THE AUSTRALIAN REGION.
General Zoological Characteristics of the Australian Region (p. 390)--
Summary of the Australian Vertebrata (p. 397)--Supposed Land-connection
between Australia and South America (p. 398)--Insects (p. 403)--
Land-shells (p. 407)--Australian Sub-regions (p. 408)--Austro-Malayan
Sub-region (p. 409)--Papua, or the New Guinea Group (p. 409)--The
Moluccas (p. 417)--Insects--Peculiarities of the Moluccan Fauna (p. 420)
--Timor Group (p. 422)--Celebes Group (p. 426)--Origin of the Fauna of
Celebes (p. 436)--Australia and Tasmania, or the Australian Sub-region
(p. 438)--The Pacific Islands, or Polynesian Sub-region (p. 442)--Fiji,
Tonga, and Samoa Islands (p. 443)--Society and Marquesas Islands (p. 443)
--Ladrone and Caroline Islands (p. 444)--New Caledonia and the New
Hebrides (p. 444)--Sandwich Islands (p. 445)--Reptiles of the Polynesian
Sub-region (p. 448)--New Zealand Sub-region (p. 449)--Islets of the New
Zealand Sub-region (p. 453)--Reptiles, Amphibia, and Fresh-water Fishes
(p. 456)--Insects (p. 457)--The Ancient Fauna of New Zealand (p. 459)--
The Origin of the New Zealand Fauna (p. 459)--Causes of the Poverty of
Insect-life in New Zealand: its Influence on the Character of the Flora
(p. 462)--Concluding Remarks on the Early History of the Australian
Region (p. 464)--Table I. Families of Animals inhabiting the Australian
Region (p. 468)--Table II. Genera of Terrestrial Mammalia and Birds of
the Australian Region (p. 473) 387-485
Index to Vol. I 489-503
MAPS AND ILLUSTRATIONS IN VOL. I.
1. Map of the World, showing the Zoo-Geographical Regions and the
contour of the Ocean-bed _Frontispiece_
_To face page_
2. Map of the Palæarctic Region 181
3. Plate I. The Alps of Central Europe with Characteristic
Animals 195
4. Plate II. Characteristic Mammalia of Western Tartary 218
5. Plate III. Characteristic Animals of North China 226
6. Map of the Ethiopian Region 251
7. Plate IV. Characteristic Animals of East Africa 261
8. Plate V. Scene in West Africa with Characteristic Animals 264
9. Plate VI. Scene in Madagascar with Characteristic Animals 278
10. Map of the Oriental Region 315
11. Plate VII. Scene in Nepaul with Characteristic Animals 331
12. Plate VIII. A Forest in Borneo with Characteristic Mammalia 337
13. Plate IX. A Malacca Forest with some of its Peculiar Birds 340
14. Map of the Australian Region 387
15. Plate X. Scene in New Guinea with Characteristic Animals 415
16. Plate XI. The Characteristic Mammalia of Tasmania 439
17. Plate XII. The Plains of New South Wales with Characteristic
Animals 442
18. Plate XIII. Scene in New Zealand with some of its Remarkable
Birds 455
ERRATA IN VOL. I.
I have detected several misprints and small errors in the final impression,
and Dr. Meyer, who has translated the work into German, has kindly
communicated all that he has noticed. It is not thought necessary to give
here all the smaller orthographical errors, most of which will be corrected
in the Index. The following seem, however, to be of sufficient importance
to justify me in asking my readers to correct them in their copies.
Page 93, 12 lines from foot, _for_ Hocco _read_ Hoazin.
" 97, line 2, _for_ Hocco _read_ Hoazin.
" 147, 13 lines from foot, _for_ three-handed _read_ three-banded.
" 177, line 6, _for_ Lycænidæ _read_ Zygænidæ.
" 183, line 20, _for_ third _read_ fourth.
" 238, line 18, _for_ Spirigidea _read_ Sphingidea.
" 242, _insert_ | 92a | Tamias | 1 | All Northern Asia | N. America.
" 245, last line, _insert in 2nd column_ (6).
" 309, line 20, _for_ Motacilla _read_ Budytes.
" 327, 12 lines from foot, _after_ Hindostan _read_ and.
" 331, last line, for _Icthyopsis_ read _Icthyophis_.
" 340, line 15, for _Edolius_ read _Bhringa_.
" 348, line 17, _for_ Flores _read_ New Guinea.
" 371, 11 lines from foot, _for_ and Borneo _read_ Borneo and
Philippines.
" 391, 10 lines from foot, _after_ Celebes _add_ and the Papuan
Islands.
" 391, 9 lines from foot, _omit_ New Guinea or.
" 414, 6 lines from foot, for _Epimachus_ read _Seleucides_.
" 415, line 10 _for_ ditto _read_ ditto.
" 427, line 20, _after_ Celebes _add_ and on some of the Philippine
Islands.
" 427, 5 lines from foot, _for_ tusks _read_ jaw.
" 462, 15 lines from foot, _for_ p. 156 _read_ p. 166.
" 474, 9 lines from foot, _after_ Celebes _add_ Papua.
THE
GEOGRAPHICAL DISTRIBUTION
OF ANIMALS.
PART I.
_THE PRINCIPLES AND GENERAL PHENOMENA OF DISTRIBUTION._
{3}CHAPTER I.
INTRODUCTORY.
It is a fact within the experience of most persons, that the various
species of animals are not uniformly dispersed over the surface of the
country. If we have a tolerable acquaintance with any district, be it a
parish, a county, or a larger extent of territory, we soon become aware
that each well-marked portion of it has some peculiarities in its animal
productions. If we want to find certain birds or certain insects, we have
not only to choose the right season but to go to the right place. If we
travel beyond our district in various directions we shall almost certainly
meet with something new to us; some species which we were accustomed to see
almost daily will disappear, others which we have never seen before will
make their appearance. If we go very far, so as to be able to measure our
journey by degrees of latitude and longitude and to perceive important
changes of climate and vegetation, the differences in the forms of animal
life will become greater; till at length we shall come to a country where
almost everything will be new, all the familiar creatures of our own
district being replaced by others more or less differing from them.
If we have been observant during our several journeys, and have combined
and compared the facts we have collected, it will become apparent that the
change we have witnessed has been of two distinct kinds. In our own and
immediately surrounding districts, particular species appeared and
disappeared because {4}the soil, the aspect, or the vegetation, was adapted
to them or the reverse. The marshes, the heaths, the woods and forests, the
chalky downs, the rocky mountains, had each their peculiar inhabitants,
which reappeared again and again as we came to tracts of country suitable
for them. But as we got further away we began to find that localities very
similar to those we had left behind were inhabited by a somewhat different
set of species; and this difference increased with distance,
notwithstanding that almost identical external conditions might be often
met with. The first class of changes is that of _stations_; the second that
of _habitats_. The one is a _local_, the other a _geographical_ phenomenon.
The whole area over which a particular animal is found may consist of any
number of _stations_, but rarely of more than one _habitat_. Stations,
however, are often so extensive as to include the entire range of many
species. Such are the great seas and oceans, the Siberian or the Amazonian
forests, the North African deserts, the Andean or the Himalayan highlands.
There is yet another difference in the nature of the change we have been
considering. The new animals which we meet with as we travel in any
direction from our starting point, are some of them very much like those we
have left behind us, and can be at once referred to familiar types; while
others are altogether unlike anything we have seen at home. When we reach
the Alps we find another kind of squirrel, in Southern Italy a distinct
mole, in Southern Europe fresh warblers and unfamiliar buntings. We meet
also with totally new forms; as the glutton and the snowy owl in Northern,
the genet and the hoopoe in Southern, and the saiga antelope and collared
pratincole in Eastern Europe. The first series are examples of what are
termed _representative species_, the second of distinct groups or _types_
of animals. The one represents a comparatively recent modification, and an
origin in or near the locality where it occurs; the other is a result of
very ancient changes both organic and inorganic, and is connected with some
of the most curious and difficult of the problems we shall have to discuss.
{5}Having thus defined our subject, let us glance at the opinions that have
generally prevailed as to the nature and causes of the phenomena presented
by the geographical distribution of animals.
It was long thought, and is still a popular notion, that the manner in
which the various kinds of animals are dispersed over the globe is almost
wholly due to diversities of climate and of vegetation. There is indeed
much to favour this belief. The arctic regions are strongly characterised
by their white bears and foxes, their reindeer, ermine, and walruses, their
white ptarmigan, owls, and falcons; the temperate zone has its foxes and
wolves, its rabbits, sheep, beavers, and marmots, its sparrows and its song
birds; while tropical regions alone produce apes and elephants, parrots and
peacocks, and a thousand strange quadrupeds and brilliant birds which are
found nowhere in the cooler regions. So the camel, the gazelle and the
ostrich live in the desert; the bison on the prairie; the tapir, the deer,
and the jaguar in forests. Mountains and marshes, plains and rocky
precipices, have each their animal inhabitants; and it might well be
thought, in the absence of accurate inquiry, that these and other
differences would sufficiently explain why most of the regions and
countries into which the earth is popularly divided should have certain
animals peculiar to them and should want others which are elsewhere
abundant.
A more detailed and accurate knowledge of the productions of different
portions of the earth soon showed that this explanation was quite
insufficient; for it was found that countries exceedingly similar in
climate and all physical features may yet have very distinct animal
populations. The equatorial parts of Africa and South America, for example,
are very similar in climate and are both covered with luxuriant forests,
yet their animal life is widely different; elephants, apes, leopards,
guinea-fowls and touracos in the one, are replaced by tapirs,
prehensile-tailed monkeys, jaguars, curassows and toucans in the other.
Again, parts of South Africa and Australia are wonderfully similar in their
soil and climate; yet one has lions, antelopes, zebras and giraffes; the
other only kangaroos, wombats, {6}phalangers and mice. In like manner parts
of North America and Europe are very similar in all essentials of soil
climate and vegetation, yet the former has racoons, opossums, and
humming-birds; while the latter possesses moles, hedgehogs and true
flycatchers. Equally striking are the facts presented by the distribution
of many large and important groups of animals. Marsupials (opossums,
phalangers &c.) are found from temperate Van Diemen's land to the tropical
islands of New Guinea and Celebes, and in America from Chili to Virginia.
No crows exist in South America, while they inhabit every other part of the
world, not excepting Australia. Antelopes are found only in Africa and
Asia; the sloths only in South America; the true lemurs are confined to
Madagascar, and the birds-of-paradise to New Guinea.
If we examine more closely the distribution of animals in any extensive
region, we find that different, though closely allied species, are often
found on the opposite sides of any considerable barrier to their migration.
