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."
XVII. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. By Hans Gadow, M.A., Ph.D.,
F.R.S.
Strickland Curator and Lecturer on Zoology in the University of
Cambridge.
The first general ideas about geographical distribution may be found
in some of the brilliant speculations contained in Buffon's "Histoire
Naturelle". The first special treatise on the subject was however
written in 1777 by E.A.W. Zimmermann, Professor of Natural Science
at Brunswick, whose large volume, "Specimen Zoologiae Geographicae
Quadrupedum"..., deals in a statistical way with the mammals; important
features of the large accompanying map of the world are the ranges
of mountains and the names of hundreds of genera indicating their
geographical range. In a second work he laid special stress on
domesticated animals with reference to the spreading of the various
races of Mankind.
In the following year appeared the "Philosophia Entomologica" by J.C.
Fabricius, who was the first to divide the world into eight regions. In
1803 G.R. Treviranus ("Biologie oder Philosophie der lebenden Natur",
Vol. II. Gottingen, 1803.) devoted a long chapter of his great work on
"Biologie" to a philosophical and coherent treatment of the distribution
of the whole animal kingdom. Remarkable progress was made in 1810 by F.
Tiedemann ("Anatomie und Naturgeschichte der Vogel". Heidelberg, 1810.)
of Heidelberg. Few, if any, of the many subsequent Ornithologists seem
to have appreciated, or known of, the ingenious way in which Tiedemann
marshalled his statistics in order to arrive at general conclusions.
There are, for instance, long lists of birds arranged in accordance
with their occurrence in one or more continents: by correlating the
distribution of the birds with their food he concludes "that the
countries of the East Indian flora have no vegetable feeders in common
with America," and "that it is probably due to the great peculiarity of
the African flora that Africa has few phytophagous kinds in common with
other countries, whilst zoophagous birds have a far more independent,
often cosmopolitan, distribution." There are also remarkable chapters
on the influence of environment, distribution, and migration, upon the
structure of the Birds! In short, this anatomist dealt with some of the
fundamental causes of distribution.
Whilst Tiedemann restricted himself to Birds, A. Desmoulins in 1822
wrote a short but most suggestive paper on the Vertebrata, omitting
the birds; he combated the view recently proposed by the entomologist
Latreille that temperature was the main factor in distribution. Some of
his ten main conclusions show a peculiar mixture of evolutionary ideas
coupled with the conception of the stability of species: whilst each
species must have started from but one creative centre, there may be
several "analogous centres of creation" so far as genera and families
are concerned. Countries with different faunas, but lying within the
same climatic zones, are proof of the effective and permanent existence
of barriers preventing an exchange between the original creative
centres.
The first book dealing with the "geography and classification" of the
whole animal kingdom was written by W. Swainson ("A Treatise on
the Geography and Classification of Animals", Lardner's "Cabinet
Cyclopaedia" London, 1835.) in 1835. He saw in the five races of Man
the clue to the mapping of the world into as many "true zoological
divisions," and he reconciled the five continents with his mystical
quinary circles.
Lyell's "Principles of Geology" should have marked a new epoch, since
in his "Elements" he treats of the past history of the globe and the
distribution of animals in time, and in his "Principles" of their
distribution in space in connection with the actual changes undergone by
the surface of the world. But as the sub-title of his great work "Modern
changes of the Earth and its inhabitants" indicates, he restricted
himself to comparatively minor changes, and, emphatically believing
in the permanency of the great oceans, his numerous and careful
interpretations of the effect of the geological changes upon the
dispersal of animals did after all advance the problem but little.
Hitherto the marine faunas had been neglected. This was remedied by E.
Forbes, who established nine homozoic zones, based mainly on the study
of the mollusca, the determining factors being to a great extent
the isotherms of the sea, whilst the 25 provinces were given by the
configuration of the land. He was followed by J.D. Dana, who, taking
principally the Crustacea as a basis, and as leading factors the mean
temperatures of the coldest and of the warmest months, established
five latitudinal zones. By using these as divisors into an American,
Afro-European, Oriental, Arctic and Antarctic realm, most of which were
limited by an eastern and western land-boundary, he arrived at about
threescore provinces.
