of acting as ferments or enzymes for
their own synthesis. Thus a given type of nucleus will continue to
synthesise other nuclein of its own kind. This determines the continuity
of a species; since each species has, probably, its own specific nuclein or
nuclear material. But it also shows us that whoever claims to have
succeeded in making living matter from inanimate will have to prove that he
has succeeded in producing nuclein material which acts as a ferment for its
own synthesis and thus reproduces itself. Nobody has thus far succeeded in
this, although nothing warrants us in taking it for granted that this task
is beyond the power of science.
XV. THE VALUE OF COLOUR IN THE STRUGGLE FOR LIFE.
By E.B. POULTON.
Hope Professor of Zoology in the University of Oxford.
INTRODUCTION.
The following pages have been written almost entirely from the historical
stand-point. Their principal object has been to give some account of the
impressions produced on the mind of Darwin and his great compeer Wallace by
various difficult problems suggested by the colours of living nature. In
order to render the brief summary of Darwin's thoughts and opinions on the
subject in any way complete, it was found necessary to say again much that
has often been said before. No attempt has been made to display as a whole
the vast contribution of Wallace; but certain of its features are
incidentally revealed in passages quoted from Darwin's letters. It is
assumed that the reader is familiar with the well-known theories of
Protective Resemblance, Warning Colours, and Mimicry both Batesian and
Mullerian. It would have been superfluous to explain these on the present
occasion; for a far more detailed account than could have been attempted in
these pages has recently appeared. (Poulton, "Essays on Evolution" Oxford,
1908, pages 293-382.) Among the older records I have made a point of
bringing together the principal observations scattered through the note-
books and collections of W.J. Burchell. These have never hitherto found a
place in any memoir dealing with the significance of the colours of
animals.
INCIDENTAL COLOURS.
Darwin fully recognised that the colours of living beings are not
necessarily of value as colours, but that they may be an incidental result
of chemical or physical structure. Thus he wrote to T. Meehan, Oct. 9,
1874: "I am glad that you are attending to the colours of dioecious
flowers; but it is well to remember that their colours may be as
unimportant to them as those of a gall, or, indeed, as the colour of an
amethyst or ruby is to these gems." ("More Letters of Charles Darwin",
Vol. I. pages 354, 355. See also the admirable account of incidental
colours in "Descent of Man" (2nd edition), 1874, pages 261, 262.)
Incidental colours remain as available assets of the organism ready to be
turned to account by natural selection. It is a probable speculation that
all pigmentary colours were originally incidental; but now and for immense
periods of time the visible tints of animals have been modified and
arranged so as to assist in the struggle with other organisms or in
courtship. The dominant colouring of plants, on the other hand, is an
essential element in the paramount physiological activity of chlorophyll.
In exceptional instances, however, the shapes and visible colours of plants
may be modified in order to promote concealment.
TELEOLOGY AND ADAPTATION.
In the department of Biology which forms the subject of this essay, the
adaptation of means to an end is probably more evident than in any other;
and it is therefore of interest to compare, in a brief introductory
section, the older with the newer teleological views.
The distinctive feature of Natural Selection as contrasted with other
attempts to explain the process of Evolution is the part played by the
struggle for existence. All naturalists in all ages must have known
something of the operations of "Nature red in tooth and claw"; but it was
left for this great theory to suggest that vast extermination is a
necessary condition of progress, and even of maintaining the ground already
gained.
Realising that fitness is the outcome of this fierce struggle, thus turned
to account for the first time, we are sometimes led to associate the
recognition of adaptation itself too exclusively with Natural Selection.
Adaptation had been studied with the warmest enthusiasm nearly forty years
before this great theory was given to the scientific world, and it is
difficult now to realise the impetus which the works of Paley gave to the
study of Natural History. That they did inspire the naturalists of the
early part of the last century is clearly shown in the following passages.
In the year 1824 the Ashmolean Museum at Oxford was intrusted to the care
of J.S. Duncan of New College. He was succeeded in this office by his
brother, P.B. Duncan, of the same College, author of a History of the
Museum, which shows very clearly the influence of Paley upon the study of
nature, and the dominant position given to his teachings: "Happily at this
time (1824) a taste for the study of natural history had been excited in
the University by Dr Paley's very interesting work on Natural Theology, and
the very popular lectures of Dr Kidd on Comparative Anatomy, and Dr
Buckland on Geology." In the arrangement of the contents of the Museum the
illustration of Paley's work was given the foremost place by J.S. Duncan:
"The first division proposes to familiarize the eye to those relations of
all natural objects which form the basis of argument in Dr Paley's Natural
Theology; to induce a mental habit of associating the view of natural
phenomena with the conviction that they are the media of Divine
manifestation; and by such association to give proper dignity to every
branch of natural science." ((From "History and Arrangement of the
Ashmolean Museum" by P.B. Duncan: see pages vi, vii of "A Catalogue of the
Ashmolean Museum", Oxford, 1836.)
The great naturalist, W.J. Burchell, in his classical work shows the same
recognition of adaptation in nature at a still earlier date. Upon the
subject of collections he wrote ("Travels in the Interior of Southern
Africa", London, Vol. I. 1822, page 505. The references to Burchell's
observations in the present essay are adapted from the author's article in
"Report of the British and South African Associations", 1905, Vol. III.
pages 57-110.): "It must not be supposed that these charms (the pleasures
of Nature) are produced by the mere discovery of new objects: it is the
harmony with which they have been adapted by the Creator to each other, and
to the situations in which they are found, which delights the observer in
countries where Art has not yet introduced her discords." The remainder of
the passage is so admirable that I venture to quote it: "To him who is
satisfied with amassing collections of curious objects, simply for the
pleasure of possessing them, such objects can afford, at best, but a
childish gratification, faint and fleeting; while he who extends his view
beyond the narrow field of nomenclature, beholds a boundless expanse, the
exploring of which is worthy of the philosopher, and of the best talents of
a reasonable being."
On September 14, 1811, Burchell was at Zand Valley (Vlei), or Sand Pool, a
few miles south-west of the site of Prieska, on the Orange River. Here he
found a Mesembryanthemum (M. turbiniforme, now M. truncatum) and also a
"Gryllus" (Acridian), closely resembling the pebbles with which their
locality was strewn. He says of both of these, "The intention of Nature,
in these instances, seems to have been the same as when she gave to the
Chameleon the power of accommodating its color, in a certain degree, to
that of the object nearest to it, in order to compensate for the deficiency
of its locomotive powers. By their form and colour, this insect may pass
unobserved by those birds, which otherwise would soon extirpate a species
so little able to elude its pursuers, and this juicy little
Mesembryanthemum may generally escape the notice of cattle and wild
animals." (Loc. cit. pages 310, 311. See Sir William Thiselton-Dyer
"Morphological Notes", XI.; "Protective Adaptations", I.; "Annals of
Botany", Vol. XX. page 124. In plates VII., VIII. and IX. accompanying
this article the author represents the species observed by Burchell,
together with others in which analogous adaptations exist. He writes:
"Burchell was clearly on the track on which Darwin reached the goal. But
the time had not come for emancipation from the old teleology. This,
however, in no respect detracts from the merit or value of his work. For,
as Huxley has pointed out ("Life and Letters of Thomas Henry Huxley",
London, 1900, I. page 457), the facts of the old teleology are immediately
transferable to Darwinism, which simply supplies them with a natural in
place of a supernatural explanation.") Burchell here seems to miss, at
least in part, the meaning of the relationship between the quiescence of
the Acridian and its cryptic colouring. Quiescence is an essential element
in the protective resemblance to a stone--probably even more indispensable
than the details of the form and colouring. Although Burchell appears to
overlook this point he fully recognised the community between protection by
concealment and more aggressive modes of defence; for, in the passage of
which a part is quoted above, he specially refers to some earlier remarks
on page 226 of his Vol. I. We here find that even when the oxen were
resting by the Juk rivier (Yoke river), on July 19, 1811, Burchell observed
"Geranium spinosum, with a fleshy stem and large white flowers...; and a
succulent species of Pelargonium...so defended by the old panicles, grown
to hard woody thorns, that no cattle could browze upon it." He goes on to
say, "In this arid country, where every juicy vegetable would soon be eaten
up by the wild animals, the Great Creating Power, with all-provident
wisdom, has given to such plants either an acrid or poisonous juice, or
sharp thorns, to preserve the species from annihilation..." All these
modes of defence, especially adapted to a desert environment, have since
been generally recognised, and it is very interesting to place beside
Burchell's statement the following passage from a letter written by Darwin,
Aug. 7, 1868, to G.H. Lewes; "That Natural Selection would tend to produce
the most formidable thorns will be admitted by every one who has observed
the distribution in South America and Africa (vide Livingstone) of thorn-
bearing plants, for they always appear where the bushes grow isolated and
are exposed to the attacks of mammals. Even in England it has been noticed
that all spine-bearing and sting-bearing plants are palatable to
quadrupeds, when the thorns are crushed." ("More Letters", I. page 308.)
ADAPTATION AND NATURAL SELECTION.
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.
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 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 butterflies--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 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 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 sexes, they "are the result
of sexual 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 sexes: "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 analogous 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.
MIMICRY,--BATESIAN OR PSEUDAPOSEMATIC, MULLERIAN OR SYNAPOSEMATIC.
The existence of superficial resemblances between animals of various
degrees of affinity must have been observed for hundreds of years. Among
the early examples, the best known to me have been found in the manuscript
note-books and collections of W.J. Burchell, the great traveller in Africa
(1810-15) and Brazil (1825-30). The most interesting of his records on
this subject are brought together in the following paragraphs.
Conspicuous among well-defended insects are the dark steely or iridescent
greenish blue fossorial wasps or sand-wasps, Sphex and the allied genera.
Many Longicorn beetles mimic these in colour, slender shape of body and
limbs, rapid movements, and the readiness with which they take to flight.
On Dec. 21, 1812, Burchell captured one such beetle (Promeces viridis) at
Kosi Fountain on the journey from the source of the Kuruman River to
Klaarwater. It is correctly placed among the Longicorns in his catalogue,
but opposite to its number is the comment "Sphex! totus purpureus."
In our own country the black-and-yellow colouring of many stinging insects,
especially the ordinary wasps, affords perhaps the commonest model for
mimicry. It is reproduced with more or less accuracy on moths, flies and
beetles. Among the latter it is again a Longicorn which offers one of the
best-known, although by no means one of the most perfect, examples. The
appearance of the well-known "wasp-beetle" (Clytus arietis) in the living
state is sufficiently suggestive to prevent the great majority of people
from touching it. In Burchell's Brazilian collection there is a nearly
allied species (Neoclytus curvatus) which appears to be somewhat less wasp-
like than the British beetle. The specimen bears the number "1188," and
the date March 27, 1827, when Burchell was collecting in the neighbourhood
of San Paulo. Turning to the corresponding number in the Brazilian note-
book we find this record: "It runs rapidly like an ichneumon or wasp, of
which it has the appearance."