Thus, on the two sides of the Andes and Rocky Mountains in America, almost
all the mammalia, birds, and insects are of distinct species. To a less
extent, the Alps and Pyrenees form a similar barrier, and even great rivers
and river plains, as those of the Amazon and Ganges, separate more or less
distinct groups of animals. Arms of the sea are still more effective, if
they are permanent; a circumstance in some measure indicated by their
depth. Thus islands far away from land almost always have very peculiar
animals found nowhere else; as is strikingly the case in Madagascar and New
Zealand, and to a less degree in the West India islands. But shallow
straits, like the English Channel or the Straits of Malacca, are not found
to have the same effect, the animals being nearly or quite identical on
their opposite shores. A change of climate or a change of vegetation may
form an equally effective barrier to migration. Many tropical and polar
animals are pretty accurately limited by certain isothermal lines; and the
limits of the great forests in most parts of the world strictly determine
the ranges of many species.
Naturalists have now arrived at the conclusion, that by some {7}slow
process of development or transmutation, all animals have been produced
from those which preceded them; and the old notion that every species was
specially created as they now exist, at a particular time and in a
particular spot, is abandoned as opposed to many striking facts, and
unsupported by any evidence. This modification of animal forms took place
very slowly, so that the historical period of three or four thousand years
has hardly produced any perceptible change in a single species. Even the
time since the last glacial epoch, which on the very lowest estimate must
be from 50,000 to 100,000 years, has only served to modify a few of the
higher animals into very slightly different species. The changes of the
forms of animals appear to have accompanied, and perhaps to have depended
on, changes of physical geography, of climate, or of vegetation; since it
is evident that an animal which is well adapted to one condition of things
will require to be slightly changed in constitution or habits, and
therefore generally in form, structure, or colour, in order to be equally
well adapted to a changed condition of surrounding circumstances. Animals
multiply so rapidly, that we may consider them as continually trying to
extend their range; and thus any new land raised above the sea by
geological causes becomes immediately peopled by a crowd of competing
inhabitants, the strongest and best adapted of which alone succeed in
maintaining their position.
If we keep in view these facts--that the minor features of the earth's
surface are everywhere slowly changing; that the forms, and structure, and
habits of all living things are also slowly changing; while the great
features of the earth, the continents, and oceans, and loftiest mountain
ranges, only change after very long intervals and with extreme slowness; we
must see that the present distribution of animals upon the several parts of
the earth's surface is the final product of all these wonderful revolutions
in organic and inorganic nature. The greatest and most radical differences
in the productions of any part of the globe must be dependent on isolation
by the most effectual and most permanent barriers. That ocean which has
remained broadest and deepest from the most remote geological epoch {8}will
separate countries the productions of which most widely and radically
differ; while the most recently-depressed seas, or the last-formed mountain
ranges, will separate countries the productions of which are almost or
quite identical. It will be evident, therefore, that the study of the
distribution of animals and plants may add greatly to our knowledge of the
past history of our globe. It may reveal to us, in a manner which no other
evidence can, which are the oldest and most permanent features of the
earth's surface, and which the newest. It may indicate the existence of
islands or continents now sunk beneath the ocean, and which have left no
record of their existence save the animal and vegetable productions which
have migrated to adjacent lands. It thus becomes an important adjunct to
geology, which can rarely do more than determine what lands have been
raised above the waters, under what conditions and at what period; but can
seldom ascertain anything of the position or extent of those which have
sunk beneath it. Our present study may often enable us, not only to say
where lands must have recently disappeared, but also to form some judgment
as to their extent, and the time that has elapsed since their submersion.
Having thus briefly sketched the nature and objects of the subject we have
to study, it will be necessary--before entering on a detailed examination
of the zoological features of the different parts of the earth, and of the
distribution of the orders, families, and genera of animals--to examine
certain preliminary facts and principles essential for our guidance. We
must first inquire what are the powers of multiplication and dispersal of
the various groups of animals, and the nature of the barriers that most
effectually limit their range. We have then to consider the effects of
changes in physical geography and in climate; to examine the nature and
extent of such changes as have been known to occur; to determine what
others are possible or probable; and to ascertain the various modes in
which such changes affect the structure, the distribution, or the very
existence of animals.
{9}Two subjects of a different nature must next engage our attention. We
have to deal with two vast masses of facts, each involving countless
details, and requiring subdivision and grouping to be capable of
intelligible treatment. All the continents and their chief subdivisions,
and all the more important islands of the globe, have to be compared as
regards their various animal forms. To do this effectively we require a
natural division of the earth especially adapted to our purpose; and we
shall have to discuss at some length the reasons for the particular system
adopted,--a discussion which must to some extent anticipate and summarize
the conclusions of the whole work. We have also to deal with many hundreds
of families and many thousands of genera of animals, and here too a true
and natural classification is of great importance. We must therefore give a
connected view of the classification adopted in the various classes of
animals dealt with.
And lastly, as the existing distribution of animals is the result and
outcome of all preceding changes of the earth and of its inhabitants, we
require as much knowledge as we can get of the animals of each country
during past geological epochs, in order to interpret the facts we shall
accumulate. We shall, therefore, enter upon a somewhat detailed sketch of
the various forms of extinct animals that have lived upon the earth during
the Tertiary period; discuss their migrations at various epochs, the
changes of physical geography that they imply, and the extent to which they
enable us to determine the birthplace of certain families and genera.
The preliminary studies above enumerated will, it is believed, enable us to
see the bearing of many facts in the distribution of animals that would
otherwise be insoluble problems; and, what is hardly less valuable, will
teach us to estimate the comparative importance of the various groups of
animals, and to avoid the common error of cutting the gordian knot of each
difficulty by vast hypothetical changes in existing continents and
oceans--probably the most permanent features of our globe.
{10}CHAPTER II.
THE MEANS OF DISPERSAL AND THE MIGRATIONS OF ANIMALS.
All animals are capable of multiplying so rapidly, that if a single pair
were placed in a continent with abundance of food and no enemies, they
might fully stock it in a very short time. Thus, a bird which produces ten
pairs of young during its lifetime (and this is far below the fertility of
many birds) will, if we take its life at five years, increase to a hundred
millions in about forty years, a number sufficient to stock a large
country. Many fishes and insects are capable of multiplying several
thousandfold each year, so that in a few years they would reach billions
and trillions. Even large and slow breeding mammals, which have only one at
a birth but continue to breed from eight to ten successive years, may
increase from a single pair to ten millions in less than forty years.
But as animals rarely have an unoccupied country to breed in, and as the
food in any one district is strictly limited, their natural tendency is to
roam in every direction in search of fresh pastures, or new hunting
grounds. In doing so, however, they meet with many obstacles. Rocks and
mountains have to be climbed, rivers or marshes to be crossed, deserts or
forests to be traversed; while narrow straits or wider arms of the sea
separate islands from the main land or continents from each other. We have
now to inquire what facilities the different classes of animals have for
overcoming these obstacles, and what kind of barriers are most effectual in
checking their progress.
_Means of Dispersal of Mammalia._--Many of the largest mammalia are able to
roam over whole continents and are hardly {11}stopped by any physical
obstacles. The elephant is almost equally at home on plains and mountains,
and it even climbs to the highest summit of Adam's Peak in Ceylon, which is
so steep and rocky as to be very difficult of ascent for man. It traverses
rivers with great ease and forces its way through the densest jungle. There
seems therefore to be no limit to its powers of wandering, but the
necessity of procuring food and its capacity of enduring changes of
climate. The tiger is another animal with great powers of dispersal. It
crosses rivers and sometimes even swims over narrow straits of the sea, and
it can endure the severe cold of North China and Tartary as well as the
heats of the plains of Bengal. The rhinoceros, the lion, and many of the
ruminants have equal powers of dispersal; so that wherever there is land
and sufficient food, there are no limits to their possible range. Other
groups of animals are more limited in their migrations. The apes, lemurs,
and many monkeys are so strictly adapted to an arboreal life that they can
never roam far beyond the limits of the forest vegetation. The same may be
said of the squirrels, the opossums, the arboreal cats, and the sloths,
with many other groups of less importance. Deserts or open country are
equally essential to the existence of others. The camel, the hare, the
zebra, the giraffe and many of the antelopes could not exist in a forest
country any more than could the jerboas or the prairie marmots.
There are other animals which are confined to mountains, and could not
extend their range into lowlands or forests. The goats and the sheep are
the most striking group of this kind, inhabiting many of the highest
mountains of the globe; of which the European ibex and mouflon are striking
examples. Rivers are equally necessary to the existence of others, as the
beaver, otter, water-vole and capybara; and to such animals high
mountain-ranges or deserts must form an absolutely impassable barrier.
_Climate as a Limit to the Range of Mammals._--Climate appears to limit the
range of many animals, though there is some reason to believe that in many
cases it is not the climate itself so much as the change of vegetation
consequent on climate which produces the effect. The quadrumana appear to
be limited by climate, {12}since they inhabit almost all the tropical
regions but do not range more than about 10° beyond the southern and 12°
beyond the northern tropic, while the great bulk of the species are found
only within an equatorial belt about 30° wide. But as these animals are
almost exclusively fruit-eaters, their distribution depends as much on
vegetation as on temperature; and this is strikingly shown by the fact that
the _Semnopithecus schistaceus_ inhabits the Himalayan mountains to a
height of 11,000 feet, where it has been seen leaping among fir-trees
loaded with snow-wreaths! Some northern animals are bounded by the
isothermal of 32°. Such are the polar bear and the walrus, which cannot
live in a state of nature far beyond the limits of the frozen ocean; but as
they live in confinement in temperate countries, their range is probably
limited by other conditions than temperature.
We must not therefore be too hasty in concluding, that animals which we now
see confined to a very hot or a very cold climate are incapable of living
in any other. The tiger was once considered a purely tropical animal, but
it inhabits permanently the cold plains of Manchuria and the Amoor, a
country of an almost arctic winter climate. Few animals seem to us more
truly inhabitants of hot countries than the elephants and rhinoceroses; yet
in Post-tertiary times they roamed over the whole of the northern
continents to within the arctic circle; and we know that the climate was
then as cold as it is now, from their entire bodies being preserved in ice.
Some change must recently have occurred either in the climate, soil, or
vegetation of Northern Asia which led to the extinction of these
forerunners of existing tropical species; and we must always bear in mind
that similar changes may have acted upon other species which we now find
restricted within narrow limits, but which may once have roamed over a wide
and varied territory.