In 1853 appeared L.K. Schmarda's ("Die geographische Verbreitung der
Thiere", Wien, 1853.) two volumes, embracing the whole subject. Various
centres of creation being, according to him, still traceable, he formed
the hypothesis that these centres were originally islands, which later
became enlarged and joined together to form the great continents, so
that the original faunas could overlap and mix whilst still remaining
pure at their respective centres. After devoting many chapters to the
possible physical causes and modes of dispersal, he divided the land
into 21 realms which he shortly characterises, e.g. Australia as the
only country inhabited by marsupials, monotremes and meliphagous birds.
Ten main marine divisions were diagnosed in a similar way. Although some
of these realms were not badly selected from the point of view of being
applicable to more than one class of animals, they were obviously too
numerous for general purposes, and this drawback was overcome, in 1857,
by P.L. Sclater. ("On the general Geographical Distribution of the
members of the class Aves", "Proc. Linn. Soc." (Zoology II. 1858, pages
130-145.)) Starting with the idea, that "each species must have been
created within and over the geographical area, which it now occupies,"
he concluded "that the most natural primary ontological divisions of the
Earth's surface" were those six regions, which since their adoption
by Wallace in his epoch-making work, have become classical. Broadly
speaking, these six regions are equivalent to the great masses of land;
they are convenient terms for geographical facts, especially since the
Palaearctic region expresses the unity of Europe with the bulk of Asia.
Sclater further brigaded the regions of the Old World as Palaeogaea and
the two Americas as Neogaea, a fundamental mistake, justifiable to a
certain extent only since he based his regions mainly upon the present
distribution of the Passerine birds.
Unfortunately these six regions are not of equal value. The Indian
countries and the Ethiopian region (Africa south of the Sahara) are
obviously nothing but the tropical, southern continuations or appendages
of one greater complex. Further, the great eastern mass of land is so
intimately connected with North America that this continent has much
more in common with Europe and Asia than with South America. Therefore,
instead of dividing the world longitudinally as Sclater had done,
Huxley, in 1868 ("On the classification and distribution of the
Alectoromorphae and Heteromorphae", "Proc. Zool. Soc." 1868, page
294.), gave weighty reasons for dividing it transversely. Accordingly
he established two primary divisions, Arctogaea or the North world in
a wider sense, comprising Sclater's Indian, African, Palaearctic and
Neartic regions; and Notogaea, the Southern world, which he divided
into (1) Austro-Columbia (an unfortunate substitute for the neotropical
region), (2) Australasia, and (3) New Zealand, the number of big regions
thus being reduced to three but for the separation of New Zealand upon
rather negative characters. Sclater was the first to accept these four
great regions and showed, in 1874 ("The geographical distribution of
Mammals", "Manchester Science Lectures", 1874.), that they were well
borne out by the present distribution of the Mammals.
Although applicable to various other groups of animals, for instance to
the tailless Amphibia and to Birds (Huxley himself had been led to found
his two fundamental divisions on the distribution of the Gallinaceous
birds), the combination of South America with Australia was gradually
found to be too sweeping a measure. The obvious and satisfactory
solution was provided by W.T. Blanford (Anniversary address (Geological
Society, 1889), "Proc. Geol. Soc." 1889-90, page 67; "Quart. Journ."
XLVI 1890.), who in 1890 recognised three main divisions, namely
Australian, South American, and the rest, for which the already existing
terms (although used partly in a new sense, as proposed by an anonymous
writer in "Natural Science", III. page 289) "Notogaea," "Neogaea" and
"Arctogaea" have been gladly accepted by a number of English writers.
After this historical survey of the search for larger and largest or
fundamental centres of animal creation, which resulted in the mapping
of the world into zoological regions and realms of after all doubtful
value, we have to return to the year 1858. The eleventh and twelfth
chapters of "The Origin of Species" (1859), dealing with "Geographical
Distribution," are based upon a great amount of observation, experiment
and reading. As Darwin's main problem was the origin of species,
nature's way of making species by gradual changes from others previously
existing, he had to dispose of the view, held universally, of the
independent creation of each species and at the same time to insist upon
a single centre of creation for each species; and in order to emphasise
his main point, the theory of descent, he had to disallow convergent, or
as they were then called, analogous forms. To appreciate the difficulty
of his position we have to take the standpoint of fifty years ago, when
the immutability of the species was an axiom and each was supposed to
have been created within or over the geographical area which it now
occupies. If he once admitted that a species could arise from many
individuals instead of from one pair, there was no way of shutting the
door against the possibility that these individuals may have been so
numerous that they occupied a very large district, even so large that
it had become as discontinuous as the distribution of many a species
actually is. Such a concession would at once be taken as an admission of
multiple, independent, origin instead of descent in Darwin's sense.
