Darwin and Modern Science (1909)
Edited by A.C. Seward
XVII. GEOGRAPHICAL DISTRIBUTION OF ANIMALS.
By By HANS GADOW, M.A., Ph.D., F.R.S.
Strickland Curator and Lecturer on Zoology in the University of Cambridge.
he 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.
THE RECONSTRUCTION OF THE GEOGRAPHY OF SUCCESSIVE EPOCHS.
A promising method is the study by the specialist of a large, widely
distributed group of animals from an evolutionary point of view. Good
examples of this method are afforded by A.E. Ortmann's ("The geographical
distribution of Freshwater Decapods and its bearing upon ancient
geography", "Proc. Amer. Phil. Soc." Vol. 41, 1902.) exhaustive paper and
by A.W. Grabau's "Phylogeny of Fusus and its Allies" ("Smithsonian Misc.
Coll." 44, 1904.) After many important groups of animals have been treated
in this way--as yet sparingly attempted--the results as to hypothetical
land-connections etc. are sure to be corrective and supplementary, and
their problems will be solved, since they are not imaginary.
The same problems are attacked, in the reverse way, by starting with the
whole fauna of a country and thence, so to speak, letting the research
radiate. Some groups will be considered as autochthonous, others as
immigrants, and the directions followed by them will be inquired into; the
search may lead far and in various directions, and by comparison of
results, by making compound maps, certain routes will assume definite
shape, and if they lead across straits and seas they are warrants to search
for land-connections in the past. (A fair sample of this method is C.H.
Eigenmann's "The Freshwater Fishes of South and Middle America", "Popular
Science Monthly", Vol. 68, 1906.) There are now not a few maps purporting
to show the outlines of land and water at various epochs. Many of these
attempts do not tally with each other, owing to the lamentable deficiencies
of geological and fossil data, but the bolder the hypothetical outlines are
drawn, the better, and this is preferable to the insertion of bays and
similar detail which give such maps a fallacious look of certainty where
none exists. Moreover it must be borne in mind that, when we draw a broad
continental belt across an ocean, this belt need never have existed in its
entirety at any one time. The features of dispersal, intended to be
explained by it, would be accomplished just as well by an unknown number of
islands which have joined into larger complexes while elsewhere they
subsided again: like pontoon-bridges which may be opened anywhere, or like
a series of superimposed dissolving views of land and sea-scapes. Hence
the reconstructed maps of Europe, the only continent tolerably known, show
a considerable number of islands in puzzling changes, while elsewhere, e.g.
in Asia, we have to be satisfied with sweeping generalisations.
At present about half-a-dozen big connections are engaging our attention,
leaving as comparatively settled the extent and the duration of such minor
"bridges" as that between Africa and Madagascar, Tasmania and Australia,
the Antilles and Central America, Europe and North Africa. (Not a few of
those who are fascinated by, and satisfied with, the statistical aspect of
distribution still have a strong dislike to the use of "bridges" if these
lead over deep seas, and they get over present discontinuous occurrences by
a former "universal or sub-universal distribution" of their groups. This
is indeed an easy method of cutting the knot, but in reality they shunt the
question only a stage or two back, never troubling to explain how their
groups managed to attain to that sub-universal range; or do they still
suppose that the whole world was originally one paradise where everything
lived side by side, until sin and strife and glacial epochs left nothing
but scattered survivors?
The permanence of the great ocean-basins had become a dogma since it was
found that a universal elevation of the land to the extent of 100 fathoms
would produce but little changes, and when it was shown that even the 1000
fathom-line followed the great masses of land rather closely, and still
leaving the great basins (although transgression of the sea to the same
extent would change the map of the world beyond recognition), by general
consent one mile was allowed as the utmost speculative limit of subsidence.
Naturally two or three miles, the average depth of the oceans, seems
enormous, and yet such a difference in level is as nothing in comparison
with the size of the Earth. On a clay model globe ten feet in diameter an
ocean bed three miles deep would scarcely be detected, and the highest
mountains would be smaller than the unavoidable grains in the glazed
surface of our model. There are but few countries which have not be
submerged at some time or other.)