_Valleys and Rivers as Barriers to Mammals._--To animals which thrive best
in dry and hilly regions, a broad level and marshy valley must often prove
an effectual barrier. The difference of vegetation and of insect life,
together with an unhealthy atmosphere, no doubt often checks migration if
it is attempted. Thus {13}many animals are restricted to the slopes of the
Himalayas or to the mountains of Central India, the flat valley of the
Ganges forming a limit to their range. In other cases, however, it is the
river rather than the valley which is the barrier. In the great Amazonian
plains many species of monkeys, birds, and even insects are found up to the
river banks on one side but do not cross to the other. Thus in the lower
part of the Rio Negro two monkeys, the _Jacchus bicolor_ and the
_Brachiurus couxiou_, are found on the north bank of the river but never on
the south, where a red-whiskered _Pithecia_ is alone found. Higher up
_Ateles paniscus_ extends to the north bank of the river while _Lagothrix
humboldtii_ comes down to the south bank; the former being a native of
Guiana, the latter of Ecuador. The range of the birds of the genus
_Psophia_ or trumpeters, is also limited by the rivers Amazon, Madeira, Rio
Negro and some others; so that in these cases we are able to define the
limits of distribution with an unusual degree of accuracy, and there is
little doubt the same barriers also limit a large number of other species.
_Arms of the Sea as Barriers to Mammals._--Very few mammals can swim over
any considerable extent of sea, although many can swim well for short
distances. The jaguar traverses the widest streams in South America, and
the bear and bison cross the Mississippi; and there can be no doubt that
they could swim over equal widths of salt water, and if accidentally
carried out to sea might sometimes succeed in reaching islands many miles
distant. Contrary to the common notion pigs can swim remarkably well. Sir
Charles Lyell tells us in his "Principles of Geology" that during the
floods in Scotland in 1829, some pigs only six months old that were carried
out to sea, swam five miles and got on shore again. He also states, on the
authority of the late Edward Forbes, that a pig jumped overboard to escape
from a terrier in the Grecian Archipelago, and swam safely to shore many
miles distant. These facts render it probable that wild pigs, from their
greater strength and activity, might under favourable circumstances cross
arms of the sea twenty or thirty miles wide; and there are facts in the
distribution of this tribe of animals which seem to indicate that they have
sometimes done so. Deer {14}take boldly to the water and can swim
considerable distances, but we have no evidence to show how long they could
live at sea or how many miles they could traverse. Squirrels, rats, and
lemmings often migrate from northern countries in bands of thousands and
hundreds of thousands, and pass over rivers, lakes and even arms of the
sea, but they generally perish in the saltwater. Admitting, however, the
powers of most mammals to swim considerable distances, we have no reason to
believe that any of them could traverse without help straits of upwards of
twenty miles in width, while in most cases a channel of half that distance
would prove an effectual barrier.
_Ice-floes and Driftwood as Aiding the Dispersal of Mammals._--In the
arctic regions icebergs originate in glaciers which descend into the sea,
and often bear masses of gravel, earth, and even some vegetation on their
surfaces; and extensive level ice-fields break away and float southwards.
These might often carry with them such arctic quadrupeds as frequent the
ice, or even on rare occasions true land-animals, which might sometimes be
stranded on distant continents or islands. But a more effectual because a
more wide-spread agent, is to be found in the uprooted trees and rafts of
driftwood often floated down great rivers and carried out to sea. Such
rafts or islands are sometimes seen drifting a hundred miles from the mouth
of the Ganges with living trees erect upon them; and the Amazon, the
Orinoco, Mississippi, Congo, and most great rivers produce similar rafts.
Spix and Martius declare that they saw at different times on the Amazon,
monkeys, tiger-cats, and squirrels, being thus carried down the stream. On
the Parana, pumas, squirrels, and many other quadrupeds have been seen on
these rafts; and Admiral W. H. Smyth informed Sir C. Lyell that among the
Philippine islands after a hurricane, he met with floating masses of wood
with trees growing upon them, so that they were at first mistaken for
islands till it was found that they were rapidly drifting along. Here
therefore, we have ample means for carrying all the smaller and especially
the arboreal mammals out to sea; and although in most cases they would
perish there, yet in some favourable instances strong winds or {15}unusual
tidal currents might carry them safely to shores perhaps several hundred
miles from their native country. The fact of green trees so often having
been seen erect on these rafts is most important; for they would act as a
sail by which the raft might he propelled in one direction for several days
in succession, and thus at last reach a shore to which a current alone
would never have carried it.
There are two groups of mammals which have quite exceptional means of
dispersal--the bats which fly, and the cetacea, seals, &c., which swim. The
former are capable of traversing considerable spaces of sea, since two
North American species either regularly or occasionally visit the Bermudas,
a distance of 600 miles from the mainland. The oceanic mammals (whales and
porpoises) seem to have no barrier but temperature; the polar species being
unable to cross the equator, while the tropical forms are equally unfitted
for the cold polar waters. The shore-feeding manatees, however, can only
live where they find food; and a long expanse of rocky coast would probably
be as complete a barrier to them as a few hundred miles of open ocean. The
amphibious seals and walruses seem many of them to be capable of making
long sea journeys, some of the species being found on islands a thousand
miles apart, but none of the arctic are identical with the antartic
species.
The otters with one exception are freshwater animals, and we have no reason
to believe they could or would traverse any great distances of salt water.
In fact, they would be less liable to dispersal across arms of the sea than
purely terrestrial species, since their powers of swimming would enable
them to regain the shore if accidentally carried out to sea by a sudden
flood.
_Means of Dispersal of Birds._--It would seem at first sight that no
barriers could limit the range of birds, and that they ought to be the most
ubiquitous of living things, and little fitted therefore to throw any light
on the laws or causes of the geographical distribution of animals. This,
however, is far from being the case; many groups of birds are almost as
strictly limited by barriers as the mammalia; and from their larger numbers
and the avidity with which they have been collected, they furnish
{16}materials of the greatest value for our present study. The different
groups of birds offer remarkable contrasts in the extent of their range,
some being the most cosmopolite of the higher animals, while others are
absolutely confined to single spots on the earth's surface. The petrels
(_Procellariidæ_) and the gulls (_Laridæ_) are among the greatest
wanderers; but most of the species are confined to one or other of the
great oceans, or to the arctic or antarctic seas, a few only being found
with scarcely any variation over almost the whole globe. The sandpipers and
plovers wander along the shores as far as do the petrels over the ocean.
Great numbers of them breed in the arctic regions and migrate as far as
India and Australia, or down to Chili and Brazil; the species of the old
and new worlds, however, being generally distinct. In striking contrast to
these wide ranges we find many of the smaller perching birds, with some of
the parrots and pigeons, confined to small islands of a few square miles in
extent, or to single valleys or mountains on the mainland.
_Dispersal of Birds by Winds._--Those groups of birds which possess no
powers of flight, such as the ostrich, cassowary, and apteryx, are in
exactly the same position as mammalia as regards their means of dispersal,
or are perhaps even inferior to them; since, although they are able to
cross rivers by swimming, it is doubtful if they could remain so long in
the water as most land quadrupeds. A very large number of short-winged
birds, such as toucans, pittas, and wrens, are perhaps worse off; for they
can fly very few miles at a time, and on falling into the water would soon
be drowned. It is only the strong-flying species that can venture to cross
any great width of sea; and even these rarely do so unless compelled by
necessity to migrate in search of food, or to a more genial climate. Small
and weak birds are, however, often carried accidentally across great widths
of ocean by violent gales. This is well exemplified by the large numbers of
stragglers from North America, which annually reach the Bermudas. No less
than sixty-nine species of American birds have occurred in Europe, most of
them in Britain and Heligoland. They consist chiefly of migratory birds
which in autumn {17}return along the eastern coasts of the United States,
and often fly from point to point across bays and inlets. They are then
liable to be blown out to sea by storms, which are prevalent at this
season; and it is almost always at this time of year that their occurrence
has been noted on the shores of Europe. It may, however, be doubted whether
this is not an altogether modern phenomenon, dependent on the number of
vessels constantly on the Atlantic which afford resting-places to the
wanderers; as it is hardly conceivable that such birds as titlarks,
cuckoos, wrens, warblers, and rails, could remain on the wing without food
or rest, the time requisite to pass over 2,000 miles of ocean. It is
somewhat remarkable that no European birds reach the American coast but a
few which pass by way of Iceland and Greenland; whereas a considerable
number do reach the Azores, fully half way across; so that their absence
can hardly be due to the prevailing winds being westerly. The case of the
Azores is, however, an argument for the unassisted passage of birds for
that distance; since two of the finches are peculiar 'species,' but closely
allied to European forms, so that their progenitors must, probably, have
reached the islands before the Atlantic was a commercial highway.
_Barriers to the Dispersal of Birds._--We have seen that, as a rule, wide
oceans are an almost absolute barrier to the passage of most birds from one
continent to another; but much narrower seas and straits are also very
effectual barriers where the habits of the birds are such as to preserve
them from being carried away by storms. All birds which frequent thickets
and forests, and which feed near or on the ground, are secure from such
accidents; and they are also restricted in their range by the extent of the
forests they inhabit. In South America a large number of the birds have
their ranges determined by the extent of the forest country, while others
are equally limited to the open plains. Such species are also bounded by
mountain ranges whenever these rise above the woody region. Great rivers,
such as the Amazon, also limit the range of many birds, even when there
would seem to be no difficulty in their crossing them. The supply of food,
and the kind of vegetation, soil, and climate {18}best suited to a bird's
habits, are probably the causes which mark out the exact limits of the
range of each species; to which must be added the prevalence of enemies of
either the parent birds, the eggs, or the young. In the Malay Archipelago
pigeons abound most where monkeys do not occur; and in South America the
same birds are comparatively scarce in the forest plains where monkeys are
very abundant, while they are plentiful on the open plains and campos, and
on the mountain plateaux, where these nest-hunting quadrupeds are rarely
found. Some birds are confined to swamps, others to mountains; some can
only live on rocky streams, others on deserts or grassy plains.
_The Phenomena of Migration._--The term "migration" is often applied to the
periodical or irregular movements of all animals; but it may be questioned
whether there are any regular migrants but birds and fishes. The annual or
periodical movements of mammalia are of a different class. Monkeys ascend
the Himalayas in summer to a height of 10,000 to 12,000 feet, and descend
again in winter. Wolves everywhere descend from the mountains to the
lowlands in severe weather. In dry seasons great herds of antelopes move
southwards towards the Cape of Good Hope. The well-known lemmings, in
severe winters, at long intervals, move down from the mountains of
Scandinavia in immense numbers, crossing lakes and rivers, eating their way
through haystacks, and surmounting every obstacle till they reach the sea,
whence very few return. The alpine hare, the arctic fox, and many other
animals, exhibit similar phenomena on a smaller scale; and generally it may
be said, that whenever a favourable succession of seasons has led to a
great multiplication of any species, it must on the pressure of hunger seek
food in fresh localities. For such movements as these we have no special
term. The summer and winter movements best correspond to true migration,
but they are always on a small scale, and of limited extent; the other
movements are rather temporary incursions than true migrations.