The formidable, well-defended ants are as freely mimicked by other insects
as the sand-wasps, ordinary wasps and bees. Thus on February 17, 1901, Guy
A.K. Marshall captured, near Salisbury, Mashonaland, three similar species
of ants (Hymenoptera) with a bug (Hemiptera) and a Locustid (Orthoptera),
the two latter mimicking the former. All the insects, seven in number,
were caught on a single plant, a small bushy vetch. ("Trans. Ent. Soc.
Lond." 1902, page 535, plate XIX. figs. 53-59.)
This is an interesting recent example from South Africa, and large numbers
of others might be added--the observations of many naturalists in many
lands; but nearly all of them known since that general awakening of
interest in the subject which was inspired by the great hypotheses of H.W.
Bates and Fritz Muller. We find, however, that Burchell had more than once
recorded the mimetic resemblance to ants. An extremely ant-like bug (the
larva of a species of Alydus) in his Brazilian collection is labelled
"1141," with the date December 8, 1826, when Burchell was at the Rio das
Pedras, Cubatao, near Santos. In the note-book the record is as follows:
"1141 Cimex. I collected this for a Formica."
Some of the chief mimics of ants are the active little hunting spiders
belonging to the family Attidae. Examples have been brought forward during
many recent years, especially by my friends Dr and Mrs Peckham, of
Milwaukee, the great authorities on this group of Araneae. Here too we
find an observation of the mimetic resemblance recorded by Burchell, and
one which adds in the most interesting manner to our knowledge of the
subject. A fragment, all that is now left, of an Attid spider, captured on
June 30, 1828, at Goyaz, Brazil, bears the following note, in this case on
the specimen and not in the note-book: "Black...runs and seems like an ant
with large extended jaws." My friend Mr R.I. Pocock, to whom I have
submitted the specimen, tells me that it is not one of the group of species
hitherto regarded as ant-like, and he adds, "It is most interesting that
Burchell should have noticed the resemblance to an ant in its movements.
This suggests that the perfect imitation in shape, as well as in movement,
seen in many species was started in forms of an appropriate size and colour
by the mimicry of movement alone." Up to the present time Burchell is the
only naturalist who has observed an example which still exhibits this
ancestral stage in the evolution of mimetic likeness.
Following the teachings of his day, Burchell was driven to believe that it
was part of the fixed and inexorable scheme of things that these strange
superficial resemblances existed. Thus, when he found other examples of
Hemipterous mimics, including one (Luteva macrophthalma) with "exactly the
manners of a Mantis," he added the sentence, "In the genus Cimex (Linn.)
are to be found the outward resemblances of insects of many other genera
and orders" (February 15, 1829). Of another Brazilian bug, which is not to
be found in his collection, and cannot therefore be precisely identified,
he wrote: "Cimex...Nature seems to have intended it to imitate a Sphex,
both in colour and the rapid palpitating and movement of the antennae"
(November 15, 1826). At the same time it is impossible not to feel the
conviction that Burchell felt the advantage of a likeness to stinging
insects and to aggressive ants, just as he recognised the benefits
conferred on desert plants by spines and by concealment. Such an
interpretation of mimicry was perfectly consistent with the theological
doctrines of his day. (See Kirby and Spence, "An Introduction to
Entomology" (1st edition), London, Vol. II. 1817, page 223.)
The last note I have selected from Burchell's manuscript refers to one of
the chief mimics of the highly protected Lycid beetles. The whole
assemblage of African insects with a Lycoid colouring forms a most
important combination and one which has an interesting bearing upon the
theories of Bates and Fritz Muller. This most wonderful set of mimetic
forms, described in 1902 by Guy A.K. Marshall, is composed of flower-
haunting beetles belonging to the family Lycidae, and the heterogeneous
group of varied insects which mimic their conspicuous and simple scheme of
colouring. The Lycid beetles, forming the centre or "models" of the whole
company, are orange-brown in front for about two-thirds of the exposed
surface, black behind for the remaining third. They are undoubtedly
protected by qualities which make them excessively unpalatable to the bulk
of insect-eating animals. Some experimental proof of this has been
obtained by Mr Guy Marshall. What are the forms which surround them?
According to the hypothesis of Bates they would be, at any rate mainly,
palatable hard-pressed insects which only hold their own in the struggle
for life by a fraudulent imitation of the trade-mark of the successful and
powerful Lycidae. According to Fritz Muller's hypothesis we should expect
that the mimickers would be highly protected, successful and abundant
species, which (metaphorically speaking) have found it to their advantage
to possess an advertisement, a danger-signal, in common with each other,
and in common with the beetles in the centre of the group.
How far does the constitution of this wonderful combination--the largest
and most complicated as yet known in all the world--convey to us the idea
of mimicry working along the lines supposed by Bates or those suggested by
Muller? Figures 1 to 52 of Mr Marshall's coloured plate ("Trans. Ent. Soc.
Lond." 1902, plate XVIII. See also page 517, where the group is analysed.)
represent a set of forty-two or forty-three species or forms of insects
captured in Mashonaland, and all except two in the neighbourhood of
Salisbury. The combination includes six species of Lycidae; nine beetles
of five groups all specially protected by nauseous qualities, Telephoridae,
Melyridae, Phytophaga, Lagriidae, Cantharidae; six Longicorn beetles; one
Coprid beetle; eight stinging Hymenoptera; three or four parasitic
Hymenoptera (Braconidae, a group much mimicked and shown by some
experiments to be distasteful); five bugs (Hemiptera, a largely unpalatable
group); three moths (Arctiidae and Zygaenidae, distasteful families); one
fly. In fact the whole combination, except perhaps one Phytophagous, one
Coprid and the Longicorn beetles, and the fly, fall under the hypothesis of
Muller and not under that of Bates. And it is very doubtful whether these
exceptions will be sustained: indeed the suspicion of unpalatability
already besets the Longicorns and is always on the heels,--I should say the
hind tarsi--of a Phytophagous beetle.
This most remarkable group which illustrates so well the problem of mimicry
and the alternative hypotheses proposed for its solution, was, as I have
said, first described in 1902. Among the most perfect of the mimetic
resemblances in it is that between the Longicorn beetle, Amphidesmus
analis, and the Lycidae. It was with the utmost astonishment and pleasure
that I found this very resemblance had almost certainly been observed by
Burchell. A specimen of the Amphidesmus exists in his collection and it
bears "651." Turning to the same number in the African Catalogue we find
that the beetle is correctly placed among the Longicorns, that it was
captured at Uitenhage on Nov. 18, 1813, and that it was found associated
with Lycid beetles in flowers ("consocians cum Lycis 78-87 in floribus").
Looking up Nos. 78-87 in the collection and catalogue, three species of
Lycidae are found, all captured on Nov. 18, 1813, at Uitenhage. Burchell
recognised the wide difference in affinity, shown by the distance between
the respective numbers; for his catalogue is arranged to represent
relationships. He observed, what students of mimicry are only just
beginning to note and record, the coincidence between model and mimic in
time and space and in habits. We are justified in concluding that he
observed the close superficial likeness although he does not in this case
expressly allude to it.
One of the most interesting among the early observations of superficial
resemblance between forms remote in the scale of classification was made by
Darwin himself, as described in the following passage from his letter to
Henslow, written from Monte Video, Aug. 15, 1832: "Amongst the lower
animals nothing has so much interested me as finding two species of
elegantly coloured true Planaria inhabiting the dewy forest! The false
relation they bear to snails is the most extraordinary thing of the kind I
have ever seen." ("More Letters", I. page 9.)
Many years later, in 1867, he wrote to Fritz Muller suggesting that the
resemblance of a soberly coloured British Planarian to a slug might be due
to mimicry. ("Life and Letters", III. page 71.)
The most interesting copy of Bates's classical memoir on Mimicry
("Contributions to an Insect Fauna of the Amazon Valley". "Trans. Linn.
Soc." Vol. XXIII. 1862, page 495.), read before the Linnean Society in
1861, is that given by him to the man who has done most to support and
extend the theory. My kind friend has given that copy to me; it bears the
inscription:
"Mr A.R. Wallace from his old travelling companion the Author."
Only a year and a half after the publication of the "Origin", we find that
Darwin wrote to Bates on the subject which was to provide such striking
evidence of the truth of Natural Selection: "I am glad to hear that you
have specially attended to 'mimetic' analogies--a most curious subject; I
hope you publish on it. I have for a long time wished to know whether what
Dr Collingwood asserts is true--that the most striking cases generally
occur between insects inhabiting the same country." (The letter is dated
April 4, 1861. "More Letters", I. page 183.)
The next letter, written about six months later, reveals the remarkable
fact that the illustrious naturalist who had anticipated Edward Forbes in
the explanation of arctic forms on alpine heights ("I was forestalled in
only one important point, which my vanity has always made me regret,
namely, the explanation by means of the Glacial period of the presence of
the same species of plants and of some few animals on distant mountain
summits and in the arctic regions. This view pleased me so much that I
wrote it out in extenso, and I believe that it was read by Hooker some
years before E. Forbes published his celebrated memoir on the subject. In
the very few points in which we differed, I still think that I was in the
right. I have never, of course, alluded in print to my having
independently worked out this view." "Autobiography, Life and Letters", I.
page 88.), had also anticipated H.W. Bates in the theory of Mimicry: "What
a capital paper yours will be on mimetic resemblances! You will make quite
a new subject of it. I had thought of such cases as a difficulty; and
once, when corresponding with Dr Collingwood, I thought of your
explanation; but I drove it from my mind, for I felt that I had not
knowledge to judge one way or the other." (The letter is dated Sept. 25,
1861: "More Letters", I. page 197.)
Bates read his paper before the Linnean Society, Nov. 21, 1861, and
Darwin's impressions on hearing it were conveyed in a letter to the author
dated Dec. 3: "Under a general point of view, I am quite convinced (Hooker
and Huxley took the same view some months ago) that a philosophic view of
nature can solely be driven into naturalists by treating special subjects
as you have done. Under a special point of view, I think you have solved
one of the most perplexing problems which could be given to solve." ("Life
and Letters", II. page 378.) The memoir appeared in the following year,
and after reading it Darwin wrote as follows, Nov. 20, 1862: "...In my
opinion it is one of the most remarkable and admirable papers I ever read
in my life...I am rejoiced that I passed over the whole subject in the
"Origin", for I should have made a precious mess of it. You have most
clearly stated and solved a wonderful problem...Your paper is too good to
be largely appreciated by the mob of naturalists without souls; but, rely
on it, that it will have LASTING value, and I cordially congratulate you on
your first great work. You will find, I should think, that Wallace will
fully appreciate it." ("Life and Letters", II. pages 391-393.) Four days
later, Nov. 24, Darwin wrote to Hooker on the same subject: "I have now
finished his paper...' it seems to me admirable. To my mind the act of
segregation of varieties into species was never so plainly brought forward,
and there are heaps of capital miscellaneous observations." ("More
Letters", I. page 214.)