For the so-called multiple, independently repeated creation of species
as an explanation of their very wide and often quite discontinuous
distribution, he substituted colonisation from the nearest and readiest
source together with subsequent modification and better adaptation to
their new home.
He was the first seriously to call attention to the many accidental
means, "which more properly should be called occasional means of
distribution," especially to oceanic islands. His specific, even
individual, centres of creation made migrations all the more necessary,
but their extent was sadly baulked by the prevailing dogma of the
permanency of the oceans. Any number of small changes ("many islands
having existed as halting places, of which not a wreck now remains"
("The Origin of Species" (1st edition), page 396.).) were conceded
freely, but few, if any, great enough to permit migration of truly
terrestrial creatures. The only means of getting across the gaps was by
the principle of the "flotsam and jetsam," a theory which Darwin took
over from Lyell and further elaborated so as to make it applicable to
many kinds of plants and animals, but sadly deficient, often grotesque,
in the case of most terrestrial creatures.
Another very fertile source was Darwin's strong insistence upon the
great influence which the last glacial epoch must have had upon the
distribution of animals and plants. Why was the migration of northern
creatures southwards of far-reaching and most significant importance?
More northerners have established themselves in southern lands than
vice versa, because there is such a great mass of land in the north
and greater continents imply greater intensity of selection. "The
productions of real islands have everywhere largely yielded to
continental forms." (Ibid. page 380.)... "The Alpine forms have almost
everywhere largely yielded to the more dominant forms generated in the
larger areas and more efficient workshops of the North."
Let us now pass in rapid survey the influence of the publication of "The
Origin of Species" upon the study of Geographical Distribution in its
wider sense.
Hitherto the following thought ran through the minds of most writers:
Wherever we examine two or more widely separated countries their
respective faunas are very different, but where two faunas can come into
contact with each other, they intermingle. Consequently these faunas
represent centres of creation, whence the component creatures have
spread peripherally so far as existing boundaries allowed them to do so.
This is of course the fundamental idea of "regions." There is not one
of the numerous writers who considered the possibility that these
intermediate belts might represent not a mixture of species but
transitional forms, the result of changes undergone by the most
peripheral migrants in adaptation to their new surroundings. The usual
standpoint was also that of Pucheran ("Note sur l'equateur zoologique",
"Rev. et Mag. de Zoologie", 1855; also several other papers, ibid. 1865,
1866, and 1867.) in 1855. But what a change within the next ten years!
Pucheran explains the agreement in coloration between the desert and
its fauna as "une harmonie post-etablie"; the Sahara, formerly a marine
basin, was peopled by immigrants from the neighbouring countries, and
these new animals adapted themselves to the new environment. He also
discusses, among other similar questions, the Isthmus of Panama with
regard to its having once been a strait. From the same author may be
quoted the following passage as a strong proof of the new influence:
"By the radiation of the contemporaneous faunas, each from one centre,
whence as the various parts of the world successively were formed and
became habitable, they spread and became modified according to the local
physical conditions."
The "multiple" origin of each species as advocated by Sclater and
Murray, although giving the species a broader basis, suffered from the
same difficulties. There was only one alternative to the old
orthodox view of independent creation, namely the bold acceptance of
land-connections to an extent for which geological and palaeontological
science was not yet ripe. Those who shrank from either view, gave up
the problem as mysterious and beyond the human intellect. This was the
expressed opinion of men like Swainson, Lyell and Humboldt. Only Darwin
had the courage to say that the problem was not insoluble. If we admit
"that in the long course of time the individuals of the same species,
and likewise of allied species, have proceeded from some one source;
then I think 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." We can thus
understand how it is that in some countries the inhabitants "are linked
to the extinct beings which formerly inhabited the same continent."
We can see why two areas, having nearly the same physical conditions,
should often be inhabited by very different forms of life,... and "we can
see why in two areas, however distant from each other, there should be a
correlation, in the presence of identical species... and of distinct but
representative species." ("The Origin of Species" (1st edition), pages
408, 409.)