The annual movements of many fishes are more strictly analogous to the
migration of birds, since they take place in large bodies and often to
considerable distances, and are {19}immediately connected with the process
of reproduction. Some, as the salmon, enter rivers; others, as the herring
and mackerel, approach the coast in the breeding season; but the exact
course of their migrations is unknown, and owing to our complete ignorance
of the area each species occupies in the ocean, and the absence of such
barriers and of such physical diversities as occur on the land, they are of
far less interest and less connected with our present study than the
movements of birds, to which we shall now confine ourselves.
_Migrations of Birds._--In all the temperate parts of the globe there are a
considerable number of birds which reside only a part of the year,
regularly arriving and leaving at tolerably fixed epochs. In our own
country many northern birds visit us in winter, such as the fieldfare,
redwing, snow-bunting, turnstone, and numerous ducks and waders; with a
few, like the black redstart, and (according to Rev. C. A. Johns) some of
the woodcocks from the south. In the summer a host of birds appear--the
cuckoo, the swifts and swallows, and numerous warblers, being the most
familiar,--which stay to build their nests and rear their young, and then
leave us again. These are true migrants; but a number of other birds visit
us occasionally, like the waxwing, the oriole, and the beefeater, and can
only be classed as stragglers, which, perhaps from too rapid multiplication
one year and want of food the next, are driven to extend their ordinary
range of migration to an unusual degree. We will now endeavour to sketch
the chief phenomena of migration in different countries.
_Europe._--It is well ascertained that most of the birds that spend their
spring and summer in the temperate parts of Europe pass the winter in North
Africa and Western Asia. The winter visitants, on the other hand, pass the
summer in the extreme north of Europe and Asia, many of them having been
found to breed in Lapland. The arrival of migratory birds from the south is
very constant as to date, seldom varying more than a week or two, without
any regard to the weather at the time; but the departure is less constant,
and more dependent on the weather. Thus the swallow always comes to us
about the middle {20}of April, however cold it may be, while its departure
may take place from the end of September to late in October, and is said by
Forster to occur on the first N. or N.E. wind after the 20th of September.
Almost all the migratory birds of Europe go southward to the Mediterranean,
move along its coasts east or west, and cross over in three places only;
either from the south of Spain, in the neighbourhood of Gibraltar, from
Sicily over Malta, or to the east by Greece and Cyprus. They are thus
always in sight of land. The passage of most small birds (and many of the
larger ones too) takes place at night; and they only cross the
Mediterranean when the wind is steady from near the east or west, and when
there is moonlight.
It is a curious fact, but one that seems to be well authenticated, that the
males often leave before the females, and both before the young birds,
which in considerable numbers migrate later and alone. These latter,
however, seldom go so far as the old ones; and numbers of young birds do
not cross the Mediterranean, but stay in the south of Europe. The same rule
applies to the northward migration; the young birds stopping short of the
extreme arctic regions, to which the old birds migrate.[1] When old and
young go together, however, the old birds take the lead. In the south of
Europe few of the migratory birds stay to breed, but pass on to more
temperate zones; thus, in the south of France, out of 350 species only 60
breed there. The same species is often sedentary in one part of Europe and
migratory in another; thus, the chaffinch is a constant resident in
England, Germany, and the middle of France; but a migrant in the south of
France and in Holland: the rook visits the south of France in winter only:
the _Falco tinnunculus_ is both a resident and a migrant in the south of
France, according to M. Marcel de Serres, there being two regular passages
every year, while a certain number always remain.
{21}We see, then, that migration is governed by certain intelligible laws;
and that it varies in many of its details, even in the same species,
according to changed conditions. It may be looked upon as an exaggeration
of a habit common to all locomotive animals, of moving about in search of
food. This habit is greatly restricted in quadrupeds by their inability to
cross the sea or even to pass through the highly-cultivated valleys of such
countries as Europe; but the power of flight in birds enables them to cross
every kind of country, and even moderate widths of sea; and as they mostly
travel at night and high in the air, their movements are difficult to
observe, and are supposed to be more mysterious than they perhaps are. In
the tropics birds move about to different districts according as certain
fruits become ripe, certain insects abundant, or as flooded tracts dry up.
On the borders of the tropics and the temperate zone extends a belt of
country of a more or less arid character, and liable to be parched at the
summer solstice. In winter and early spring its northern margin is verdant,
but it soon becomes burnt up, and most of its birds necessarily migrate to
the more fertile regions to the north of them. They thus follow the spring
or summer as it advances from the south towards the pole, feeding on the
young flower buds, the abundance of juicy larvæ, and on the ripening
fruits; and as soon as these become scarce they retrace their steps
homewards to pass the winter. Others whose home is nearer the pole are
driven south by cold, hunger, and darkness, to more hospitable climes,
returning northward in the early summer. As a typical example of a
migratory bird, let us take the nightingale. During the winter this bird
inhabits almost all North Africa, Asia Minor, and the Jordan Valley. Early
in April it passes into Europe by the three routes already mentioned, and
spreads over France, Britain, Denmark, and the south of Sweden, which it
reaches by the beginning of May. It does not enter Brittany, the Channel
Islands, or the western part of England, never visiting Wales, except the
extreme south of Glamorganshire, and rarely extending farther north than
Yorkshire. It spreads over Central Europe, through Austria and Hungary to
Southern Russia and the warmer parts of Siberia, {22}but it nevertheless
breeds in the Jordan Valley, so that in some places it is only the surplus
population that migrates. In August and September, all who can return to
their winter quarters.
Migrations of this type probably date back from at least the period when
there was continuous land along the route passed over; and it is a
suggestive fact that this land connection is known to have existed in
recent geological times. Britain was connected with the Continent during,
and probably before, the glacial epoch; and Gibraltar, as well as Sicily
and Malta, were also recently united with Africa, as is proved by the
fossil elephants and other large mammalia found in their caverns, by the
comparatively shallow water still existing in this part of the
Mediterranean while the remainder is of oceanic profundity, and by the
large amount of identity in the species of land animals still inhabiting
the opposite shores of the Mediterranean. The submersion of these two
tracts of land (which were perhaps of considerable extent) would be a slow
process, and from year to year the change might be hardly perceptible. It
is easy to see how the migration that had once taken place over continuous
land would be kept up, first over lagoons and marshes, then over a narrow
channel, and subsequently over a considerable sea, no one generation of
birds ever perceiving any difference in the route.
There is, however, no doubt that the sea-passage is now very dangerous to
many birds. Quails cross in immense flocks, and great numbers are drowned
at sea whenever the weather is unfavourable. Some individuals always stay
through the winter in the south of Europe, and a few even in England and
Ireland; and were the sea to become a little wider the migration would
cease, and the quail, like some other birds, would remain divided between
south Europe and north Africa. Aquatic birds are observed to follow the
routes of great rivers and lakes, and the shores of the sea. One great body
reaches central Europe by way of the Danube from the shores of the Black
Sea; another ascends the Rhone Valley from the Gulf of Lyons.
{23}_India and China._--In the peninsula of India and in China great
numbers of northern birds arrive during September and October, and leave
from March to May. Among the smaller birds are wagtails, pipits, larks,
stonechats, warblers, thrushes, buntings, shrikes, starlings, hoopoes, and
quails. Some species of cranes and storks, many ducks, and great numbers of
_Scolopacidæ_ also visit India in winter; and to prey upon these come a
band of rapacious birds--the peregrine falcon, the hobby, kestrel, common
sparrowhawk, harrier, and the short-eared owl. These birds are almost all
natives of Europe and Western Asia; they spread over all northern and
central India, mingling with the sedentary birds of the oriental fauna, and
give to the ornithology of Hindostan at this season quite a European
aspect. The peculiar species of the higher Himalayas do not as a rule
descend to the plains in winter, but merely come lower down the mountains;
and in southern India and Ceylon comparatively few of these migratory birds
appear.
In China the migratory birds follow generally the coast line, coming
southwards in winter from eastern Siberia and northern Japan; while a few
purely tropical forms travel northwards in summer to Japan, and on the
mainland as far as the valley of the Amoor.
_North America._--The migrations of birds in North America have been
carefully studied by resident naturalists, and present some interesting
features. The birds of the eastern parts of North America are pre-eminently
migratory, a much smaller proportion being permanent residents than in
corresponding latitudes in Europe. Thus, in Massachusetts there are only
about 30 species of birds which are resident all the year, while the
regular summer visitors are 106. Comparing with this our own country,
though considerably further north, the proportions are reversed; there
being 140 residents and 63 summer visitors. This difference is clearly due
to the much greater length and severity of the winter, and the greater heat
of summer, in America than with us. The number of permanent residents
increases pretty regularly as we go southward; but the number of birds at
any locality during the breeding season seems to increase as we go
{24}northward as far as Canada, where, according to Mr. Allen, more species
breed than in the warm Southern States. Even in the extreme north, beyond
the limit of forests, there are no less than 60 species which breed; in
Canada about 160; while in Carolina there are only 135, and in Louisiana,
130. The extent of the migration varies greatly, some species only going a
few degrees north and south, while others migrate annually from the tropics
to the extreme north of the continent; and every gradation occurs between
these extremes. Among those which migrate furthest are the species of
_Dendroeca_, and other American flycatching warblers (_Mniotiltidæ_), many
of which breed on the shores of Hudson's Bay, and spend the winter in
Mexico or the West Indian islands.
The great migratory movement of American birds is almost wholly confined to
the east coast; the birds of the high central plains and of California
being for the most part sedentary, or only migrating for short distances.
All the species which reach South America, and most of those which winter
in Mexico and Guatemala, are exclusively eastern species; though a few
Rocky Mountain birds range southward along the plateaux of Mexico and
Guatemala, but probably not as regular annual migrants.
In America as in Europe birds appear in spring with great regularity, while
the time of the autumnal return is less constant. More curious is the fact,
also observed in both hemispheres, that they do not all return by the same
route followed in going northwards, some species being constant visitors to
certain localities in spring but not in autumn, others in autumn but not in
spring.
