Darwin and Modern Science

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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.