Darwin's reluctance to assume great geological changes, such as a
land-connection of Europe with North America, is easily explained by the
fact that he restricted himself to the distribution of the present and
comparatively recent species. "I do not believe that it will ever be
proved that within the recent period continents which are now quite
separate, have been continuously, or almost continuously, united with
each other, and with the many existing oceanic islands." (Ibid. page
357.) Again, "believing... that our continents have long remained in
nearly the same relative position, though subjected to large, but
partial oscillations of level," that means to say within the period of
existing species, or "within the recent period." (Ibid. page. 370.) The
difficulty was to a great extent one of his own making. Whilst almost
everybody else believed in the immutability of the species, which
implies an enormous age, logically since the dawn of creation, to him
the actually existing species as the latest results of evolution, were
necessarily something very new, so young that only the very latest of
the geological epochs could have affected them. It has since come to
our knowledge that a great number of terrestrial "recent" species, even
those of the higher classes of Vertebrates, date much farther back than
had been thought possible. Many of them reach well into the Miocene, a
time since which the world seems to have assumed the main outlines of
the present continents.
In the year 1866 appeared A. Murray's work on the "Geographical
Distribution of Mammals", a book which has perhaps received less
recognition than it deserves. His treatment of the general introductory
questions marks a considerable advance of our problem, although, and
partly because, he did not entirely agree with Darwin's views as laid
down in the first edition of "The Origin of Species", which after all
was the great impulse given to Murray's work. Like Forbes he did not
shrink from assuming enormous changes in the configuration of the
continents and oceans because the theory of descent, with its necessary
postulate of great migrations, required them. He stated, for instance,
"that a Miocene Atlantis sufficiently explains the common distribution
of animals and plants in Europe and America up to the glacial epoch."
And next he considers how, and by what changes, the rehabilitation and
distribution of these lands themselves were effected subsequent to
that period. Further, he deserves credit for having cleared up a
misunderstanding of the idea of specific centres of creation. Whilst for
instance Schmarda assumed without hesitation that the same species,
if occurring at places separated by great distances, or apparently
insurmountable barriers, had been there created independently (multiple
centres), Lyell and Darwin held that each species had only one single
centre, and with this view most of us agree, but their starting point
was to them represented by one individual, or rather one single pair.
According to Murray, on the other hand, this centre of a species is
formed by all the individuals of a species, all of which equally undergo
those changes which new conditions may impose upon them. In this respect
a new species has a multiple origin, but this in a sense very different
from that which was upheld by L. Agassiz. As Murray himself puts it: "To
my multiple origin, communication and direct derivation is essential.
The species is compounded of many influences brought together through
many individuals, and distilled by Nature into one species; and, being
once established it may roam and spread wherever it finds the conditions
of life not materially different from those of its original centre."
(Murray, "The Geographical Distribution of Mammals", page 14. London,
1866.) This declaration fairly agrees with more modern views, and
it must be borne in mind that the application of the single-centre
principle to the genera, families and larger groups in the search for
descent inevitably leads to one creative centre for the whole animal
kingdom, a condition as unwarrantable as the myth of Adam and Eve being
the first representatives of Mankind.
It looks as if it had required almost ten years for "The Origin
of Species" to show its full effect, since the year 1868 marks the
publication of Haeckel's "Naturliche Schoepfungsgeschichte" in addition
to other great works. The terms "Oecology" (the relation of organisms
to their environment) and "Chorology" (their distribution in space) had
been given us in his "Generelle Morphologie" in 1866. The fourteenth
chapter of the "History of Creation" is devoted to the distribution of
organisms, their chorology, with the emphatic assertion that "not
until Darwin can chorology be spoken of as a separate science, since
he supplied the acting causes for the elucidation of the hitherto
accumulated mass of facts." A map (a "hypothetical sketch") shows the
monophyletic origin and the routes of distribution of Man.