Some interesting cases have been observed in America of a gradual
alteration in the extent of the migration of certain birds. A Mexican
swallow (_Hirundo lunifrons_) first appeared in Ohio in 1815. Year by year
it increased the extent of its range till by 1845 it had reached Maine and
Canada; and it is now quoted by American writers as extending its annual
migrations to Hudson's Bay. An American wren (_Troglodytes ludovicianus_)
is another bird which has spread considerably northwards since {25}the time
of the ornithologist Wilson; and the rice-bird, or "Bob-o'-link," of the
Americans, continually widens its range as rice and wheat are more
extensively cultivated. This bird winters in Cuba and other West Indian
Islands, and probably also in Mexico. In April it enters the Southern
States and passes northward, till in June it reaches Canada and extends
west to the Saskatchewan River in 54° north latitude.
_South Temperate America._--The migratory birds of this part of the world
have been observed by Mr. Hudson at Buenos Ayres. As in Europe and North
America, there are winter and summer visitors, from Patagonia and the
tropics respectively. Species of _Pyrocephalus_, _Milvulus_, swallows, and
a hummingbird, are among the most regular of the summer visitors. They are
all insectivorous birds. From Patagonia species of _Tænioptera_,
_Cinclodes_, and _Centrites_, come in winter, with two gulls, two geese,
and six snipes and plovers. Five species of swallows appear at Buenos Ayres
in spring, some staying to breed, others passing on to more temperate
regions farther south. As a rule the birds which come late and leave early
are the most regular. Some are very irregular in their movements, the
_Molothrus bonariensis_, for example, sometimes leaves early in autumn,
sometimes remains all the winter. Some resident birds also move in winter
to districts where they are never seen in summer.
_General Remarks on Migration._--The preceding summary of the main facts of
migration (which might have been almost indefinitely extended, owing to the
great mass of detailed information that exists on the subject) appears to
accord with the view already suggested, that the "instinct" of migration
has arisen from the habit of wandering in search of food common to all
animals, but greatly exaggerated in the case of birds by their powers of
flight and by the necessity for procuring a large amount of soft insect
food for their unfledged young. Migration in its simple form may be best
studied in North America, where it takes place over a continuous land
surface with a considerable change of climate from south to north. We have
here (as probably in Europe and elsewhere) every grade of migration, from
species which merely shift the northern and southern {26}limits of their
range a few hundred miles, so that in the central parts of the area the
species is a permanent resident, to others which move completely over 1,000
miles of latitude, so that in all the intervening districts they are only
known as birds of passage. Now, just as the rice-bird and the Mexican
swallow have extended their migrations, owing to favourable conditions
induced by human agency; so we may presume that large numbers of species
would extend their range where favourable conditions arose through natural
causes. If we go back only as far as the height of the glacial epoch, there
is reason to believe that all North America, as far south as about 40°
north latitude, was covered with an almost continuous and perennial
ice-sheet. At this time the migratory birds would extend up to this barrier
(which would probably terminate in the midst of luxuriant vegetation, just
as the glaciers of Switzerland now often terminate amid forests and
corn-fields), and as the cold decreased and the ice retired almost
imperceptibly year by year, would follow it up farther and farther
according as the peculiarities of vegetation and insect-food were more or
less suited to their several constitutions. It is an ascertained fact that
many individual birds return year after year to build their nests in the
same spot. This shows a strong local attachment, and is, in fact, the
faculty or feeling on which their very existence probably depends. For were
they to wander at random each year, they would almost certainly not meet
with places so well suited to them, and might even get into districts where
they or their young would inevitably perish. It is also a curious fact that
in so many cases the old birds migrate first, leaving the young ones
behind, who follow some short time later, but do not go so far as their
parents. This is very strongly opposed to the notion of an imperative
instinct. The old birds have been before, the young have not; and it is
only when the old ones have all or nearly all gone that the young go too,
probably following some of the latest stragglers. They wander, however,
almost at random, and the majority are destroyed before the next spring.
This is proved by the fact that the birds which return in spring are as a
rule not more numerous than those which came the {27}preceding spring,
whereas those which went away in autumn were two or three times as
numerous. Those young birds that do get back, however, have learnt by
experience, and the next year they take care to go with the old ones. The
most striking fact in favour of the "instinct" of migration is the
"agitation," or excitement, of confined birds at the time when their wild
companions are migrating. It seems probable, however, that this is what may
be called a social excitement, due to the anxious cries of the migrating
birds; a view supported by the fact stated by Marcel de Serres, that the
black swan of Australia, when domesticated in Europe, sometimes joins wild
swans in their northward migration. We must remember too that migration at
the proper time is in many cases absolutely essential to the existence of
the species; and it is therefore not improbable that some strong social
emotion should have been gradually developed in the race, by the
circumstance that all who for want of such emotion did not join their
fellows inevitably perished.
The mode by which a passage originally overland has been converted into one
over the sea offers no insuperable difficulties, as has already been
pointed out. The long flights of some birds without apparently stopping on
the way is thought to be inexplicable, as well as their finding their
nesting-place of the previous year from a distance of many hundreds or even
a thousand miles. But the observant powers of animals are very great; and
birds flying high in the air may be guided by the physical features of the
country spread out beneath them in a way that would be impracticable to
purely terrestrial animals.
It is assumed by some writers that the breeding-place of a species is to be
considered as its true home rather than that to which it retires in winter;
but this can hardly be accepted as a rule of universal application. A bird
can only breed successfully where it can find sufficient food for its
young; and the reason probably why so many of the smaller birds leave the
warm southern regions to breed in temperate or even cold latitudes, is
because caterpillars and other soft insect larvæ are there abundant at the
proper time, while in their winter home the {28}larvæ have all changed into
winged insects. But this favourable breeding district will change its
position with change of climate; and as the last great change has been one
of increased warmth in all the temperate zones, it is probable that many of
the migratory birds are comparatively recent visitors. Other changes may
however have taken place, affecting the vegetation and consequently the
insects of a district; and we have seldom the means of determining in any
particular case in what direction the last extension of range occurred. For
the purposes of the study of geographical distribution therefore, we must,
except in special cases, consider the true range of a species to comprise
all the area which it occupies regularly for any part of the year, while
all those districts which it only visits at more or less distant intervals,
apparently driven by storms or by hunger, and where it never regularly or
permanently settles, should not be included as forming part of its area of
distribution.
_Means of Dispersal of Reptiles and Amphibia._--If we leave out of
consideration the true marine groups--the turtles and sea-snakes--reptiles
are scarcely more fitted for traversing seas and oceans than are mammalia.
We accordingly find that in those oceanic islands which possess no
indigenous mammals, land reptiles are also generally wanting. The several
groups of these animals, however, differ considerably both in their means
of dispersal and in their power of resisting adverse conditions. Snakes are
most dependent on climate, becoming very scarce in temperate and cold
climates and entirely ceasing at 62° north latitude, and they do not ascend
very lofty mountains, ceasing at 6,000 feet elevation in the Alps. Some
inhabit deserts, others swamps and marshes, while many are adapted for a
life in forests. They swim rivers easily, but apparently have no means of
passing the sea, since they are very rarely found on oceanic islands.
Lizards are also essentially tropical, but they go somewhat farther north
than snakes, and ascend higher on the mountains, reaching 10,000 feet in
the Alps. They possess too some unknown means (probably in the egg-state)
of passing over the ocean, since they are found to inhabit many islands
where there are neither mammalia nor snakes.
{29}The amphibia are much less sensitive to cold than are true reptiles,
and they accordingly extend much farther north, frogs being found within
the arctic circle. Their semi-aquatic life also gives them facilities for
dispersal, and their eggs are no doubt sometimes carried by aquatic birds
from one pond or stream to another. Salt water is fatal to them as well as
to their eggs, and hence it arises that they are seldom found in those
oceanic islands from which mammalia are absent. Deserts and oceans would
probably form the most effectual barriers to their dispersal; whereas both
snakes and lizards abound in deserts, and have some means of occasionally
passing the ocean which frogs and salamanders do not seem to possess.
_Means of Dispersal of Fishes._--The fact that the same species of
freshwater fish often inhabit distinct river systems, proves that they have
some means of dispersal over land. The many authentic accounts of fish
falling from the atmosphere, indicate one of the means by which they may be
transferred from one river basin to another, viz., by hurricanes and
whirlwinds, which often carry up considerable quantities of water and with
it fishes of small size. In volcanic countries, also, the fishes of
subterranean streams may sometimes be thrown up by volcanic explosions, as
Humboldt relates happened in South America. Another mode by which fishes
may be distributed is by their eggs being occasionally carried away by
aquatic birds; and it is stated by Gmelin that geese and ducks during their
migrations feed on the eggs of fish, and that some of these pass through
their bodies with their vitality unimpaired.[2] Even water-beetles flying
from one pond to another might occasionally carry with them some of the
smaller eggs of fishes. But it is probable that fresh-water fish are also
enabled to migrate by changes of level causing streams to alter their
course and carry their waters into adjacent basins. On plateaux the sources
of distinct river systems often approach each other, and the same thing
occurs with lateral tributaries on the lowlands near their mouths. Such
changes, although small in extent, and occurring only at long intervals,
would {30}act very powerfully in modifying the distribution of fresh-water
fish.
Sea fish would seem at first sight to have almost unlimited means of
dispersal, but this is far from being the case. Temperature forms a
complete barrier to a large number of species, cold water being essential
to many, while others can only dwell in the warmth of the tropics. Deep
water is another barrier to large numbers of species which are adapted to
shores and shallows; and thus the Atlantic is quite as impassable a gulf to
most fishes as it is to birds. Many sea fishes migrate to a limited extent
for the purpose of depositing their spawn in favourable situations. The
herring, an inhabitant of the deep sea, comes in shoals to our coast in the
breeding season; while the salmon quits the northern seas and enters our
rivers, mounting upwards to the clear cold water near their sources to
deposit its eggs. Keeping in mind the essential fact that changes of
temperature and of depth are the main barriers to the dispersal of fish, we
shall find little difficulty in tracing the causes that have determined
their distribution.
