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 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.