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.