_Means of Dispersal of Mollusca._--The marine, fresh-water, and land
mollusca are three groups whose powers of dispersal and consequent
distribution are very different, and must be separately considered. The
_Pteropoda_, the _Ianthina_, and other groups of floating molluscs, drift
about in mid-ocean, and their dispersal is probably limited chiefly by
temperature, but perhaps also by the presence of enemies or the scarcity of
proper food. The univalve and bivalve mollusca, of which the whelk and the
cockle may be taken as types, move so slowly in their adult state, that we
should expect them to have an exceedingly limited distribution; but the
young of all these are free swimming embryos, and they thus have a powerful
means of dispersal, and are carried by tides and currents so as ultimately
to spread over every shore and shoal that offers conditions favourable for
their development. The fresh water molluscs, which one might at first
suppose could not range beyond their own river-basin, are yet very widely
distributed in common with almost all other fresh water productions; and
Mr. Darwin has shown that this is {31}due to the fact, that ponds and
marshes are constantly frequented by wading and swimming birds which are
pre-eminently wanderers, and which frequently carry away with them the
seeds of plants, and the eggs of molluscs and aquatic insects. Fresh water
molluscs just hatched were found to attach themselves to a duck's foot
suspended in an aquarium; and they would thus be easily carried from one
lake or river to another, and by the help of different species of aquatic
birds, might soon spread all over the globe. Even a water-beetle has been
caught with a small living shell (_Ancylus_) attached to it; and these fly
long distances and are liable to be blown out to sea, one having been
caught on board the _Beagle_ when forty-five miles from land. Although
fresh water molluscs and their eggs must frequently be carried out to sea,
yet this cannot lead to their dispersal, since salt water is almost
immediately fatal to them; and we are therefore forced to conclude that the
apparently insignificant and uncertain means of dispersal above alluded to
are really what have led to their wide distribution. The true land-shells
offer a still more difficult case, for they are exceedingly sensitive to
the influence of salt water; they are not likely to be carried by aquatic
birds, and yet they are more or less abundant all over the globe,
inhabiting the most remote oceanic islands. It has been found, however,
that land-shells have the power of lying dormant a long time. Some have
lived two years and a half shut up in pill boxes; and one Egyptian desert
snail came to life after having been glued down to a tablet in the British
Museum for four years!
We are indebted to Mr. Darwin for experiments on the power of land shells
to resist sea water, and he found that when they had formed a membranous
diaphragm over the mouth of the shell they survived many days' immersion
(in one case fourteen days); and another experimenter, quoted by Mr.
Darwin, found that out of one hundred land shells immersed for a fortnight
in the sea, twenty-seven recovered. It is therefore quite possible for them
to be carried in the chinks of drift wood for many hundred miles across the
sea, and this is probably one of the most effectual modes of their
dispersal. Very young shells would also {32}sometimes attach themselves to
the feet of birds walking or resting on the ground, and as many of the
waders often go far inland, this may have been one of the methods of
distributing species of land shells; for it must always be remembered that
nature can afford to wait, and that if but once in a thousand years a
single bird should convey two or three minute snails to a distant island,
this is all that is required for us to find that island well stocked with a
great and varied population of land shells.
_Means of Dispersal of Insects and the Barriers which Limit their
Range._--Winged insects, as a whole, have perhaps more varied means of
dispersal over the globe than any other highly organised animals. Many of
them can fly immense distances, and the more delicate ones are liable to be
carried by storms and hurricanes over a wide expanse of ocean. They are
often met with far out at sea. Hawk-moths frequently fly on board ships as
they approach the shores of tropical countries, and they have sometimes
been captured more than 250 miles from the nearest land. Dragon-flies came
on board the _Adventure_ frigate when fifty miles off the coast of South
America. A southerly wind brought flies in myriads to Admiral Smyth's ship
in the Mediterranean when he was 100 miles distant from the coast of
Africa. A large Indian beetle (_Chrysochroa ocellata_) was quite recently
caught alive in the Bay of Bengal by Captain Payne of the barque _William
Mansoon_, 273 miles from the nearest land. Darwin caught a locust 370 miles
from land; and in 1844 swarms of locusts several miles in extent, and as
thick as the flakes in a heavy snowstorm, visited Madeira. These must have
come with perfect safety more than 300 miles; and as they continued flying
over the island for a long time, they could evidently have travelled to a
much greater distance. Numbers of living beetles belonging to seven genera,
some aquatic and some terrestrial, were caught by Mr. Darwin in the open
sea, seventeen miles from the coast of South America, and they did not seem
injured by the salt water. Almost all the accidental causes that lead to
the dispersal of the higher animals would be still more favourable for
insects. Floating trees could carry hundreds of insects for one bird or
mammal; and so many of the larvæ, eggs, {33}and pupæ of insects have their
abode in solid timber, that they might survive being floated immense
distances. Great numbers of tropical insects have been captured in the
London docks, where they have been brought in foreign timber; and some have
emerged from furniture after remaining torpid for many years. Most insects
have the power of existing weeks or months without food, and some are very
tenacious of life. Many beetles will survive immersion for hours in strong
spirit; and water a few degrees below the boiling point will not always
kill them. We can therefore easily understand how, in the course of ages
insects may become dispersed by means which would be quite inadequate in
the case of the higher animals. The drift-wood and tropical fruits that
reach Ireland and the Orkneys; the double cocoa-nuts that cross the Indian
ocean from the Seychelle Islands to the coast of Sumatra; the winds that
carry volcanic dust and ashes for thousands of miles; the hurricanes that
travel in their revolving course over wide oceans; all indicate means by
which a few insects may, at rare intervals be carried to remote regions,
and become the progenitors of a group of allied forms.
But the dispersal of insects requires to be looked at from another point of
view. They are, of all animals, perhaps the most wonderfully adapted for
special conditions; and are so often fitted to fill one place in nature and
one only, that the barriers against their permanent displacement are almost
as numerous and as effective as their means of dispersal. Hundreds of
species of lepidoptera, for example, can subsist in the larva state only on
one species of plant; so that even if the perfect insects were carried to a
new country, the continuance of the race would depend upon the same or a
closely allied plant being abundant there. Other insects require succulent
vegetable food all the year round, and are therefore confined to tropical
regions; some can live only in deserts, others in forests; some are
dependent on water-plants, some on mountain-vegetation. Many are so
intimately connected with other insects during some part of their existence
that they could not live without them; such are the parasitical hymenoptera
and diptera, and those mimicking species whose welfare depends upon their
being {34}mistaken for something else. Then again, insects have enemies in
every stage of their existence--the egg, the larva, the pupa, and the
perfect form; and the abundance of any one of these enemies may render
their survival impossible in a country otherwise well suited to them. Ever
bearing in mind these two opposing classes of facts, we shall not be
surprised at the enormous range of some groups of insects, and at the
extreme localization of others; and shall be able to give a rational
account of many phenomena of distribution that would otherwise seem quite
unintelligible.
{35}CHAPTER III.
DISTRIBUTION AS AFFECTED BY THE CONDITIONS AND CHANGES OF THE EARTH'S
SURFACE.
The distribution of animals over the earth's surface, is evidently
dependent in great measure upon those grand and important characteristics
of our globe, the study of which is termed physical geography. The
proportion of land and water; the outlines and distribution of continents;
the depth of seas and oceans; the position of islands; the height,
direction, and continuity of mountain chains; the position and extent of
deserts, lakes, and forests; the direction and velocity of ocean currents,
as well as of prevalent winds and hurricanes; and lastly, the distribution
of heat and cold, of rain, snow, and ice, both in their means and in their
extremes, have all to be considered when we endeavour to account for the
often unequal and unsymmetrical manner in which animals are dispersed over
the globe. But even this knowledge is insufficient unless we inquire
further as to the evidence of permanence possessed by each of these
features, in order that we may give due weight to the various causes that
have led to the existing facts of animal distribution.
_Land and Water._--The well-known fact that nearly three-fourths of the
surface of the earth is occupied by water, and but a little more than
one-fourth by land, is important as indicating the vast extent of ocean by
which many of the continents and islands are separated from each other. But
there is another fact {36}which greatly increases its importance, namely,
that the mean height of the land is very small compared with the mean depth
of the sea. It has been estimated by Humboldt that the mean height of all
the land surface does not exceed a thousand feet, owing to the comparative
narrowness of mountain ranges and the great extent of alluvial plains and
valleys; the ocean bed, on the contrary, not only descends deeper than the
tops of the highest mountains rise above its surface, but these profound
depths are broad sunken plains, while the shallows correspond to the
mountain ranges, so that its mean depth is, as nearly as can be estimated,
twelve thousand feet.[3] Hence, as the area of water is three times that of
the land, the total cubical contents of the land, above the sea level,
would be only that of the waters which are below that level. The important
result follows, that whereas it is scarcely possible that in past times the
amount of land surface should ever greatly have exceeded that which now
exists, it is just possible that all the land may have been at some time
submerged; and therefore in the highest degree probable that among the
continual changes of land and sea that have been always going on, the
amount of land surface has often been much less than it is now. For the
same reason it is probable that there have been times when large masses of
land have been more isolated from the rest than they are at present; just
as South America would be if North America were submerged, or as Australia
would become if the Malay Archipelago were to sink beneath the ocean. It is
also very important to bear in mind the fact insisted on by Sir Charles
Lyell, that the shallow parts of the ocean are almost always in the
vicinity of land; and that an amount of elevation that would make little
difference to the bed of the ocean, would raise up extensive tracts of dry
land in the vicinity of existing continents. It is almost certain,
therefore, that changes in the distribution of land and sea must have taken
place more frequently by additions to, or {37}modifications of pre-existing
land, than by the upheaval of entirely new continents in mid-ocean. These
two principles will throw light upon two constantly recurring groups of
facts in the distribution of animals,--the restriction of peculiar forms to
areas not at present isolated,--and on the other hand, the occurrence of
allied forms in lands situated on opposite shores of the great oceans.
_Continental Areas._--Although the dry land of the earth's surface is
distributed with so much irregularity, that there is more than twice as
much north of the equator as there is south of it, and about twice as much
in the Asiatic as in the American hemisphere; and, what is still more
extraordinary, that on a hemisphere of which a point in St. George's
Channel between England and Ireland is the centre, the land is nearly equal
in extent to the water, while in the opposite hemisphere it is in the
proportion of only one-eighth,--yet the whole of the land is almost
continuous. It consists essentially of only three masses: the American, the
Asia-African, and the Australian. The two former are only separated by
thirty-six miles of shallow sea at Behring's Straits, so that it is
possible to go from Cape Horn to Singapore or the Cape of Good Hope without
ever being out of sight of land; and owing to the intervention of the
numerous islands of the Malay Archipelago the journey might be continued
under the same conditions as far as Melbourne and Hobart Town. This curious
fact, of the almost perfect continuity of all the great masses of land
notwithstanding their extremely irregular shape and distribution, is no
doubt dependent on the circumstances just alluded to; that the great depth
of the oceans and the slowness of the process of upheaval, has almost
always produced the new lands either close to, or actually connected with
pre-existing lands; and this has necessarily led to a much greater
uniformity in the distribution of organic forms, than would have prevailed
had the continents been more completely isolated from each other.