CONNECTION OF SOUTH EASTERN ASIA WITH AUSTRALIA. Neumayr's Sino-Australian
continent during mid-Mesozoic times was probably a much changing
Archipelago, with final separations subsequent to the Cretaceous period.
Henceforth Australasia was left to its own fate, but for a possible
connection with the antarctic continent.
AFRICA, MADAGASCAR, INDIA. The "Lemuria" of Sclater and Haeckel cannot
have been more than a broad bridge in Jurassic times; whether it was ever
available for the Lemurs themselves must depend upon the time of its
duration, the more recent the better, but it is difficult to show that it
lasted into the Miocene.
AFRICA AND SOUTH AMERICA. Since the opposite coasts show an entire absence
of marine fossils and deposits during the Mesozoic period, whilst further
north and south such are known to exist and are mostly identical on either
side, Neumayr suggested the existence of a great Afro-Son American mass of
land during the Jurassic epoch. Such land is almost a necessity and is
supported by many facts; it would easily explain the distribution of
numerous groups of terrestrial creatures. Moreover to the north of this
hypothetical land, somewhere across from the Antilles and Guiana to North
Africa and South Western Europe, existed an almost identical fauna of
Corals and Molluscs, indicating either a coast-line or a series of islands
interrupted by shallow seas, just as one would expect if, and when, a
Brazil-Ethiopian mass of land were breaking up. Lastly from Central
America to the Mediterranean stretches one of the Tertiary tectonic lines
of the geologists. Here also the great question is how long this continent
lasted. Apparently the South Atlantic began to encroach from the south so
that by the later Cretaceous epoch the land was reduced to a comparatively
narrow Brazil-West Africa, remnants of which persisted certainly into the
early Tertiary, until the South Atlantic joined across the equator with the
Atlantic portion of the "Thetys," leaving what remained of South America
isolated from the rest of the world.
ANTARCTIC CONNECTIONS. Patagonia and Argentina seem to have joined
Antartica during the Cretaceous epoch, and this South Georgian bridge had
broken down again by mid-Tertiary times when South America became
consolidated. The Antarctic continent, presuming that it existed, seems
also to have been joined, by way of Tasmania, with Australia, also during
the Cretaceous epoch, and it is assumed that the great Australia-Antarctic-
Patagonian land was severed first to the south of Tasmania and then at the
South Georgian bridge. No connection, and this is important, is indicated
between Antarctica and either Africa or Madagascar.
So far we have followed what may be called the vicissitudes of the great
Permo-Carboniferous Gondwana land in its fullest imaginary extent, an
enormous equatorial and south temperate belt from South America to Africa,
South India and Australia, which seems to have provided the foundation of
the present Southern continents, two of which temporarily joined
Antarctica, of which however we know nothing except that it exists now.
Let us next consider the Arctic and periarctic lands. Unfortunately very
little is known about the region within the arctic circle. If it was all
land, or more likely great changing archipelagoes, faunistic exchange
between North America, Europe and Siberia would present no difficulties,
but there is one connection which engages much attention, namely a land
where now lies the North temperate and Northern part of the Atlantic ocean.
How far south did it ever extend and what is the latest date of a direct
practicable communication, say from North Western Europe to Greenland?
Connections, perhaps often interrupted, e.g. between Greenland and
Labrador, at another time between Greenland and Scandinavia, seem to have
existed at least since the Permo-Carboniferous epoch. If they existed also
in late Cretaceous and in Tertiary times, they would of course easily
explain exchanges which we know to have repeatedly taken place between
America and Europe, but they are not proved thereby, since most of these
exchanges can almost as easily have occurred across the polar regions, and
others still more easily by repeated junction of Siberia with Alaska.
Let us now describe a hypothetical case based on the supposition of
connecting bridges. Not to work in a circle, we select an important group
which has not served as a basis for the reconstruction of bridges; and it
must be a group which we feel justified in assuming to be old enough to
have availed itself of ancient land-connections.