Darwin was here referring to the tendency of similar varieties of the same
species to pair together, and on Nov. 25 he wrote to Bates asking for
fuller information on this subject. ("More Letters", I. page 215. See
also parts of Darwin's letter to Bates in "Life and Letters", II. page
392.) If Bates's opinion were well founded, sexual selection would bear a
most important part in the establishment of such species. (See Poulton,
"Essays on Evolution", 1908, pages 65, 85-88.) It must be admitted,
however, that the evidence is as yet quite insufficient to establish this
conclusion. It is interesting to observe how Darwin at once fixed on the
part of Bates's memoir which seemed to bear upon sexual selection. A
review of Bates's theory of Mimicry was contributed by Darwin to the
"Natural History Review" (New Ser. Vol. III. 1863, page 219.) and an
account of it is to be found in the "Origin" (Edition 1872, pages 375-378.)
and in "The Descent of Man". (Edition 1874, pages 323-325.)
Darwin continually writes of the value of hypothesis as the inspiration of
inquiry. We find an example in his letter to Bates, Nov. 22, 1860: "I
have an old belief that a good observer really means a good theorist, and I
fully expect to find your observations most valuable." ("More Letters", I.
page 176.) Darwin's letter refers to many problems upon which Bates had
theorised and observed, but as regards Mimicry itself the hypothesis was
thought out after the return of the letter from the Amazons, when he no
longer had the opportunity of testing it by the observation of living
Nature. It is by no means improbable that, had he been able to apply this
test, Bates would have recognised that his division of butterfly
resemblances into two classes,--one due to the theory of mimicry, the other
to the influence of local conditions,--could not be sustained.
Fritz Muller's contributions to the problem of Mimicry were all made in
S.E. Brazil, and numbers of them were communicated, with other observations
on natural history, to Darwin, and by him sent to Professor R. Meldola who
published many of the facts. Darwin's letters to Meldola (Poulton,
"Charles Darwin and the theory of Natural Selection", London, 1896, pages
199-218.) contain abundant proofs of his interest in Muller's work upon
Mimicry. One deeply interesting letter (Loc. cit. pages 201, 202.) dated
Jan. 23, 1872, proves that Fritz Muller before he originated the theory of
Common Warning Colours (Synaposematic Resemblance or Mullerian Mimicry),
which will ever be associated with his name, had conceived the idea of the
production of mimetic likeness by sexual selection.
Darwin's letter to Meldola shows that he was by no means inclined to
dismiss the suggestion as worthless, although he considered it daring.
"You will also see in this letter a strange speculation, which I should not
dare to publish, about the appreciation of certain colours being developed
in those species which frequently behold other forms similarly ornamented.
I do not feel at all sure that this view is as incredible as it may at
first appear. Similar ideas have passed through my mind when considering
the dull colours of all the organisms which inhabit dull-coloured regions,
such as Patagonia and the Galapagos Is." A little later, on April 5, he
wrote to Professor August Weismann on the same subject: "It may be
suspected that even the habit of viewing differently coloured surrounding
objects would influence their taste, and Fritz Muller even goes so far as
to believe that the sight of gaudy butterflies might influence the taste of
distinct species." ("Life and Letters", III. page 157.)
This remarkable suggestion affords interesting evidence that F. Muller was
not satisfied with the sufficiency of Bates's theory. Nor is this
surprising when we think of the numbers of abundant conspicuous butterflies
which he saw exhibiting mimetic likenesses. The common instances in his
locality, and indeed everywhere in tropical America, were anything but the
hard-pressed struggling forms assumed by the theory of Bates. They
belonged to the groups which were themselves mimicked by other butterflies.
Fritz Muller's suggestion also shows that he did not accept Bates's
alternative explanation of a superficial likeness between models
themselves, based on some unknown influence of local physico-chemical
forces. At the same time Muller's own suggestion was subject to this
apparently fatal objection, that the sexual selection he invoked would tend
to produce resemblances in the males rather than the females, while it is
well known that when the sexes differ the females are almost invariably
more perfectly mimetic than the males and in a high proportion of cases are
mimetic while the males are non-mimetic.
The difficulty was met several years later by Fritz Muller's well-known
theory, published in 1879 ("Kosmos", May 1879, page 100.), and immediately
translated by Meldola and brought before the Entomological Society.
("Proc. Ent. Soc. Lond." 1879, page xx.) Darwin's letter to Meldola dated
June 6, 1879, shows "that the first introduction of this new and most
suggestive hypothesis into this country was due to the direct influence of
Darwin himself, who brought it before the notice of the one man who was
likely to appreciate it at its true value and to find the means for its
presentation to English naturalists." ("Charles Darwin and the Theory of
Natural Selection", page 214.) Of the hypothesis itself Darwin wrote "F.
Muller's view of the mutual protection was quite new to me." (Ibid. page
213.) The hypothesis of Mullerian mimicry was at first strongly opposed.
Bates himself could never make up his mind to accept it. As the Fellows
were walking out of the meeting at which Professor Meldola explained the
hypothesis, an eminent entomologist, now deceased, was heard to say to
Bates: "It's a case of save me from my friends!" The new ideas
encountered and still encounter to a great extent the difficulty that the
theory of Bates had so completely penetrated the literature of natural
history. The present writer has observed that naturalists who have not
thoroughly absorbed the older hypothesis are usually far more impressed by
the newer one than are those whose allegiance has already been rendered.
The acceptance of Natural Selection itself was at first hindered by similar
causes, as Darwin clearly recognised: "If you argue about the non-
acceptance of Natural Selection, it seems to me a very striking fact that
the Newtonian theory of gravitation, which seems to every one now so
certain and plain, was rejected by a man so extraordinarily able as
Leibnitz. The truth will not penetrate a preoccupied mind." (To Sir J.
Hooker, July 28, 1868, "More Letters", I. page 305. See also the letter to
A.R. Wallace, April 30, 1868, in "More Letters" II. page 77, lines 6-8 from
top.)
There are many naturalists, especially students of insects, who appear to
entertain an inveterate hostility to any theory of mimicry. Some of them
are eager investigators in the fascinating field of geographical
distribution, so essential for the study of Mimicry itself. The changes of
pattern undergone by a species of Erebia as we follow it over different
parts of the mountain ranges of Europe is indeed a most interesting
inquiry, but not more so than the differences between e.g. the Acraea
johnstoni of S.E. Rhodesia and of Kilimanjaro. A naturalist who is
interested by the Erebia should be equally interested by the Acraea; and so
he would be if the student of mimicry did not also record that the
characteristics which distinguish the northern from the southern
individuals of the African species correspond with the presence, in the
north but not in the south, of certain entirely different butterflies.
That this additional information should so greatly weaken, in certain
minds, the appeal of a favourite study, is a psychological problem of no
little interest. This curious antagonism is I believe confined to a few
students of insects. Those naturalists who, standing rather farther off,
are able to see the bearings of the subject more clearly, will usually
admit the general support yielded by an ever-growing mass of observations
to the theories of Mimicry propounded by H.W. Bates and Fritz Muller. In
like manner natural selection itself was in the early days often best
understood and most readily accepted by those who were not naturalists.
Thus Darwin wrote to D.T. Ansted, Oct. 27, 1860: "I am often in despair in
making the generality of NATURALISTS even comprehend me. Intelligent men
who are not naturalists and have not a bigoted idea of the term species,
show more clearness of mind." ("More Letters", I. page 175.)
Even before the "Origin" appeared Darwin anticipated the first results upon
the mind of naturalists. He wrote to Asa Gray, Dec. 21, 1859: "I have
made up my mind to be well abused; but I think it of importance that my
notions should be read by intelligent men, accustomed to scientific
argument, though NOT naturalists. It may seem absurd, but I think such men
will drag after them those naturalists who have too firmly fixed in their
heads that a species is an entity." ("Life and Letters" II. page 245.)
Mimicry was not only one of the first great departments of zoological
knowledge to be studied under the inspiration of natural Selection, it is
still and will always remain one of the most interesting and important of
subjects in relation to this theory as well as to evolution. In mimicry we
investigate the effect of environment in its simplest form: we trace the
effects of the pattern of a single species upon that of another far removed
from it in the scale of classification. When there is reason to believe
that the model is an invader from another region and has only recently
become an element in the environment of the species native to its second
home, the problem gains a special interest and fascination. Although we
are chiefly dealing with the fleeting and changeable element of colour we
expect to find and we do find evidence of a comparatively rapid evolution.
The invasion of a fresh model is for certain species an unusually sudden
change in the forces of the environment and in some instances we have
grounds for the belief that the mimetic response has not been long delayed.
MIMICRY AND SEX.
Ever since Wallace's classical memoir on mimicry in the Malayan Swallowtail
butterflies, those naturalists who have written on the subject have
followed his interpretation of the marked prevalence of mimetic resemblance
in the female sex as compared with the male. They have believed with
Wallace that the greater dangers of the female, with slower flight and
often alighting for oviposition, have been in part met by the high
development of this special mode of protection. The fact cannot be
doubted. It is extremely common for a non-mimetic male to be accompanied
by a beautifully mimetic female and often by two or three different forms
of female, each mimicking a different model. The male of a polymorphic
mimetic female is, in fact, usually non-mimetic (e.g. Papilio dardanus =
merope), or if a mimic (e.g. the Nymphaline genus Euripus), resembles a
very different model. On the other hand a non-mimetic female accompanied
by a mimetic male is excessively rare. An example is afforded by the
Oriental Nymphaline, Cethosia, in which the males of some species are rough
mimics of the brown Danaines. In some of the orb-weaving spiders the males
mimic ants, while the much larger females are non-mimetic. When both sexes
mimic, it is very common in butterflies and is also known in moths, for the
females to be better and often far better mimics than the males.