The isthmuses which connect Africa with Asia, and North with South America,
are, however, so small and insignificant compared with the vast extent of
the countries they unite that {38}we can hardly consider them to form more
than a nominal connection. The Isthmus of Suez indeed, being itself a
desert, and connecting districts which for a great distance are more or
less desert also, does not effect any real union between the luxuriant
forest-clad regions of intertropical Asia and Africa. The Isthmus of Panama
is a more effectual line of union, since it is hilly, well watered, and
covered with luxuriant vegetation; and we accordingly find that the main
features of South American zoology are continued into Central America and
Mexico. In Asia a great transverse barrier exists, dividing that continent
into a northern and southern portion; and as the lowlands occur on the
south and the highlands on the north of the great mountain range, which is
situated not far beyond the tropic, an abrupt change of climate is
produced; so that a belt of about a hundred miles wide, is all that
intervenes between a luxuriant tropical region and an almost arctic waste.
Between the northern part of Asia, and Europe, there is no barrier of
importance; and it is impossible to separate these regions as regards the
main features of animal life. Africa, like Asia, has a great transverse
barrier, but it is a desert instead of a mountain chain; and it is found
that this desert is a more effectual barrier to the diffusion of animals
than the Mediterranean Sea; partly because it coincides with the natural
division of a tropical from a temperate climate, but also on account of
recent geological changes which we shall presently allude to. It results
then from this outline sketch of the earth's surface, that the primary
divisions of the geographer correspond approximately with those of the
zoologist. Some large portion of each of the popular divisions forms the
nucleus of a zoological region; but the boundaries are so changed that the
geographer would hardly recognise them: it has, therefore, been found
necessary to give them those distinct names which will be fully explained
in our next chapter.
_Recent Changes in the Continental Areas._--The important fact has been now
ascertained, that a considerable portion of the Sahara south of Algeria and
Morocco was under water at a very recent epoch. Over much of this area
sea-shells, identical with those now living in the Mediterranean, are
abundantly scattered, {39}not only in depressions below the level of the
sea but up to a height of 900 feet above it. Borings for water made by the
French government have shown, that these shells occur twenty feet deep in
the sand; and the occurrence of abundance of salt, sometimes even forming
considerable hills, is an additional proof of the disappearance of a large
body of salt water. The common cockle is one of the most abundant of the
shells found; and the Rev. H. B. Tristram discovered a new fish, in a salt
lake nearly 300 miles inland, but which has since been found to inhabit the
Gulf of Guinea. Connected with this proof of recent elevation in the
Sahara, we have most interesting indications of subsidence in the area of
the Mediterranean, which were perhaps contemporaneous. Sicily and Malta are
connected with Africa by a submerged bank from 300 to 1,200 feet below the
surface; while the depth of the Mediterranean, both to the east and west,
is enormous, in some parts more than 13,000 feet; and another submerged
bank with a depth of 1,000 feet occurs at the straits of Gibraltar. In
caves in Sicily, remains of the living African elephant have been found by
Baron Anca; and in other caves Dr. Falconer discovered remains of the
_Elephas antiquus_ and of two species of _Hippopotamus_. In Malta, three
species of elephant have been discovered by Captain Spratt; a large one
closely allied to _E. antiquus_ and two smaller ones not exceeding five
feet high when adult. These facts clearly indicate, that when North Africa
was separated by a broad arm of the sea from the rest of the continent, it
was probably connected with Europe; and this explains why zoologists find
themselves obliged to place it along with Europe in the same zoological
region.
Besides this change in the level of the Sahara and the Mediterranean basin,
Europe has undergone many fluctuations in its physical geography in very
recent times. In Wales, abundance of sea-shells of living species have been
found at an elevation of 1,300 feet; and in Sardinia there is proof of an
elevation of 300 feet since the human epoch; and these are only samples of
many such changes of level. But these changes, though very important
locally and as connected with geological problems, need not be further
noticed here; as they were not of a {40}nature to affect the larger
features of the earth's surface or to determine the boundaries of great
zoological regions.
The only other recent change of great importance which can be adduced to
illustrate our present subject, is that which has taken place between North
and South America. The living marine shells of the opposite coasts of the
isthmus of Panama, as well as the corals and fishes, are generally of
distinct species, but some are identical and many are closely allied; the
West Indian fossil shells and corals of the Miocene period, however, are
found to be largely identical with those of the Pacific coast. The fishes
of the Atlantic and Pacific shores of America are as a rule very distinct;
but Dr. Günther has recently shown that a considerable number of species
inhabiting the seas on opposite sides of the isthmus are absolutely
identical. These facts certainly indicate, that during the Miocene epoch a
broad channel separated North and South America; and it seems probable that
a series of elevations and subsidences have taken place uniting and
separating them at different epochs; the most recent submersion having
lasted but a short time, and thus, while allowing the passage of abundance
of locomotive fishes, not admitting of much change in the comparatively
stationary mollusca.
_The Glacial Epoch as affecting the Distribution of Animals._--The
remarkable refrigeration of climate in the northern hemisphere within the
epoch, of existing species, to which the term Glacial epoch is applied,
together with the changes of level that accompanied and perhaps assisted to
produce it, has been one of the chief agents in determining many of the
details of the existing distribution of animals in temperate zones. A
comparison of the effects produced by existing glaciers with certain
superficial phenomena in the temperate parts of Europe and North America,
renders it certain that between the Newer Pliocene and the Recent epochs, a
large portion of the northern hemisphere must have been covered with a
sheet of ice several thousand feet thick, like that which now envelopes the
interior of Greenland. Much further south the mountains were covered with
perpetual snow, and sent glaciers down every valley; and all the {41}great
valleys on the southern side of the Alps poured down streams of ice which
stretched far out into the plains of Northern Italy, and have left their
débris in the form of huge mountainous moraines, in some cases more than a
thousand feet high. In Canada and New Hampshire the marks of moving ice are
found on the tops of mountains from 3,000 to 5,000 feet high; and the whole
surface of the country around and to the north of the great lakes is scored
by glaciers. Wherever the land was submerged during a part of this cold
period, a deposit called boulder-clay, or glacial-drift has been formed.
This is a mass of sand, clay, or gravel, full of angular or rounded stones
of all sizes, up to huge blocks as large as a cottage; and especially
characterized by these stones being distributed confusedly through it, the
largest being as often near the top as near the bottom, and never sorted
into layers of different sizes as in materials carried by water. Such
deposits are known to be formed by glaciers and icebergs; when deposited on
the land by glaciers they form moraines, when carried into water and thus
spread with more regularity over a wider area they form drift. This drift
is rarely found except where there is other evidence of ice-action, and
never south of the 40th parallel of latitude, to which in the northern
hemisphere signs of ice-action extend. In the southern hemisphere, in
Patagonia and in New Zealand, exactly similar phenomena occur.
A very interesting confirmation of the reality of this cold epoch is
derived from the study of fossil remains. Both the plants and animals of
the Miocene period indicate that the climate of Central Europe was
decidedly warmer or more equable than it is now; since the flora closely
resembled that of the Southern United States, with a likeness also to that
of Eastern Asia and Australia. Many of the shells were of tropical genera;
and there were numbers of large mammalia allied to the elephant,
rhinoceros, and tapir. At the same time, or perhaps somewhat earlier, a
temperate climate extended into the arctic regions, and allowed a
magnificent vegetation of shrubs and forest trees, some of them evergreen,
to flourish within twelve degrees of the Pole. In the Pliocene period we
find ourselves {42}among forms implying a climate very little different
from the present; and our own Crag formation furnishes evidence of a
gradual refrigeration of climate; since its three divisions, the Coralline,
Red, and Norwich Crags, show a decreasing number of southern, and an
increasing number of northern species, as we approach the Glacial epoch.
Still later than these we have the shells of the drift, almost all of which
are northern and many of them arctic species. Among the mammalia indicative
of cold, are the mammoth and the reindeer. In gravels and cave-deposits of
Post-Pliocene date we find the same two animals, which soon disappear as
the climate approached its present condition; and Professor Forbes has
given a list of fifty shells which inhabited the British seas before the
Glacial epoch and inhabit it still, but are all wanting in the glacial
deposits. The whole of these are found in the Newer Pliocene strata of
Sicily and the south of Europe, where they escaped destruction during the
glacial winter.
There are also certain facts in the distribution of plants, which are so
well explained by the Glacial epoch that they may be said to give an
additional confirmation to it. All over the northern hemisphere within the
glaciated districts, the summits of lofty mountains produce plants
identical with those of the polar regions. In the celebrated case of the
White Mountains in New Hampshire, United States (latitude 45°), all the
plants on the summit are arctic species, none of which exist in the
lowlands for near a thousand miles further north. It has also been remarked
that the plants of each mountain are more especially related to those of
the countries directly north of it. Thus, those of the Pyrenees and of
Scotland are Scandinavian, and those of the White Mountains are all species
found in Labrador. Now, remembering that we have evidence of an exceedingly
mild and uniform climate in the arctic regions during the Miocene period
and a gradual refrigeration from that time, it is evident that with each
degree of change more and more hardy plants would be successively driven
southwards; till at last the plains of the temperate zone would be
inhabited by plants, which were once confined to alpine heights or to the
arctic regions. {43}As the icy mantle gradually melted off the face of the
earth these plants would occupy the newly exposed soil, and would thus
necessarily travel in two directions, back towards the arctic circle and up
towards the alpine peaks. The facts are thus exactly explained by a cause
which independent evidence has proved to be a real one, and every such
explanation is an additional proof of the reality of the cause. But this
explanation implies, that in cases where the Glacial epoch cannot have so
acted alpine plants should not be northern plants; and a striking proof of
this is to be found on the Peak of Teneriffe, a mountain 12,000 feet high.
In the uppermost 4,500 feet of this mountain above the limit of trees, Von
Buch found only eleven species of plants, eight of which were peculiar; but
the whole were allied to those found at lower elevations. On the Alps or
Pyrenees at this elevation, there would be a rich flora comprising hundreds
of arctic plants; and the absence of anything corresponding to them in this
case, in which their ingress was cut off by the sea, is exactly what the
theory leads us to expect.
_Changes of Vegetation as affecting the Distribution of Animals._--As so
many animals are dependent on vegetation, its changes immediately affect
their distribution. A remarkable example of this is afforded by the
pre-historic condition of Denmark, as interpreted by means of the peat-bogs
and kitchen-middens. This country is now celebrated for its beech-trees;
oaks and pines being scarce; and it is known to have had the same
vegetation in the time of the Romans. In the peat-bogs, however, are found
deposits of oak trees; and deeper still pines alone occur. Now the
kitchen-middens tell us much of the natural history of Denmark in the early
Stone period; and a curious confirmation of the fact that Denmark like
Norway was then chiefly covered with pine forests is obtained by the
discovery, that the Capercailzie was then abundant, a bird which feeds
almost exclusively on the young shoots and seeds of pines and allied
plants. The cause of this change in the vegetation is unknown; but from the
known fact that when forests are destroyed trees, of a different kind
usually occupy the ground, we may suppose that some such change as a
temporary submergence might cause an entirely {44}different vegetation and
a considerably modified fauna to occupy the country.