Natural Selection may be all-mighty, all-sufficient, but it requires
time, so much that the countless aeons required for the evolution of the
present fauna were soon felt to be one of the most serious drawbacks of
the theory. Therefore every help to ease and shorten this process should
have been welcomed. In 1868 M. Wagner (The first to formulate clearly
the fundamental idea of a theory of migration and its importance in
the origin of new species was L. von Buch, who in his "Physikalische
Beschreibung der Canarischen Inseln", written in 1825, wrote as follows:
"Upon the continents the individuals of the genera by spreading far,
form, through differences of the locality, food and soil, varieties
which finally become constant as new species, since owing to the
distances they could never be crossed with other varieties and thus
be brought back to the main type. Next they may again, perhaps upon
different roads, return to the old home where they find the old
type likewise changed, both having become so different that they can
interbreed no longer. Not so upon islands, where the individuals shut
up in narrow valleys or within narrow districts, can always meet one
another and thereby destroy every new attempt towards the fixing of a
new variety." Clearly von Buch explains here why island types remain
fixed, and why these types themselves have become so different from
their continental congeners.--Actually von Buch is aware of a most
important point, the difference in the process of development which
exists between a new species b, which is the result of an ancestral
species a having itself changed into b and thereby vanished itself, and
a new species c which arose through separation out of the same ancestral
a, which itself persists as such unaltered. Von Buch's prophetic view
seems to have escaped Lyell's and even Wagner's notice.) came to the
rescue with his "Darwin'sche Theorie und das Migrations-Gesetz der
Organismen". (Leipzig, 1868.) He shows that migration, i.e. change of
locality, implies new environmental conditions (never mind whether
these be new stimuli to variation, or only acting as their selectors or
censors), and moreover secures separation from the original stock and
thus eliminates or lessens the reactionary dangers of panmixia. Darwin
accepted Wagner's theory as "advantageous." Through the heated polemics
of the more ardent selectionists Wagner's theory came to grow into an
alternative instead of a help to the theory of selectional evolution.
Separation is now rightly considered a most important factor by modern
students of geographical distribution.
For the same year, 1868, we have to mention Huxley, whose Arctogaea and
Notogaea are nothing less than the reconstructed main masses of land
of the Mesozoic period. Beyond doubt the configuration of land at that
remote period has left recognisable traces in the present continents,
but whether they can account for the distribution of such a much later
group as the Gallinaceous birds is more than questionable. In any case
he took for his text a large natural group of birds, cosmopolitan as
a whole, but with a striking distribution. The Peristeropodes, or
pigeon-footed division, are restricted to the Australian and Neotropical
regions, in distinction to the Alectoropodes (with the hallux inserted
at a level above the front toes) which inhabit the whole of the
Arctogaea, only a few members having spread into the South World.
Further, as Asia alone has its Pheasants and allies, so is Africa
characterised by its Guinea-fowls and relations, America has the Turkey
as an endemic genus, and the Grouse tribe in a wider sense has its
centre in the holarctic region: a splendid object lesson of descent,
world-wide spreading and subsequent differentiation. Huxley, by the way,
was the first--at least in private talk--to state that it will be for
the morphologist, the well-trained anatomist, to give the casting vote
in questions of geographical distribution, since he alone can determine
whether we have to deal with homologous, or analogous, convergent,
representative forms.
It seems late to introduce Wallace's name in 1876, the year of the
publication of his standard work. ("The Geographical Distribution of
Animals", 2 vols. London, 1876.) We cannot do better than quote the
author's own words, expressing the hope that his "book should bear a
similar relation to the eleventh and twelfth chapters of the "Origin of
Species" as Darwin's "Animals and Plants under Domestication" does to
the first chapter of that work," and to add that he has amply succeeded.
Pleading for a few primary centres he accepts Sclater's six regions and
does not follow Huxley's courageous changes which Sclater himself had
accepted in 1874. Holding the view of the permanence of the oceans he
accounts for the colonisation of outlying islands by further elaborating
the views of Lyell and Darwin, especially in his fascinating "Island
Life", with remarkable chapters on the Ice Age, Climate and Time and
other fundamental factors. His method of arriving at the degree
of relationship of the faunas of the various regions is eminently
statistical. Long lists of genera determine by their numbers the
affinity and hence the source of colonisation. In order to make sure
of his material he performed the laborious task of evolving a new
classification of the host of Passerine birds. This statistical method
has been followed by many authors, who, relying more upon quantity than
quality, have obscured the fact that the key to the present distribution
lies in the past changes of the earth's surface. However, with Wallace
begins the modern study of the geographical distribution of animals and
the sudden interest taken in this subject by an ever widening circle of
enthusiasts far beyond the professional brotherhood.