Although still believing that Wallace's hypothesis in large part accounts
for the facts briefly summarised above, the present writer has recently
been led to doubt whether it offers a complete explanation. Mimicry in the
male, even though less beneficial to the species than mimicry in the
female, would still surely be advantageous. Why then is it so often
entirely restricted to the female? While the attempt to find an answer to
this question was haunting me, I re-read a letter written by Darwin to
Wallace, April 15, 1868, containing the following sentences: "When female
butterflies are more brilliant than their males you believe that they have
in most cases, or in all cases, been rendered brilliant so as to mimic some
other species, and thus escape danger. But can you account for the males
not having been rendered equally brilliant and equally protected? Although
it may be most for the welfare of the species that the female should be
protected, yet it would be some advantage, certainly no disadvantage, for
the unfortunate male to enjoy an equal immunity from danger. For my part,
I should say that the female alone had happened to vary in the right
manner, and that the beneficial variations had been transmitted to the same
sex alone. Believing in this, I can see no improbability (but from analogy
of domestic animals a strong probability) that variations leading to beauty
must often have occurred in the males alone, and been transmitted to that
sex alone. Thus I should account in many cases for the greater beauty of
the male over the female, without the need of the protective principle."
("More Letters", II. pages 73, 74. On the same subject--"the gay-coloured
females of Pieris" (Perrhybris (Mylothris) pyrrha of Brazil), Darwin wrote
to Wallace, May 5, 1868, as follows: "I believe I quite follow you in
believing that the colours are wholly due to mimicry; and I further believe
that the male is not brilliant from not having received through inheritance
colour from the female, and from not himself having varied; in short, that
he has not been influenced by selection." It should be noted that the male
of this species does exhibit a mimetic pattern on the under surface. "More
Letters" II. page 78.)
The consideration of the facts of mimicry thus led Darwin to the conclusion
that the female happens to vary in the right manner more commonly than the
male, while the secondary sexual characters of males supported the
conviction "that from some unknown cause such characters (viz. new
characters arising in one sex and transmitted to it alone) apparently
appear oftener in the male than in the female." (Letter from Darwin to
Wallace, May 5, 1867, "More Letters", II. Page 61.)
Comparing these conflicting arguments we are led to believe that the first
is the stronger. Mimicry in the male would be no disadvantage but an
advantage, and when it appears would be and is taken advantage of by
selection. The secondary sexual characters of males would be no advantage
but a disadvantage to females, and, as Wallace thinks, are withheld from
this sex by selection. It is indeed possible that mimicry has been
hindered and often prevented from passing to the males by sexual selection.
We know that Darwin was much impressed ("Descent of Man", page 325.) by
Thomas Belt's daring and brilliant suggestion that the white patches which
exist, although ordinarily concealed, on the wings of mimetic males of
certain Pierinae (Dismorphia), have been preserved by preferential mating.
He supposed this result to have been brought about by the females
exhibiting a deep-seated preference for males that displayed the chief
ancestral colour, inherited from periods before any mimetic pattern had
been evolved in the species. But it has always appeared to me that Belt's
deeply interesting suggestion requires much solid evidence and repeated
confirmation before it can be accepted as a valid interpretation of the
facts. In the present state of our knowledge, at any rate of insects and
especially of Lepidoptera, it is probable that the female is more apt to
vary than the male and that an important element in the interpretation of
prevalent female mimicry is provided by this fact.
In order adequately to discuss the question of mimicry and sex it would be
necessary to analyse the whole of the facts, so far as they are known in
butterflies. On the present occasion it is only possible to state the
inferences which have been drawn from general impressions,--inferences
which it is believed will be sustained by future inquiry.
(1) Mimicry may occasionally arise in one sex because the differences
which distinguish it from the other sex happen to be such as to afford a
starting-point for the resemblance. Here the male is at no disadvantage as
compared with the female, and the rarity of mimicry in the male alone (e.g.
Cethosia) is evidence that the great predominance of female mimicry is not
to be thus explained.
(2) The tendency of the female to dimorphism and polymorphism has been of
great importance in determining this predominance. Thus if the female
appear in two different forms and the male in only one it will be twice as
probable that she will happen to possess a sufficient foundation for the
evolution of mimicry.
(3) The appearance of melanic or partially melanic forms in the female has
been of very great service, providing as it does a change of ground-colour.
Thus the mimicry of the black generally red-marked American "Aristolochia
swallowtails" (Pharmacophagus) by the females of Papilio swallowtails was
probably begun in this way.
(4) It is probably incorrect to assume with Haase that mimicry always
arose in the female and was later acquired by the male. Both sexes of the
third section of swallowtails (Cosmodesmus) mimic Pharmacophagus in
America, far more perfectly than do the females of Papilio. But this is
not due to Cosmodesmus presenting us with a later stage of history begun in
Papilio; for in Africa Cosmodesmus is still mimetic (of Danainae) in both
sexes although the resemblances attained are imperfect, while many African
species of Papilio have non-mimetic males with beautifully mimetic females.
The explanation is probably to be sought in the fact that the females of
Papilio are more variable and more often tend to become dimorphic than
those of Cosmodesmus, while the latter group has more often happened to
possess a sufficient foundation for the origin of the resemblance in
patterns which, from the start, were common to male and female.
(5) In very variable species with sexes alike, mimicry can be rapidly
evolved in both sexes out of very small beginnings. Thus the reddish marks
which are common in many individuals of Limenitis arthemis were almost
certainly the starting-point for the evolution of the beautifully mimetic
L. archippus. Nevertheless in such cases, although there is no reason to
suspect any greater variability, the female is commonly a somewhat better
mimic than the male and often a very much better mimic. Wallace's
principle seems here to supply the obvious interpretation.
(6) When the difference between the patterns of the model and presumed
ancestor of the mimic is very great, the female is often alone mimetic;
when the difference is comparatively small, both sexes are commonly
mimetic. The Nymphaline genus Hypolimnas is a good example. In Hypolimnas
itself the females mimic Danainae with patterns very different from those
preserved by the non-mimetic males: in the sub-genus Euralia, both sexes
resemble the black and white Ethiopian Danaines with patterns not very
dissimilar from that which we infer to have existed in the non-mimetic
ancestor.
(7) Although a melanic form or other large variation may be of the utmost
importance in facilitating the start of a mimetic likeness, it is
impossible to explain the evolution of any detailed resemblance in this
manner. And even the large colour variation itself may well be the
expression of a minute and "continuous" change in the chemical and physical
constitution of pigments.
SEXUAL SELECTION (EPIGAMIC CHARACTERS).
We do not know the date at which the idea of Sexual Selection arose in
Darwin's mind, but it was probably not many years after the sudden flash of
insight which, in October 1838, gave to him the theory of Natural
Selection. An excellent account of Sexual Selection occupies the
concluding paragraph of Part I. of Darwin's Section of the Joint Essay on
Natural Selection, read July 1st, 1858, before the Linnean Society.
("Journ. Proc. Linn. Soc." Vol. III. 1859, page 50.) The principles are so
clearly and sufficiently stated in these brief sentences that it is
appropriate to quote the whole: "Besides this natural means of selection,
by which those individuals are preserved, whether in their egg, or larval,
or mature state, which are best adapted to the place they fill in nature,
there is a second agency at work in most unisexual animals, tending to
produce the same effect, namely, the struggle of the males for the females.
These struggles are generally decided by the law of battle, but in the case
of birds, apparently, by the charms of their song, by their beauty or their
power of courtship, as in the dancing rock-thrush of Guiana. The most
vigorous and healthy males, implying perfect adaptation, must generally
gain the victory in their contests. This kind of selection, however, is
less rigorous than the other; it does not require the death of the less
successful, but gives to them fewer descendants. The struggle falls,
moreover, at a time of year when food is generally abundant, and perhaps
the effect chiefly produced would be the modification of the secondary
sexual characters, which are not related to the power of obtaining food, or
to defence from enemies, but to fighting with or rivalling other males.
The result of this struggle amongst the males may be compared in some
respects to that produced by those agriculturists who pay less attention to
the careful selection of all their young animals, and more to the
occasional use of a choice mate."
A full exposition of Sexual Selection appeared in the "The Descent of Man"
in 1871, and in the greatly augmented second edition, in 1874. It has been
remarked that the two subjects, "The Descent of Man and Selection in
Relation to Sex", seem to fuse somewhat imperfectly into the single work of
which they form the title. The reason for their association is clearly
shown in a letter to Wallace, dated May 28, 1864: "...I suspect that a
sort of sexual selection has been the most powerful means of changing the
races of man." ("More Letters", II. page 33.)
Darwin, as we know from his Autobiography ("Life and Letters", I. page
94.), was always greatly interested in this hypothesis, and it has been
shown in the preceding pages that he was inclined to look favourably upon
it as an interpretation of many appearances usually explained by Natural
Selection. Hence Sexual Selection, incidentally discussed in other
sections of the present essay, need not be considered at any length, in the
section specially allotted to it.
Although so interested in the subject and notwithstanding his conviction
that the hypothesis was sound, Darwin was quite aware that it was probably
the most vulnerable part of the "Origin". Thus he wrote to H.W. Bates,
April 4, 1861: "If I had to cut up myself in a review I would have
(worried?) and quizzed sexual selection; therefore, though I am fully
convinced that it is largely true, you may imagine how pleased I am at what
you say on your belief." ("More Letters", I. page 183.)
The existence of sound-producing organs in the males of insects was, Darwin
considered, the strongest evidence in favour of the operation of sexual
selection in this group. ("Life and Letters", III. pages 94, 138.) Such a
conclusion has received strong support in recent years by the numerous
careful observations of Dr F.A. Dixey ("Proc. Ent. Soc. Lond." 1904, page
lvi; 1905, pages xxxvii, liv; 1906, page ii.) and Dr G.B. Longstaff ("Proc.
Ent. Soc. Lond." 1905, page xxxv; "Trans. Ent. Soc. Lond." 1905, page 136;
1908, page 607.) on the scents of male butterflies. The experience of
these naturalists abundantly confirms and extends the account given by
Fritz Muller ("Jen. Zeit." Vol. XI. 1877, page 99; "Trans. Ent. Soc. Lond."
1878, page 211.) of the scents of certain Brazilian butterflies. It is a
remarkable fact that the apparently epigamic scents of male butterflies
should be pleasing to man while the apparently aposematic scents in both
sexes of species with warning colours should be displeasing to him. But
the former is far more surprising than the latter. It is not perhaps
astonishing that a scent which is ex hypothesi unpleasant to an insect-
eating Vertebrate should be displeasing to the human sense; but it is
certainly wonderful that an odour which is ex hypothesi agreeable to a
female butterfly should also be agreeable to man.
