Darwin and Modern Science (1909)
Edited by A.C. Seward
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
the greater generic and specific affinity with East Asia than with West
America." ("More Letters", I. page 434.) The recent discoveries of a
Tulip tree and a Sassafras in China afford fresh illustrations. A few
years later Asa Gray found the explanation in both areas being centres of
preservation of the Cretaceous flora from a common origin. It is
interesting to note that the paper in which this was enunciated at once
established his reputation.
In Europe the latitudinal range of the great mountain chains gave the
Miocene flora no chance of escape during the Glacial period, and the
Mediterranean appears to have equally intercepted the flow of alpine plants
to the Atlas. (John Ball in Appendix G, page 438, in "Journal of a Tour in
Morocco and the Great Atlas", J.D. Hooker and J. Ball, London, 1878.) In
Southern Europe the myrtle, the laurel, the fig and the dwarf-palm are the
sole representatives of as many great tropical families. Another great
tropical family, the Gesneraceae has left single representatives from the
Pyrenees to the Balkans; and in the former a diminutive yam still lingers.
These are only illustrations of the evidence which constantly accumulates
and which finds no rational explanation except that which Darwin has given
to it.
The theory of southward migration is the key to the interpretation of the
geographical distribution of plants. It derived enormous support from the
researches of Heer and has now become an accepted commonplace. Saporta in
1888 described the vegetable kingdom as "emigrant pour suivre une direction
determinee et marcher du nord au sud, a la recherche de regions et de
stations plus favorables, mieux appropriees aux adaptations acquises, a
meme que la temperature terrestre perd ses conditions premieres."
("Origine Paleontologique des arbres", Paris, 1888, page 28.) If, as is so
often the case, the theory now seems to be a priori inevitable, the
historian of science will not omit to record that the first germ sprang
from the brain of Darwin.
In attempting this sketch of Darwin's influence on Geographical
Distribution, I have found it impossible to treat it from an external point
of view. His interest in it was unflagging; all I could say became
necessarily a record of that interest and could not be detached from it.
He was in more or less intimate touch with everyone who was working at it.
In reading the letters we move amongst great names. With an extraordinary
charm of persuasive correspondence he was constantly suggesting,
criticising and stimulating. It is hardly an exaggeration to say that from
the quiet of his study at Down he was founding and directing a wide-world
school.
POSTSCRIPTUM.
Since this essay was put in type Dr Ernst's striking account of the "New
Flora of the Volcanic Island of Krakatau" (Cambridge, 1909.) has reached
me. All botanists must feel a debt of gratitude to Prof. Seward for his
admirable translation of a memoir which in its original form is practically
unprocurable and to the liberality of the Cambridge University Press for
its publication. In the preceding pages I have traced the laborious
research by which the methods of Plant Dispersal were established by
Darwin. In the island of Krakatau nature has supplied a crucial experiment
which, if it had occurred earlier, would have at once secured conviction of
their efficiency. A quarter of a century ago every trace of organic life
in the island was "destroyed and buried under a thick covering of glowing
stones." Now, it is "again covered with a mantle of green, the growth
being in places so luxuriant that it is necessary to cut one's way
laboriously through the vegetation." (Op. cit. page 4.) Ernst traces
minutely how this has been brought about by the combined action of wind,
birds and sea currents, as means of transport. The process will continue,
and he concludes:--"At last after a long interval the vegetation on the
desolated island will again acquire that wealth of variety and luxuriance
which we see in the fullest development which Nature has reached in the
primaeval forest in the tropics." (Op. cit. page 72.) The possibility of
such a result revealed itself to the insight of Darwin with little
encouragement or support from contemporary opinion.
One of the most remarkable facts established by Ernst is that this has not
been accomplished by the transport of seeds alone. "Tree stems and
branches played an important part in the colonisation of Krakatau by plants
and animals. Large piles of floating trees, stems, branches and bamboos
are met with everywhere on the beach above high-water mark and often
carried a considerable distance inland. Some of the animals on the island,
such as the fat Iguana (Varanus salvator) which suns itself in the beds of
streams, may have travelled on floating wood, possibly also the ancestors
of the numerous ants, but certainly plants." (Op. cit. page 56.) Darwin
actually had a prevision of this. Writing to Hooker he says:--"Would it
not be a prodigy if an unstocked island did not in the course of ages
receive colonists from coasts whence the currents flow, trees are drifted
and birds are driven by gales?" ("More Letters", I. page 483.) And ten
years earlier:--"I must believe in the...whole plant or branch being washed
into the sea; with floods and slips and earthquakes; this must continually
be happening." ("Life and Letters", II. pages 56, 57.) If we give to
"continually" a cosmic measure, can the fact be doubted? All this, in the
light of our present knowledge, is too obvious to us to admit of
discussion. But it seems to me nothing less than pathetic to see how in
the teeth of the obsession as to continental extension, Darwin fought
single-handed for what we now know to be the truth.
Guppy's heart failed him when he had to deal with the isolated case of
Agathis which alone seemed inexplicable by known means of transport. But
when we remember that it is a relic of the pre-Angiospermous flora, and is
of Araucarian ancestry, it cannot be said that the impossibility, in so
prolonged a history, of the bodily transference of cone-bearing branches or
even of trees, compels us as a last resort to fall back on continental
extension to account for its existing distribution.
When Darwin was in the Galapagos Archipelago, he tells us that he fancied
himself "brought near to the very act of creation." He saw how new species
might arise from a common stock. Krakatau shows us an earlier stage and
how by simple agencies, continually at work, that stock might be supplied.
It also shows us how the mixed and casual elements of a new colony enter
into competition for the ground and become mutually adjusted. The study of
Plant Distribution from a Darwinian standpoint has opened up a new field of
research in Ecology. The means of transport supply the materials for a
flora, but their ultimate fate depends on their equipment for the "struggle
for existence." The whole subject can no longer be regarded as a mere
statistical inquiry which has seemed doubtless to many of somewhat arid
interest. The fate of every element of the earth's vegetation has sooner
or later depended on its ability to travel and to hold its own under new
conditions. And the means by which it has secured success is an each case
a biological problem which demands and will reward the most attentive
study. This is the lesson which Darwin has bequeathed to us. It is summed
up in the concluding paragraph of the "Origin" ("Origin of Species" (6th
edition), page 429.):--"It is interesting to contemplate a tangled bank,
clothed with many plants of many kinds, with birds singing on the bushes,
with various insects flitting about, and with worms crawling through the
damp earth, and to reflect that these elaborately constructed forms, so
different from each other, and dependent upon each other in so complex a
manner, have all been produced by laws acting around us."
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