A considerable literature has since grown up, almost bewildering in its
range, diversity of aims and style of procedure. It is a chaos, with
many paths leading into the maze, but as yet very few take us to a
position commanding a view of the whole intricate terrain with its
impenetrable tangle and pitfalls.
One line of research, not initiated but greatly influenced by Wallace's
works, became so prominent as to almost constitute a period which may
be characterised as that of the search by specialists for either the
justification or the amending of his regions. As class after class of
animals was brought up to reveal the secret of the true regions, some
authors saw in their different results nothing but the faultiness of
previously established regions; others looked upon eventual agreements
as their final corroboration, especially when for instance such diverse
groups as mammals and scorpions could, with some ingenuity, be made to
harmonise. But the obvious result of all these efforts was the growing
knowledge that almost every class seemed to follow principles of its
own. The regions tallied neither in extent nor in numbers, although
most of them gravitated more and more towards three centres, namely
Australia, South America and the rest of the world. Still zoologists
persisted in the search, and the various modes and capabilities of
dispersal of the respective groups were thought sufficient explanation
of the divergent results in trying to bring the mapping of the world
under one scheme.
Contemporary literature is full of devices for the mechanical dispersal
of animals. Marine currents, warm and cold, were favoured all the
more since they showed the probable original homes of the creatures in
question. If these could not stand sea-water, they floated upon logs or
icebergs, or they were blown across by storms; fishes were lifted over
barriers by waterspouts, and there is on record even an hypothetical
land tortoise, full of eggs, which colonised an oceanic island after a
perilous sea voyage upon a tree trunk. Accidents will happen, and beyond
doubt many freaks of discontinuous distribution have to be accounted
for by some such means. But whilst sufficient for the scanty settlers of
true oceanic islands, they cannot be held seriously to account for the
rich fauna of a large continent, over which palaeontology shows us that
the immigrants have passed like waves. It should also be borne in mind
that there is a great difference between flotsam and jetsam. A current
is an extension of the same medium and the animals in it may suffer no
change during even a long voyage, since they may be brought from one
litoral to another where they will still be in the same or but slightly
altered environment. But the jetsam is in the position of a passenger
who has been carried off by the wrong train. Almost every year some
American land birds arrive at our western coasts and none of them have
gained a permanent footing although such visits must have taken place
since prehistoric times. It was therefore argued that only those groups
of animals should be used for locating and defining regions which were
absolutely bound to the soil. This method likewise gave results not
reconcilable with each other, even when the distribution of fossils
was taken into account, but it pointed to the absolute necessity of
searching for former land-connections regardless of their extent and the
present depths to which they may have sunk.
That the key to the present distribution lies in the past had been felt
long ago, but at last it was appreciated that the various classes of
animals and plants have appeared in successive geological epochs and
also at many places remote from each other. The key to the distribution
of any group lies in the configuration of land and water of that epoch
in which it made its first appearance. Although this sounds like a
platitude, it has frequently been ignored. If, for argument's sake,
Amphibia were evolved somewhere upon the great southern land-mass of
Carboniferous times (supposed by some to have stretched from South
America across Africa to Australia), the distribution of this developing
class must have proceeded upon lines altogether different from that of
the mammals which dated perhaps from lower Triassic times, when the old
south continental belt was already broken up. The broad lines of this
distribution could never coincide with that of the other, older class,
no matter whether the original mammalian centre was in the Afro-Indian,
Australian, or Brazilian portion. If all the various groups of animals
had come into existence at the same time and at the same place, then it
would be possible, with sufficient geological data, to construct a map
showing the generalised results applicable to the whole animal kingdom.
But the premises are wrong. Whatever regions we may seek to establish
applicable to all classes, we are necessarily mixing up several
principles, namely geological, historical, i.e. evolutionary, with
present day statistical facts. We might as well attempt one compound
picture representing a chick's growth into an adult bird and a child's
growth into manhood.
In short there are no general regions, not even for each class
separately, unless this class be one which is confined to a
comparatively short geological period. Most of the great classes have
far too long a history and have evolved many successive main groups.