Entirely new light upon the seasonal appearance of epigamic characters is
shed by the recent researches of C.W. Beebe ("The American Naturalist",
Vol. XLII. No. 493, Jan. 1908, page 34.), who caused the scarlet tanager
(Piranga erythromelas) and the bobolink (Dolichonyx oryzivorus) to retain
their breeding plumage through the whole year by means of fattening food,
dim illumination, and reduced activity. Gradual restoration to the light
and the addition of meal-worms to the diet invariably brought back the
spring song, even in the middle of winter. A sudden alteration of
temperature, either higher or lower, caused the birds nearly to stop
feeding, and one tanager lost weight rapidly and in two weeks moulted into
the olive-green winter plumage. After a year, and at the beginning of the
normal breeding season, "individual tanagers and bobolinks were gradually
brought under normal conditions and activities," and in every case moulted
from nuptial plumage to nuptial plumage. "The dull colours of the winter
season had been skipped." The author justly claims to have established
"that the sequence of plumage in these birds is not in any way predestined
through inheritance..., but that it may be interrupted by certain factors
in the environmental complex."
XVI. GEOGRAPHICAL DISTRIBUTION OF PLANTS.
By SIR WILLIAM THISELTON-DYER, K.C.M.G., C.I.E. Sc.D., F.R.S.
The publication of "The Origin of Species" placed the study of Botanical
Geography on an entirely new basis. It is only necessary to study the
monumental "Geographie Botanique raisonnee" of Alphonse De Candolle,
published four years earlier (1855), to realise how profound and far-
reaching was the change. After a masterly and exhaustive discussion of all
available data De Candolle in his final conclusions could only arrive at a
deadlock. It is sufficient to quote a few sentences:--
"L'opinion de Lamarck est aujourd'hui abandonee par tous les naturalistes
qui ont etudie sagement les modifications possibles des etres organises...
"Et si l'on s'ecarte des exagerations de Lamarck, si l'on suppose un
premier type de chaque genre, de chaque famille tout au moins, on se trouve
encore a l'egard de l'origine de ces types en presence de la grande
question de la creation.
"Le seul parti a prendre est donc d'envisager les etres organises comme
existant depuis certaines epoques, avec leurs qualites particulieres."
(Vol. II. page 1107.)
Reviewing the position fourteen years afterwards, Bentham remarked:--"These
views, generally received by the great majority of naturalists at the time
De Candolle wrote, and still maintained by a few, must, if adhered to,
check all further enquiry into any connection of facts with causes," and he
added, "there is little doubt but that if De Candolle were to revise his
work, he would follow the example of so many other eminent naturalists,
and...insist that the present geographical distribution of plants was in
most instances a derivative one, altered from a very different former
distribution." ("Pres. Addr." (1869) "Proc. Linn. Soc." 1868-69, page
lxviii.)
Writing to Asa Gray in 1856, Darwin gave a brief preliminary account of his
ideas as to the origin of species, and said that geographical distribution
must be one of the tests of their validity. ("Life and Letters", II. page
78.) What is of supreme interest is that it was also their starting-point.
He tells us:--"When I visited, during the voyage of H.M.S. "Beagle", the
Galapagos Archipelago,...I fancied myself brought near to the very act of
creation. I often asked myself how these many peculiar animals and plants
had been produced: the simplest answer seemed to be that the inhabitants
of the several islands had descended from each other, undergoing
modification in the course of their descent." ("The Variation of Animals
and Plants" (2nd edition), 1890, I. pages 9, 10.) We need not be surprised
then, that in writing in 1845 to Sir Joseph Hooker, he speaks of "that
grand subject, that almost keystone of the laws of creation, Geographical
Distribution." ("Life and Letters", I. page 336.)
Yet De Candolle was, as Bentham saw, unconsciously feeling his way, like
Lyell, towards evolution, without being able to grasp it. They both strove
to explain phenomena by means of agencies which they saw actually at work.
If De Candolle gave up the ultimate problem as insoluble:--"La creation ou
premiere formation des etres organises echappe, par sa nature et par son
anciennete, a nos moyens d'observation" (Loc. cit. page 1106.), he steadily
endeavoured to minimise its scope. At least half of his great work is
devoted to the researches by which he extricated himself from a belief in
species having had a multiple origin, the view which had been held by
successive naturalists from Gmelin to Agassiz. To account for the obvious
fact that species constantly occupy dissevered areas, De Candolle made a
minute study of their means of transport. This was found to dispose of the
vast majority of cases, and the remainder he accounted for by geographical
change. (Loc. cit. page 1116.)
But Darwin strenuously objected to invoking geographical change as a
solution of every difficulty. He had apparently long satisfied himself as
to the "permanence of continents and great oceans." Dana, he tells us
"was, I believe, the first man who maintained" this ("Life and Letters",
III. page 247. Dana says:--"The continents and oceans had their general
outline or form defined in earliest time," "Manual of Geology", revised
edition. Philadelphia, 1869, page 732. I have no access to an earlier
edition.), but he had himself probably arrived at it independently. Modern
physical research tends to confirm it. The earth's centre of gravity, as
pointed out by Pratt from the existence of the Pacific Ocean, does not
coincide with its centre of figure, and it has been conjectured that the
Pacific Ocean dates its origin from the separation of the moon from the
earth.
The conjecture appears to be unnecessary. Love shows that "the force that
keeps the Pacific Ocean on one side of the earth is gravity, directed more
towards the centre of gravity than the centre of the figure." ("Report of
the 77th Meeting of the British Association" (Leicester, 1907), London,
1908, page 431.) I can only summarise the conclusions of a technical but
masterly discussion. "The broad general features of the distribution of
continent and ocean can be regarded as the consequences of simple causes of
a dynamical character," and finally, "As regards the contour of the great
ocean basins, we seem to be justified in saying that the earth is
approximately an oblate spheroid, but more nearly an ellipsoid with three
unequal axes, having its surface furrowed according to the formula for a
certain spherical harmonic of the third degree" (Ibid. page 436.), and he
shows that this furrowed surface must be produced "if the density is
greater in one hemispheroid than in the other, so that the position of the
centre of gravity is eccentric." (Ibid. page 431.) Such a modelling of
the earth's surface can only be referred to a primitive period of
plasticity. If the furrows account for the great ocean basins, the
disposition of the continents seems equally to follow. Sir George Darwin
has pointed out that they necessarily "arise from a supposed primitive
viscosity or plasticity of the earth's mass. For during this course of
evolution the earth's mass must have suffered a screwing motion, so that
the polar regions have travelled a little from west to east relatively to
the equator. This affords a possible explanation of the north and south
trend of our great continents." ("Encycl. Brit." (9th edition), Vol.
XXIII. "Tides", page 379.)
It would be trespassing on the province of the geologist to pursue the
subject at any length. But as Wallace ("Island Life" (2nd edition), 1895,
page 103.), who has admirably vindicated Darwin's position, points out, the
"question of the permanence of our continents...lies at the root of all our
inquiries into the great changes of the earth and its inhabitants." But he
proceeds: "The very same evidence which has been adduced to prove the
GENERAL stability and permanence of our continental areas also goes to
prove that they have been subjected to wonderful and repeated changes in
DETAIL." (Loc. cit. page 101.) Darwin of course would have admitted this,
for with a happy expression he insisted to Lyell (1856) that "the
skeletons, at least, of our continents are ancient." ("More Letters", II.
page 135.) It is impossible not to admire the courage and tenacity with
which he carried on the conflict single-handed. But he failed to convince
Lyell. For we still find him maintaining in the last edition of the
"Principles": "Continents therefore, although permanent for whole
geological epochs, shift their positions entirely in the course of ages."
(Lyell's "Principles of Geology" (11th edition), London, 1872, I. page
258.)
Evidence, however, steadily accumulates in Darwin's support. His position
still remains inexpugnable that it is not permissible to invoke
geographical change to explain difficulties in distribution without valid
geological and physical support. Writing to Mellard Reade, who in 1878 had
said, "While believing that the ocean-depths are of enormous age, it is
impossible to reject other evidences that they have once been land," he
pointed out "the statement from the 'Challenger' that all sediment is
deposited within one or two hundred miles from the shores." ("More
Letters", II. page 146.) The following year Sir Archibald Geikie
("Geographical Evolution", "Proc. R. Geogr. Soc." 1879, page 427.) informed
the Royal Geographical Society that "No part of the results obtained by the
'Challenger' expedition has a profounder interest for geologists and
geographers than the proof which they furnish that the floor of the ocean
basins has no real analogy among the sedimentary formations which form most
of the framework of the land."
Nor has Darwin's earlier argument ever been upset. "The fact which I
pointed out many years ago, that all oceanic islands are volcanic (except
St Paul's, and now that is viewed by some as the nucleus of an ancient
volcano), seem to me a strong argument that no continent ever occupied the
great oceans." ("More Letters", II. page 146.)
Dr Guppy, who devoted several years to geological and botanical
investigations in the Pacific, found himself forced to similar conclusions.
"It may be at once observed," he says, "that my belief in the general
principle that islands have always been islands has not been shaken," and
he entirely rejects "the hypothesis of a Pacific continent." He comes
back, in full view of the problems on the spot, to the position from which,
as has been seen, Darwin started: "If the distribution of a particular
group of plants or animals does not seem to accord with the present
arrangement of the land, it is by far the safest plan, even after
exhausting all likely modes of explanation, not to invoke the intervention
of geographical changes; and I scarcely think that our knowledge of any one
group of organisms is ever sufficiently precise to justify a recourse to
hypothetical alterations in the present relations of land and sea."
("Observations of a Naturalist in the Pacific between 1896 and 1899",
London, 1903, I. page 380.) Wallace clinches the matter when he finds
"almost the whole of the vast areas of the Atlantic, Pacific, Indian, and
Southern Oceans, without a solitary relic of the great islands or
continents supposed to have sunk beneath their waves." ("Island Life",
page 105.)
Writing to Wallace (1876), Darwin warmly approves the former's "protest
against sinking imaginary continents in a quite reckless manner, as was
stated by Forbes, followed, alas, by Hooker, and caricatured by Wollaston
and (Andrew) Murray." ("Life and Letters", III. page 230.) The transport
question thus became of enormously enhanced importance. We need not be
surprised then at his writing to Lyell in 1856:--"I cannot avoid thinking
that Forbes's 'Atlantis' was an ill-service to science, as checking a close
study of means of dissemination" (Ibid. II. page 78.), and Darwin spared no
pains to extend our knowledge of them. He implores Hooker, ten years
later, to "admit how little is known on the subject," and summarises with
some satisfaction what he had himself achieved:--"Remember how recently you
and others thought that salt water would soon kill seeds...Remember that no
one knew that seeds would remain for many hours in the crops of birds and
retain their vitality; that fish eat seeds, and that when the fish are
devoured by birds the seeds can germinate, etc. Remember that every year
many birds are blown to Madeira and to the Bermudas. Remember that dust is
blown 1000 miles across the Atlantic." ("More Letters", I. page 483.)