The occurrence of one species of Peripatus in the whole of Australia,
Tasmania and New Zealand (the latter being joined to Australia by way of
New Britain in Cretaceous times but not later) puts the genus back into
this epoch, no unsatisfactory assumption to the morphologist. The apparent
absence of Peripatus in Madagascar indicates that it did not come from the
east into Africa, that it was neither Afro-Indian, nor Afro-Australian; nor
can it have started in South America. We therefore assume as its creative
centre Australia or Malaya in the Cretaceous epoch, whence its occurrence
in Sumatra, Malay Peninsula, New Britain, New Zealand and Australia is
easily explained. Then extension across Antarctica to Patagonia and Chile,
whence it could spread into the rest of South America as this became
consolidated in early Tertiary times. For getting to the Antilles and into
Mexico it would have to wait until the Miocene, but long before that time
it could arrive in Africa, there surviving as a Congolese and a Cape
species. This story is unsupported by a single fossil. Peripatus may have
been "sub-universal" all over greater Gondwana land in Carboniferous times,
and then its absence from Madagascar would be difficult to explain, but the
migrations suggested above amount to little considering that the distance
from Tasmania to South America could be covered in far less time than that
represented by the whole of the Eocene epoch alone.
There is yet another field, essentially the domain of geographical
distribution, the cultivation of which promises fair to throw much light
upon Nature's way of making species. This is the study of the organisms
with regard to their environment. Instead of revealing pedigrees or of
showing how and when the creatures got to a certain locality, it
investigates how they behaved to meet the ever changing conditions of their
habitats. There is a facies, characteristic of, and often peculiar to, the
fauna of tropical moist forests, another of deserts, of high mountains, of
underground life and so forth; these same facies are stamped upon whole
associations of animals and plants, although these may be--and in widely
separated countries generally are--drawn from totally different families of
their respective orders. It does not go to the root of the matter to say
that these facies have been brought about by the extermination of all the
others which did not happen to fit into their particular environment. One
might almost say that tropical moist forests must have arboreal frogs and
that these are made out of whatever suitable material happened to be
available; in Australia and South America Hylidae, in Africa Ranidae, since
there Hylas are absent. The deserts must have lizards capable of standing
the glare, the great changes of temperature, of running over or burrowing
into the loose sand. When as in America Iguanids are available, some of
these are thus modified, while in Africa and Asia the Agamids are drawn
upon. Both in the Damara and in the Transcaspian deserts, a Gecko has been
turned into a runner upon sand!
We cannot assume that at various epochs deserts, and at others moist
forests were continuous all over the world. The different facies and
associations were developed at various times and places. Are we to suppose
that, wherever tropical forests came into existence, amongst the stock of
humivagous lizards were always some which presented those nascent
variations which made them keep step with the similarly nascent forests,
the overwhelming rest being eliminated? This principle would imply that
the same stratum of lizards always had variations ready to fit any changed
environment, forests and deserts, rocks and swamps. The study of Ecology
indicates a different procedure, a great, almost boundless plasticity of
the organism, not in the sense of an exuberant moulding force, but of a
readiness to be moulded, and of this the "variations" are the visible
outcome. In most cases identical facies are produced by heterogeneous
convergences and these may seem to be but superficial, affecting only what
some authors are pleased to call the physiological characters; but
environment presumably affects first those parts by which the organism
comes into contact with it most directly, and if the internal structures
remain unchanged, it is not because these are less easily modified but
because they are not directly affected. When they are affected, they too
change deeply enough.
That the plasticity should react so quickly--indeed this very quickness
seems to have initiated our mistaking the variations called forth for
something performed--and to the point, is itself the outcome of the long
training which protoplasm has undergone since its creation.
In Nature's workshop he does not succeed who has ready an arsenal of tools
for every conceivable emergency, but he who can make a tool at the spur of
the moment. The ordeal of the practical test is Charles Darwin's glorious
conception of Natural Selection.
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