Darwin and Modern Science

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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.