It has always been the fashion to minimise Darwin's conclusions, and these
have not escaped objection. The advocatus diaboli has a useful function in
science. But in attacking Darwin his brief is generally found to be
founded on a slender basis of facts. Thus Winge and Knud Andersen have
examined many thousands of migratory birds and found "that their crops and
stomachs were always empty. They never observed any seeds adhering to the
feathers, beaks or feet of the birds." (R.F. Scharff, "European Animals",
page 64, London, 1907.) The most considerable investigation of the problem
of Plant Dispersal since Darwin is that of Guppy. He gives a striking
illustration of how easily an observer may be led into error by relying on
negative evidence.
"When Ekstam published, in 1895, the results of his observations on the
plants of Nova Zembla, he observed that he possessed no data to show
whether swimming and wading birds fed on berries; and he attached all
importance to dispersal by winds. On subsequently visiting Spitzbergen he
must have been at first inclined, therefore, to the opinion of Nathorst,
who, having found only a solitary species of bird (a snow-sparrow) in that
region, naturally concluded that birds had been of no importance as agents
in the plant-stocking. However, Ekstam's opportunities were greater, and
he tells us that in the craws of six specimens of Lagopus hyperboreus shot
in Spitzbergen in August he found represented almost 25 per cent. of the
usual phanerogamic flora of that region in the form of fruits, seeds,
bulbils, flower-buds, leaf-buds, etc..."
"The result of Ekstam's observations in Spitzbergen was to lead him to
attach a very considerable importance in plant dispersal to the agency of
birds; and when in explanation of the Scandinavian elements in the
Spitzbergen flora he had to choose between a former land connection and the
agency of birds, he preferred the bird." (Guppy, op. cit. II. pages 511,
512.)
Darwin objected to "continental extensions" on geological grounds, but he
also objected to Lyell that they do not "account for all the phenomena of
distribution on islands" ("Life and Letters", II. page 77.), such for
example as the absence of Acacias and Banksias in New Zealand. He agreed
with De Candolle that "it is poor work putting together the merely POSSIBLE
means of distribution." But he also agreed with him that they were the
only practicable door of escape from multiple origins. If they would not
work then "every one who believes in single centres will have to admit
continental extensions" (Ibid. II. page 82.), and that he regarded as a
mere counsel of despair:--"to make continents, as easily as a cook does
pancakes." (Ibid. II. page 74.)
The question of multiple origins however presented itself in another shape
where the solution was much more difficult. The problem, as stated by
Darwin, is this:--"The identity of many plants and animals, on mountain-
summits, separated from each other by hundreds of miles of
lowlands...without the apparent possibility of their having migrated from
one point to the other." He continues, "even as long ago as 1747, such
facts led Gmelin to conclude that the same species must have been
independently created at several distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid
attention to the Glacial period, which affords...a simple explanation of
the facts." ("Origin of Species" (6th edition) page 330.)
The "simple explanation" was substantially given by E. Forbes in 1846. It
is scarcely too much to say that it belongs to the same class of fertile
and far-reaching ideas as "natural selection" itself. It is an
extraordinary instance, if one were wanted at all, of Darwin's magnanimity
and intense modesty that though he had arrived at the theory himself, he
acquiesced in Forbes receiving the well-merited credit. "I have never," he
says, "of course alluded in print to my having independently worked out
this view." But he would have been more than human if he had not added:--
"I was forestalled in...one important point, which my vanity has always
made me regret." ("Life and Letters", I. page 88.)
Darwin, however, by applying the theory to trans-tropical migration, went
far beyond Forbes. The first enunciation to this is apparently contained
in a letter to Asa Gray in 1858. The whole is too long to quote, but the
pith is contained in one paragraph. "There is a considerable body of
geological evidence that during the Glacial epoch the whole world was
colder; I inferred that,...from erratic boulder phenomena carefully
observed by me on both the east and west coast of South America. Now I am
so bold as to believe that at the height of the Glacial epoch, AND WHEN ALL
TROPICAL PRODUCTIONS MUST HAVE BEEN CONSIDERABLY DISTRESSED, several
temperate forms slowly travelled into the heart of the Tropics, and even
reached the southern hemisphere; and some few southern forms penetrated in
a reverse direction northward." ("Life and Letters", II. page 136.) Here
again it is clear that though he credits Agassiz with having called vivid
attention to the Glacial period, he had himself much earlier grasped the
idea of periods of refrigeration.
Putting aside the fact, which has only been made known to us since Darwin's
death, that he had anticipated Forbes, it is clear that he gave the theory
a generality of which the latter had no conception. This is pointed out by
Hooker in his classical paper "On the Distribution of Arctic Plants"
(1860). "The theory of a southern migration of northern types being due to
the cold epochs preceding and during the glacial, originated, I believe,
with the late Edward Forbes; the extended one, of the trans-tropical
migration, is Mr Darwin's." ("Linn. Trans." XXIII. page 253. The attempt
appears to have been made to claim for Heer priority in what I may term for
short the arctic-alpine theory (Scharff, "European Animals", page 128). I
find no suggestion of his having hit upon it in his correspondence with
Darwin or Hooker. Nor am I aware of any reference to his having done so in
his later publications. I am indebted to his biographer, Professor
Schroter, of Zurich, for an examination of his earlier papers with an
equally negative result.) Assuming that local races have derived from a
common ancestor, Hooker's great paper placed the fact of the migration on
an impregnable basis. And, as he pointed out, Darwin has shown that "such
an explanation meets the difficulty of accounting for the restriction of so
many American and Asiatic arctic types to their own peculiar longitudinal
zones, and for what is a far greater difficulty, the representation of the
same arctic genera by most closely allied species in different longitudes."
The facts of botanical geography were vital to Darwin's argument. He had
to show that they admitted of explanation without assuming multiple origins
for species, which would be fatal to the theory of Descent. He had
therefore to strengthen and extend De Candolle's work as to means of
transport. He refused to supplement them by hypothetical geographical
changes for which there was no independent evidence: this was simply to
attempt to explain ignotum per ignotius. He found a real and, as it has
turned out, a far-reaching solution in climatic change due to cosmical
causes which compelled the migration of species as a condition of their
existence. The logical force of the argument consists in dispensing with
any violent assumption, and in showing that the principle of descent is
adequate to explain the ascertained facts.
It does not, I think, detract from the merit of Darwin's conclusions that
the tendency of modern research has been to show that the effects of the
Glacial period were less simple, more localised and less general than he
perhaps supposed. He admitted that "equatorial refrigeration...must have
been small." ("More Letters", I. page 177.) It may prove possible to
dispense with it altogether. One cannot but regret that as he wrote to
Bates:--"the sketch in the 'Origin' gives a very meagre account of my
fuller MS. essay on this subject." (Loc. cit.) Wallace fully accepted
"the effect of the Glacial epoch in bringing about the present distribution
of Alpine and Arctic plants in the NORTHERN HEMISPHERE," but rejected "the
lowering of the temperature of the tropical regions during the Glacial
period" in order to account for their presence in the SOUTHERN hemisphere.
("More Letters", II. page 25 (footnote 1).) The divergence however does
not lie very deep. Wallace attaches more importance to ordinary means of
transport. "If plants can pass in considerable numbers and variety over
wide seas and oceans, it must be yet more easy for them to traverse
continuous areas of land, wherever mountain-chains offer suitable
stations." ("Island Life" (2nd edition), London, 1895, page 512.) And he
argues that such periodical changes of climate, of which the Glacial period
may be taken as a type, would facilitate if not stimulate the process.
(Loc. cit. page 518.)
It is interesting to remark that Darwin drew from the facts of plant
distribution one of his most ingenious arguments in support of this theory.
(See "More Letters", I. page 424.) He tells us, "I was led to anticipate
that the species of the larger genera in each country would oftener present
varieties, than the species of the smaller genera." ("Origin", page 44.)
He argues "where, if we may use the expression, the manufactory of species
has been active, we ought generally to find the manufactory still in
action." (Ibid. page 45.) This proved to be the case. But the labour
imposed upon him in the study was immense. He tabulated local floras
"belting the whole northern hemisphere" ("More Letters", I. page 107.),
besides voluminous works such as De Candolle's "Prodromus". The results
scarcely fill a couple of pages. This is a good illustration of the
enormous pains which he took to base any statement on a secure foundation
of evidence, and for this the world, till the publication of his letters,
could not do him justice. He was a great admirer of Herbert Spencer, whose
"prodigality of original thought" astonished him. "But," he says, "the
reflection constantly recurred to me that each suggestion, to be of real
value to service, would require years of work." (Ibid. II. page 235.)
At last the ground was cleared and we are led to the final conclusion. "If
the difficulties be not insuperable in admitting that in the long course of
time all the individuals of the same species belonging to the same genus,
have proceeded from some one source; then all the grand leading facts of
geographical distribution are explicable on the theory of migration,
together with subsequent modification and the multiplication of new forms."
("Origin", page 360.) In this single sentence Darwin has stated a theory
which, as his son F. Darwin has said with justice, has "revolutionized
botanical geography." ("The Botanical Work of Darwin", "Ann. Bot." 1899,
page xi.) It explains how physical barriers separate and form botanical
regions; how allied species become concentrated in the same areas; how,
under similar physical conditions, plants may be essentially dissimilar,
showing that descent and not the surroundings is the controlling factor;
how insular floras have acquired their peculiarities; in short how the most
various and apparently uncorrelated problems fall easily and inevitably
into line.
The argument from plant distribution was in fact irresistible. A proof, if
one were wanted, was the immediate conversion of what Hooker called "the
stern keen intellect" ("More Letters", I. page 134.) of Bentham, by general
consent the leading botanical systematist at the time. It is a striking
historical fact that a paper of his own had been set down for reading at
the Linnean Society on the same day as Darwin's, but had to give way. In
this he advocated the fixity of species. He withdrew it after hearing
Darwin's. We can hardly realise now the momentous effect on the scientific
thought of the day of the announcement of the new theory. Years afterwards
(1882) Bentham, notwithstanding his habitual restraint, could not write of
it without emotion. "I was forced, however reluctantly, to give up my
long-cherished convictions, the results of much labour and study." The
revelation came without preparation. Darwin, he wrote, "never made any
communications to me in relation to his views and labours." But, he adds,
I...fully adopted his theories and conclusions, notwithstanding the severe
pain and disappointment they at first occasioned me." ("Life and Letters",
II. page 294.) Scientific history can have few incidents more worthy. I
do not know what is most striking in the story, the pathos or the moral
dignity of Bentham's attitude.