Let us take the mammals. Marsupials live now in Australia and in both
Americas, because they already existed in Mesozoic times; Ungulata
existed at one time or other all over the world except in Australia,
because they are post-Cretaceous; Insectivores, although as old as any
Placentalia, are cosmopolitan excepting South America and Australia;
Stags and Bears, as examples of comparatively recent Arctogaeans, are
found everywhere with the exception of Ethiopia and Australia. Each of
these groups teaches a valuable historical lesson, but when these are
combined into the establishment of a few mammalian "realms," they mean
nothing but statistical majorities. If there is one at all, Australia is
such a realm backed against the rest of the world, but as certainly it
is not a mammalian creative centre!
Well then, if the idea of generally applicable regions is a mare's nest,
as was the search for the Holy Grail, what is the object of the study
of geographical distribution? It is nothing less than the history of the
evolution of life in space and time in the widest sense. The attempt to
account for the present distribution of any group of organisms involves
the aid of every branch of science. It bids fair to become a history of
the world. It started in a mild, statistical way, restricting itself to
the present fauna and flora and to the present configuration of land
and water. Next came Oceanography concerned with the depths of the seas,
their currents and temperatures; then enquiries into climatic changes,
culminating in irreconcilable astronomical hypotheses as to glacial
epochs; theories about changes of the level of the seas, mainly from the
point of view of the physicist and astronomer. Then came more and more
to the front the importance of the geological record, hand in hand with
the palaeontological data and the search for the natural affinities, the
genetic system of the organisms. Now and then it almost seems as if the
biologists had done their share by supplying the problems and that the
physicists and geologists would settle them, but in reality it is not
so. The biologists not only set the problems, they alone can check the
offered solutions. The mere fact of palms having flourished in Miocene
Spitzbergen led to an hypothetical shifting of the axis of the world
rather than to the assumption, by way of explanation, that the palms
themselves might have changed their nature. One of the most valuable
aids in geological research, often the only means for reconstructing the
face of the earth in by-gone periods, is afforded by fossils, but only
the morphologist can pronounce as to their trustworthiness as witnesses,
because of the danger of mistaking analogous for homologous forms. This
difficulty applies equally to living groups, and it is so important that
a few instances may not be amiss.
There is undeniable similarity between the faunas of Madagascar and
South America. This was supported by the Centetidae and Dendrobatidae,
two entire "families," as also by other facts. The value of the
Insectivores, Solenodon in Cuba, Centetes in Madagascar, has been much
lessened by their recognition as an extremely ancient group and as a
case of convergence, but if they are no longer put into the same
family, this amendment is really to a great extent due to their widely
discontinuous distribution. The only systematic difference of the
Dendrobatidae from the Ranidae is the absence of teeth, morphologically
a very unimportant character, and it is now agreed, on the strength of
their distribution, that these little arboreal, conspicuously coloured
frogs, Dendrobates in South America, Mantella in Madagascar, do not form
a natural group, although a third genus, Cardioglossa in West Africa,
seems also to belong to them. If these creatures lived all on the
same continent, we should unhesitatingly look upon them as forming a
well-defined, natural little group. On the other hand the Aglossa, with
their three very divergent genera, namely Pipa in South America, Xenopus
and Hymenochirus in Africa, are so well characterised as one ancient
group that we use their distribution unhesitatingly as a hint of a
former connection between the two continents. We are indeed arguing in
vicious circles. The Ratitae as such are absolutely worthless since they
are a most heterogeneous assembly, and there are untold groups, of
the artificiality of which many a zoo-geographer had not the slightest
suspicion when he took his statistical material, the genera and
families, from some systematic catalogues or similar lists. A lamentable
instance is that of certain flightless Rails, recently extinct or
sub-fossil, on the isalnds of Mauritius, Rodriguez and Chatham. Being
flightless they have been used in support of a former huge Antarctic
continent, instead of ruling them out of court as Rails which, each in
its island, have lost the power of flight, a process which must have
taken place so recently that it is difficult, upon morphological
grounds, to justify their separation into Aphanapteryx in Mauritius,
Erythromachus in Rodriguez and Diaphorapteryx on Chatham Island.
Morphologically they may well form but one genus, since they have sprung
from the same stock and have developed upon the same lines; they are
therefore monogenetic: but since we know that they have become what they
are independently of each other (now unlike any other Rails), they are
polygenetic and therefore could not form one genus in the old Darwinian
sense. Further, they are not a case of convergence, since their ancestry
is not divergent but leads into the same stratum.