Darwin necessarily restricted himself in the "Origin" to establishing the
general principles which would account for the facts of distribution, as a
part of his larger argument, without attempting to illustrate them in
particular cases. This he appears to have contemplated doing in a separate
work. But writing to Hooker in 1868 he said:--"I shall to the day of my
death keep up my full interest in Geographical Distribution, but I doubt
whether I shall ever have strength to come in any fuller detail than in the
"Origin" to this grand subject." ("More Letters", II. page 7.) This must
be always a matter for regret. But we may gather some indication of his
later speculations from the letters, the careful publication of which by F.
Darwin has rendered a service to science, the value of which it is
difficult to exaggerate. They admit us to the workshop, where we see a
great theory, as it were, in the making. The later ideas that they contain
were not it is true public property at the time. But they were
communicated to the leading biologists of the day and indirectly have had a
large influence.
If Darwin laid the foundation, the present fabric of Botanical Geography
must be credited to Hooker. It was a happy partnership. The far-seeing,
generalising power of the one was supplied with data and checked in
conclusions by the vast detailed knowledge of the other. It may be
permitted to quote Darwin's generous acknowledgment when writing the
"Origin":--"I never did pick any one's pocket, but whilst writing my
present chapter I keep on feeling (even when differing most from you) just
as if I were stealing from you, so much do I owe to your writings and
conversation, so much more than mere acknowledgements show." ("Life and
Letters", II. page 148 (footnote).) Fourteen years before he had written
to Hooker: "I know I shall live to see you the first authority in Europe
on...Geographical Distribution." (Ibid. I. page 336.) We owe it to Hooker
that no one now undertakes the flora of a country without indicating the
range of the species it contains. Bentham tells us: "After De Candolle,
independently of the great works of Darwin...the first important addition
to the science of geographical botany was that made by Hooker in his
"Introductory Essay to the Flora of Tasmania", which, though
contemporaneous only with the "Origin of Species", was drawn up with a
general knowledge of his friend's observations and views." (Pres. Addr.
(1869), "Proc. Linn. Soc." 1868-69, page lxxiv.) It cannot be doubted that
this and the great memoir on the "Distribution of Arctic Plants" were only
less epoch-making than the "Origin" itself, and must have supplied a
powerful support to the general theory of organic evolution.
Darwin always asserted his "entire ignorance of Botany." ("More Letters",
I. page 400.) But this was only part of his constant half-humorous self-
depreciation. He had been a pupil of Henslow, and it is evident that he
had a good working knowledge of systematic botany. He could find his way
about in the literature and always cites the names of plants with
scrupulous accuracy. It was because he felt the want of such a work for
his own researches that he urged the preparation of the "Index Kewensis",
and undertook to defray the expense. It has been thought singular that he
should have been elected a "correspondant" of the Academie des Sciences in
the section of Botany, but it is not surprising that his work in
Geographical Botany made the botanists anxious to claim him. His heart
went with them. "It has always pleased me," he tells us, "to exalt plants
in the scale of organised beings." ("Life and Letters", I. page 98.) And
he declares that he finds "any proposition more easily tested in botanical
works (Ibid. II. page 99.) than in zoological."
In the "Introductory Essay" Hooker dwelt on the "continuous current of
vegetation from Scandinavia to Tasmania" ("Introductory Essay to the Flora
of Tasmania", London, 1859. Reprinted from the "Botany of the Antarctic
Expedition", Part III., "Flora of Tasmania", Vol I. page ciii.), but finds
little evidence of one in the reverse direction. "In the New World,
Arctic, Scandinavian, and North American genera and species are
continuously extended from the north to the south temperate and even
Antarctic zones; but scarcely one Antarctic species, or even genus advances
north beyond the Gulf of Mexico" (page civ.). Hooker considered that this
negatived "the idea that the Southern and Northern Floras have had common
origin within comparatively modern geological epochs." (Loc. cit.) This
is no doubt a correct conclusion. But it is difficult to explain on
Darwin's view alone, of alternating cold in the two hemispheres, the
preponderant migration from the north to the south. He suggests,
therefore, that it "is due to the greater extent of land in the north and
to the northern forms...having...been advanced through natural selection
and competition to a higher stage of perfection or dominating power."
("Origin of Species" (6th edition), page 340; cf. also "Life and Letters",
II. page 142.) The present state of the Flora of New Zealand affords a
striking illustration of the correctness of this view. It is poor in
species, numbering only some 1400, of which three-fourths are endemic.
They seem however quite unable to resist the invasion of new comers and
already 600 species of foreign origin have succeeded in establishing
themselves.
If we accept the general configuration of the earth's surface as permanent
a continuous and progressive dispersal of species from the centre to the
circumference, i.e. southwards, seems inevitable. If an observer were
placed above a point in St George's Channel from which one half of the
globe was visible he would see the greatest possible quantity of land
spread out in a sort of stellate figure. The maritime supremacy of the
English race has perhaps flowed from the central position of its home.
That such a disposition would facilitate a centrifugal migration of land
organisms is at any rate obvious, and fluctuating conditions of climate
operating from the pole would supply an effective means of propulsion. As
these became more rigorous animals at any rate would move southwards to
escape them. It would be equally the case with plants if no insuperable
obstacle interposed. This implies a mobility in plants, notwithstanding
what we know of means of transport which is at first sight paradoxical.
Bentham has stated this in a striking way: "Fixed and immovable as is the
individual plant, there is no class in which the race is endowed with
greater facilities for the widest dispersion...Plants cast away their
offspring in a dormant state, ready to be carried to any distance by those
external agencies which we may deem fortuitous, but without which many a
race might perish from the exhaustion of the limited spot of soil in which
it is rooted." (Pres. Addr.(1869), "Proc. Linn. Soc." 1868-69, pages lxvi,
lxvii.)
I have quoted this passage from Bentham because it emphasises a point which
Darwin for his purpose did not find it necessary to dwell upon, though he
no doubt assumed it. Dispersal to a distance is, so to speak, an
accidental incident in the life of a species. Lepidium Draba, a native of
South-eastern Europe, owes its prevalence in the Isle of Thanet to the
disastrous Walcheren expedition; the straw-stuffing of the mattresses of
the fever-stricken soldiers who were landed there was used by a farmer for
manure. Sir Joseph Hooker ("Royal Institution Lecture", April 12, 1878.)
tells us that landing on Lord Auckland's Island, which was uninhabited,
"the first evidence I met with of its having been previously visited by man
was the English chickweed; and this I traced to a mound that marked the
grave of a British sailor, and that was covered with the plant, doubtless
the offspring of seed that had adhered to the spade or mattock with which
the grave had been dug."
Some migration from the spot where the individuals of a species have
germinated is an essential provision against extinction. Their descendants
otherwise would be liable to suppression by more vigorous competitors. But
they would eventually be extinguished inevitably, as pointed out by
Bentham, by the exhaustion of at any rate some one necessary constituent of
the soil. Gilbert showed by actual analysis that the production of a
"fairy ring" is simply due to the using up by the fungi of the available
nitrogen in the enclosed area which continually enlarges as they seek a
fresh supply on the outside margin. Anyone who cultivates a garden can
easily verify the fact that every plant has some adaptation for varying
degrees of seed-dispersal. It cannot be doubted that slow but persistent
terrestrial migration has played an enormous part in bringing about
existing plant-distribution, or that climatic changes would intensify the
effect because they would force the abandonment of a former area and the
occupation of a new one. We are compelled to admit that as an incident of
the Glacial period a whole flora may have moved down and up a mountain
side, while only some of its constituent species would be able to take
advantage of means of long-distance transport.
I have dwelt on the importance of what I may call short-distance dispersal
as a necessary condition of plant life, because I think it suggests the
solution of a difficulty which leads Guppy to a conclusion with which I am
unable to agree. But the work which he has done taken as a whole appears
to me so admirable that I do so with the utmost respect. He points out, as
Bentham had already done, that long-distance dispersal is fortuitous. And
being so it cannot have been provided for by previous adaptation. He says
(Guppy, op. cit. II. page 99.): "It is not conceivable that an organism
can be adapted to conditions outside its environment." To this we must
agree; but, it may be asked, do the general means of plant dispersal
violate so obvious a principle? He proceeds: "The great variety of the
modes of dispersal of seeds is in itself an indication that the dispersing
agencies avail themselves in a hap-hazard fashion of characters and
capacities that have been developed in other connections." (Loc. cit. page
102.) "Their utility in these respects is an accident in the plant's
life." (Loc. cit. page 100.) He attributes this utility to a "determining
agency," an influence which constantly reappears in various shapes in the
literature of Evolution and is ultra-scientific in the sense that it bars
the way to the search for material causes. He goes so far as to doubt
whether fleshy fruits are an adaptation for the dispersal of their
contained seeds. (Loc. cit. page 102.) Writing as I am from a hillside
which is covered by hawthorn bushes sown by birds, I confess I can feel
little doubt on the subject myself. The essential fact which Guppy brings
out is that long-distance unlike short-distance dispersal is not universal
and purposeful, but selective and in that sense accidental. But it is not
difficult to see how under favouring conditions one must merge into the
other.
Guppy has raised one novel point which can only be briefly referred to but
which is of extreme interest. There are grounds for thinking that flowers
and insects have mutually reacted upon one another in their evolution.
Guppy suggests that something of the same kind may be true of birds. I
must content myself with the quotation of a single sentence. "With the
secular drying of the globe and the consequent differentiation of climate
is to be connected the suspension to a great extent of the agency of birds
as plant dispersers in later ages, not only in the Pacific Islands but all
over the tropics. The changes of climate, birds and plants have gone on
together, the range of the bird being controlled by the climate, and the
distribution of the plant being largely dependent on the bird." (Loc.cit.
II. page 221.)
Darwin was clearly prepared to go further than Hooker in accounting for the
southern flora by dispersion from the north. Thus he says: "We must, I
suppose, admit that every yard of land has been successively covered with a
beech-forest between the Caucasus and Japan." ("More Letters", II. page
9.) Hooker accounted for the dissevered condition of the southern flora by
geographical change, but this Darwin could not admit. He suggested to
Hooker that the Australian and Cape floras might have had a point of
connection through Abyssinia (Ibid. I. page 447.), an idea which was
promptly snuffed out. Similarly he remarked to Bentham (1869): "I suppose
you think that the Restiaceae, Proteaceae, etc., etc. once extended over
the whole world, leaving fragments in the south." (Ibid. I. page 380.)
Eventually he conjectured "that there must have been a Tertiary Antarctic
continent, from which various forms radiated to the southern extremities of
our present continents." ("Life and Letters", III. page 231.) But
characteristically he could not admit any land connections and trusted to
"floating ice for transporting seed." ("More Letters", I. page 116.) I am
far from saying that this theory is not deserving of serious attention,
though there seems to be no positive evidence to support it, and it
immediately raises the difficulty how did such a continent come to be
stocked?
We must, however, agree with Hooker that the common origin of the northern
and southern floras must be referred to a remote past. That Darwin had
this in his mind at the time of the publication of the "Origin" is clear
from a letter to Hooker. "The view which I should have looked at as
perhaps most probable (though it hardly differs from yours) is that the
whole world during the Secondary ages was inhabited by marsupials,
araucarias (Mem.--Fossil wood of this nature in South America), Banksia,
etc.; and that these were supplanted and exterminated in the greater area
of the north, but were left alive in the south." (Ibid. I. page 453.)
Remembering that Araucaria, unlike Banksia, belongs to the earlier Jurassic
not to the angiospermous flora, this view is a germinal idea of the widest
generality.
The extraordinary congestion in species of the peninsulas of the Old World
points to the long-continued action of a migration southwards. Each is in
fact a cul-de-sac into which they have poured and from which there is no
escape. On the other hand the high degree of specialisation in the
southern floras and the little power the species possess of holding their
own in competition or in adaptation to new conditions point to long-
continued isolation. "An island...will prevent free immigration and
competition, hence a greater number of ancient forms will survive." (Ibid.
I. page 481.) But variability is itself subject to variation. The nemesis
of a high degree of protected specialisation is the loss of adaptability.
(See Lyell, "The Geological Evidences of the Antiquity of Man", London,
1863, page 446.) It is probable that many elements of the southern flora
are doomed: there is, for example, reason to think that the singular
Stapelieae of S. Africa are a disappearing group. The tree Lobelias which
linger in the mountains of Central Africa, in Tropical America and in the
Sandwich Islands have the aspect of extreme antiquity. I may add a further
striking illustration from Professor Seward: "The tall, graceful fronds of
Matonia pectinata, forming miniature forests on the slopes of Mount Ophir
and other districts in the Malay Peninsula in association with Dipteris
conjugata and Dipteris lobbiana, represent a phase of Mesozoic life which
survives
'Like a dim picture of the drowned past.'" ("Report of the 73rd Meeting of
the British Assoc." (Southport, 1903), London, 1904, page 844.)
The Matonineae are ferns with an unusually complex vascular system and were
abundant "in the northern hemisphere during the earlier part of the
Mesozoic era."
It was fortunate for science that Wallace took up the task which his
colleague had abandoned. Writing to him on the publication of his
"Geographical Distribution of Animals" Darwin said: "I feel sure that you
have laid a broad and safe foundation for all future work on Distribution.
How interesting it will be to see hereafter plants treated in strict
relation to your views." ("More Letters", II. page 12.) This hope was
fulfilled in "Island Life". I may quote a passage from it which admirably
summarises the contrast between the northern and the southern floras.
"Instead of the enormous northern area, in which highly organised and
dominant groups of plants have been developed gifted with great colonising
and aggressive powers, we have in the south three comparatively small and
detached areas, in which rich floras have been developed with SPECIAL
adaptations to soil, climate, and organic environment, but comparatively
impotent and inferior beyond their own domain." (Wallace, "Island Life",
pages 527, 528.)
It will be noticed that in the summary I have attempted to give of the
history of the subject, efforts have been concentrated on bringing into
relation the temperate floras of the northern and southern hemispheres, but
no account has been taken of the rich tropical vegetation which belts the
world and little to account for the original starting-point of existing
vegetation generally. It must be remembered on the one hand that our
detailed knowledge of the floras of the tropics is still very incomplete
and far inferior to that of temperate regions; on the other hand
palaeontological discoveries have put the problem in an entirely new light.
Well might Darwin, writing to Heer in 1875, say: "Many as have been the
wonderful discoveries in Geology during the last half-century, I think none
have exceeded in interest your results with respect to the plants which
formerly existed in the arctic regions." ("More Letters", II. page 240.)
As early as 1848 Debey had described from the Upper Cretaceous rocks of
Aix-la-Chapelle Flowering plants of as high a degree of development as
those now existing. The fact was commented upon by Hooker ("Introd. Essay
to the Flora of Tasmania", page xx.), but its full significance seems to
have been scarcely appreciated. For it implied not merely that their
evolution must have taken place but the foundations of existing
distribution must have been laid in a preceding age. We now know from the
discoveries of the last fifty years that the remains of the Neocomian flora
occur over an area extending through 30 deg of latitude. The conclusion is
irresistible that within this was its centre of distribution and probably
of origin.
Darwin was immensely impressed with the outburst on the world of a fully
fledged angiospermous vegetation. He warmly approved the brilliant theory
of Saporta that this happened "as soon (as) flower-frequenting insects were
developed and favoured intercrossing." ("More Letters", II. page 21.)
Writing to him in 1877 he says: "Your idea that dicotyledonous plants were
not developed in force until sucking insects had been evolved seems to me a
splendid one. I am surprised that the idea never occurred to me, but this
is always the case when one first hears a new and simple explanation of
some mysterious phenomenon." ("Life and Letters", III. page 285.
Substantially the same idea had occurred earlier to F.W.A. Miquel.
Remarking that "sucking insects (Haustellata)...perform in nature the
important duty of maintaining the existence of the vegetable kingdom, at
least as far as the higher orders are concerned," he points our that "the
appearance in great numbers of haustellate insects occurs at and after the
Cretaceous epoch, when the plants with pollen and closed carpels
(Angiosperms) are found, and acquire little by little the preponderance in
the vegetable kingdom." "Archives Neerlandaises", III. (1868). English
translation in "Journ. of Bot." 1869, page 101.)
Even with this help the abruptness still remains an almost insoluble
problem, though a forecast of floral structure is now recognised in some
Jurassic and Lower Cretaceous plants. But the gap between this and the
structural complexity and diversity of angiosperms is enormous. Darwin
thought that the evolution might have been accomplished during a period of
prolonged isolation. Writing to Hooker (1881) he says: "Nothing is more
extraordinary in the history of the Vegetable Kingdom, as it seems to me,
than the APPARENTLY very sudden or abrupt development of the higher plants.
I have sometimes speculated whether there did not exist somewhere during
long ages an extremely isolated continent, perhaps near the South Pole."
("Life and Letters", III. page 248.)
The present trend of evidence is, however, all in favour of a northern
origin for flowering plants, and we can only appeal to the imperfection of
the geological record as a last resource to extricate us from the
difficulty of tracing the process. But Darwin's instinct that at some time
or other the southern hemisphere had played an important part in the
evolution of the vegetable kingdom did not mislead him. Nothing probably
would have given him greater satisfaction than the masterly summary in
which Seward has brought together the evidence for the origin of the
Glossopteris flora in Gondwana land.
"A vast continental area, of which remnants are preserved in Australia,
South Africa and South America...A tract of enormous extent occupying an
area, part of which has since given place to a southern ocean, while
detached masses persist as portions of more modern continents, which have
enabled us to read in their fossil plants and ice-scratched boulders the
records of a lost continent, in which the Mesozoic vegetation of the
northern continent had its birth." ("Encycl. Brit." (10th edition 1902),
Vol. XXXI. ("Palaeobotany; Mesozoic"), page 422.) Darwin would probably
have demurred on physical grounds to the extent of the continent, and
preferred to account for the transoceanic distribution of its flora by the
same means which must have accomplished it on land.
It must in fairness be added that Guppy's later views give some support to
the conjectural existence of the "lost continent." "The distribution of
the genus Dammara" (Agathis) led him to modify his earlier conclusions. He
tells us:--"In my volume on the geology of Vanua Levu it was shown that the
Tertiary period was an age of submergence in the Western Pacific, and a
disbelief in any previous continental condition was expressed. My later
view is more in accordance with that of Wichmann, who, on geological
grounds, contended that the islands of the Western Pacific were in a
continental condition during the Palaeozoic and Mesozoic periods, and that
their submergence and subsequent emergence took place in Tertiary times."
(Guppy, op. cit. II. page 304.)
The weight of the geological evidence I am unable to scrutinise. But
though I must admit the possibility of some unconscious bias in my own mind
on the subject, I am impressed with the fact that the known distribution of
the Glossopteris flora in the southern hemisphere is precisely paralleled
by that of Proteaceae and Restiaceae in it at the present time. It is not
unreasonable to suppose that both phenomena, so similar, may admit of the
same explanation. I confess it would not surprise me if fresh discoveries
in the distribution of the Glossopteris flora were to point to the
possibility of its also having migrated southwards from a centre of origin
in the northern hemisphere.
Darwin, however, remained sceptical "about the travelling of plants from
the north EXCEPT DURING THE TERTIARY PERIOD." But he added, "such
speculations seem to me hardly scientific, seeing how little we know of the
old floras." ("Life and Letters", III. page 247.) That in later
geological times the south has been the grave of the weakened offspring of
the aggressive north can hardly be doubted. But if we look to the
Glossopteris flora for the ancestry of Angiosperms during the Secondary
period, Darwin's prevision might be justified, though he has given us no
clue as to how he arrived at it.
It may be true that technically Darwin was not a botanist. But in two
pages of the "Origin" he has given us a masterly explanation of "the
relationship, with very little identity, between the productions of North
America and Europe." (Pages 333, 334.) He showed that this could be
accounted for by their migration southwards from a common area, and he told
Wallace that he "doubted much whether the now called Palaearctic and
Neartic regions ought to be separated." ("Life and Letters", III. page
230.) Catkin-bearing deciduous trees had long been seen to justify
Darwin's doubt: oaks, chestnuts, beeches, hazels, hornbeams, birches,
alders, willows and poplars are common both to the Old and New World.
Newton found that the separate regions could not be sustained for birds,
and he is now usually followed in uniting them as the Holartic. One feels
inclined to say in reading the two pages, as Lord Kelvin did to a
correspondent who asked for some further development of one of his papers,
It is all there. We have only to apply the principle to previous
geological ages to understand why the flora of the Southern United States
preserves a Cretaceous facies. Applying it still further we can understand
why, when the northern hemisphere gradually cooled through the Tertiary
period, the plants of the Eocene "suggest a comparison of the climate and
forests with those of the Malay Archipelago and Tropical America."
(Clement Reid, "Encycl. Brit." (10th edition), Vol. XXXI. ("Palaeobotany;
Tertiary"), page 435.) Writing to Asa Gray in 1856 with respect to the
United States flora, Darwin said that "nothing has surprised me more than
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