_Organic Changes as affecting Distribution._--We have now briefly touched
on some of the direct effects of changes in physical geography, climate,
and vegetation, on the distribution of animals; but the indirect effects of
such changes are probably of quite equal, if not of greater importance.
Every change becomes the centre of an ever-widening circle of effects. The
different members of the organic world are so bound together by complex
relations, that any one change generally involves numerous other changes,
often of the most unexpected kind. We know comparatively little of the way
in which one animal or plant is bound up with others, but we know enough to
assure us that groups the most apparently disconnected are often dependent
on each other. We know, for example, that the introduction of goats into
St. Helena utterly destroyed a whole flora of forest trees; and with them
all the insects, mollusca, and perhaps birds directly or indirectly
dependent on them. Swine, which ran wild in Mauritius, exterminated the
Dodo. The same animals are known to be the greatest enemies of venomous
serpents. Cattle will, in many districts, wholly prevent the growth of
trees; and with the trees the numerous insects dependent on those trees,
and the birds which fed upon the insects, must disappear, as well as the
small mammalia which feed on the fruits, seeds, leaves, or roots. Insects
again have the most wonderful influence on the range of mammalia. In
Paraguay a certain species of fly abounds which destroys new-born cattle
and horses; and thus neither of these animals have run wild in that
country, although they abound both north and south of it. This inevitably
leads to a great difference in the vegetation of Paraguay, and through that
to a difference in its insects, birds, reptiles, and wild mammalia. On what
causes the existence of the fly depends we do not know, but it is not
improbable that some comparatively slight changes in the temperature or
humidity of the air at a particular season, or the introduction of some
enemy might lead to its extinction or banishment. The whole face of the
country would then soon be changed: new species would {45}come in, while
many others would be unable to live there; and the immediate cause of this
great alteration would probably be quite imperceptible to us, even if we
could watch it in progress year by year. So, in South Africa, the
celebrated Tsetse fly inhabits certain districts having well defined
limits; and where it abounds no horses, dogs, or cattle can live. Yet
asses, zebras, and antelopes are unaffected by it. So long as this fly
continues to exist, there is a living barrier to the entrance of certain
animals, quite as effectual as a lofty mountain range or a wide arm of the
sea. The complex relations of one form of life with others is nowhere
better illustrated than in Mr. Darwin's celebrated case of the cats and
clover, as given in his _Origin of Species_, 6th ed., p. 57. He has
observed that both wild heartsease and red-clover are fertilized in this
country by humble-bees only, so that the production of seed depends on the
visits of these insects. A gentleman who has specially studied humble-bees
finds that they are largely kept down by field-mice, which destroy their
combs and nests. Field-mice in their turn are kept down by cats; and
probably also by owls; so that these carnivorous animals are really the
agents in rendering possible the continued existence of red-clover and wild
heartsease. For if they were absent, the field-mice having no enemies,
would multiply to such an extent as to destroy all the humble-bees; and
these two plants would then produce no seed and soon become extinct.
Mr. Darwin has also shown that one species often exterminates another
closely allied to it, when the two are brought into contact. One species of
swallow and thrush are known to have increased at the expense of allied
species. Rats, carried all over the world by commerce, are continually
extirpating other species of rats. The imported hive-bee is, in Australia,
rapidly exterminating a native stingless bee. Any slight change, therefore,
of physical geography or of climate, which allows allied species hitherto
inhabiting distinct areas to come into contact, will often lead to the
extermination of one of them; and this extermination will be effected by no
external force, by no actual enemy, but merely because the one is slightly
better {46}adapted to live, to increase, and to maintain itself under
adverse circumstances, than the other.
Now if we consider carefully the few suggestive facts here referred to (and
many others of like import are to be found in Mr. Darwin's various works),
we shall be led to conclude that the several species, genera, families, and
orders, both of animals and vegetables which inhabit any extensive region,
are bound together by a series of complex relations; so that the increase,
diminution, or extermination of any one, may set in motion a series of
actions and reactions more or less affecting a large portion of the whole,
and requiring perhaps centuries of fluctuation before the balance is
restored. The range of any species or group in such a region, will in many
cases (perhaps in most) be determined, not by physical barriers, but by the
competition of other organisms. Where barriers have existed from a remote
epoch, they will at first have kept back certain animals from coming in
contact with each other; but when the assemblage of organisms on the two
sides of the barrier have, after many ages, come to form a balanced organic
whole, the destruction of the barrier may lead to a very partial
intermingling of the peculiar forms of the two regions. Each will have
become modified in special ways adapted to the organic and physical
conditions of the country, and will form a living barrier to the entrance
of animals less perfectly adapted to those conditions. Thus while the
abolition of ancient barriers will always lead to much intermixture of
forms, much extermination and wide-spread alteration in some families of
animals; other important groups will be unable materially to alter their
range; or they may make temporary incursions into the new territory, and be
ultimately driven back to very near their ancient limits.
In order to make this somewhat difficult subject more intelligible, it may
be well to consider the probable effects of certain hypothetical conditions
of the earth's surface:--
1. If the dry land of the globe had been from the first continuous, and
nowhere divided up by such boundaries as lofty mountain ranges, wide
deserts, or arms of the sea, it seems probable that none of the larger
groups (as _orders_, _tribes_, or {47}_families_,) would have a limited
range; but, as is to some extent the case in tropical America east of the
Andes, every such group would be represented over the whole area, by
countless minute modifications of form adapted to local conditions.
2. One great physical barrier would, however, even then exist; the hot
equatorial zone would divide the faunas and floras of the colder regions of
the northern and southern hemispheres from any chance of intermixture. This
one barrier would be more effectual than it is now, since there would be no
lofty mountain ranges to serve as a bridge for the partial interchange of
northern and southern forms.
3. If such a condition of the earth as here supposed continued for very
long periods, we may conceive that the action and reaction of the various
organisms on each other, combined with the influence of very slowly
changing physical conditions, would result in an almost perfect organic
balance, which would be manifested by a great stability in the average
numbers, the local range, and the peculiar characteristics of every
species.
4. Under such a condition of things it is not improbable that the total
number of clearly differentiated specific forms might be much greater than
it is now, though the number of generic and family types might perhaps be
less; for dominant species would have had ample time to spread into every
locality where they could exist, and would then become everywhere modified
into forms best suited to the permanent local conditions.
5. Now let us consider what would be the probable effect of the
introduction of a barrier, cutting off a portion of this homogeneous and
well-balanced world. Suppose, for instance, that a subsidence took place,
cutting off by a wide arm of the sea a large and tolerably varied island.
The first and most obvious result would be that the individuals of a number
of species would be divided into two portions, while others, the limits of
whose range agreed approximately with the line of subsidence, would exist
in unimpaired numbers on the new island or on the main land. But the
species whose numbers were diminished and whose original area was also
absolutely diminished by the portion now under the sea, would not be able
to hold their {48}ground against the rival forms whose numbers were intact.
Some would probably diminish and rapidly die out; others which produced
favourable varieties, might be so modified by natural selection as to
maintain their existence under a different form; and such changes would
take place in varying modes on the two sides of the new strait.
6. But the progress of these changes would necessarily affect the other
species in contact with them. New places would be opened in the economy of
nature which many would struggle to obtain; and modification would go on in
ever-widening circles and very long periods of time might be required to
bring the whole again into a state of equilibrium.
7. A new set of factors would in the meantime have come into play. The
sinking of land and the influx of a large body of water could hardly take
place without producing important climatal changes. The temperature, the
winds, the rains, might all be affected, and more or less changed in
duration and amount. This would lead to a quite distinct movement in the
organic world. Vegetation would certainly be considerably affected, and
through this the insect tribes. We have seen how closely the life of the
higher animals is often bound up with that of insects; and thus a set of
changes might arise that would modify the numerical proportions, and even
the forms and habits of a great number of species, would completely
exterminate some, and raise others from a subordinate to a dominant
position. And all these changes would occur differently on opposite sides
of the strait, since the insular climate could not fail to differ
considerably from that of the continent.
8. But the two sets of changes, as above indicated, produced by different
modes of action of the same primary cause, would act and react on each
other; and thus lead to such a far-spreading disturbance of the organic
equilibrium as ultimately perhaps to affect in one way or another, every
form of life upon the earth.
This hypothetical case is useful as enabling us better to realize how
wide-spreading might be the effects of one of the simplest changes of
physical geography, upon a compact mass of mutually {49}adapted organisms.
In the actual state of things, the physical changes that occur and have
occurred through all geological epochs are larger and more varied. Almost
every mile of land surface has been again and again depressed beneath the
ocean; most of the great mountain chains have either originated or greatly
increased in height during the Tertiary period; marvellous alterations of
climate and vegetation have taken place over half the land-surface of the
earth; and all these vast changes have influenced a globe so cut up by seas
and oceans, by deserts and snow-clad mountains, that in many of its more
isolated land-masses ancient forms of life have been preserved, which, in
the more extensive and more varied continents have long given way to higher
types. How complex then must have been the actions and reactions such a
state of things would bring about; and how impossible must it be for us to
guess, in most cases, at the exact nature of the forces that limit the
range of some species and cause others to be rare or to become extinct! All
that we can in general hope to do is, to trace out, more or less
hypothetically, some of the larger changes in physical geography that have
occurred during the ages immediately preceeding our own, and to estimate
the effect they will probably have produced on animal distribution. We may
then, by the aid of such knowledge as to past organic mutations as the
geological record supplies us with, be able to determine the probable
birthplace and subsequent migrations of the more important genera and
families; and thus obtain some conception of that grand series of
co-ordinated changes in the earth and its inhabitants, whose final result
is seen in the forms and the geographical distribution of existing animals.
{50}CHAPTER IV.
ON ZOOLOGICAL REGIONS.
To the older school of Naturalists the native country of an animal was of
little importance, except in as far as climates differed. Animals were
supposed to be specially adapted to live in certain zones or under certain
physical conditions, and it was hardly recognised that apart from these
conditions there was any influence in locality which could materially
affect them. It was believed that, while the animals of tropical,
temperate, and arctic climates, essentially differed; those of the tropics
were essentially alike all over the world. A group of animals was said to
inhabit the "Indies;" and important differences of structure were often
overlooked from the idea, that creatures equally adapted to live in hot
countries and with certain general resemblances, would naturally be related
to each other. Thus the Toucans and Hornbills, the Humming-Birds and
Sun-Birds, and even the Tapirs and the Elephants, came to be popularly
associated as slightly modified varieties of tropical forms of life; while
to naturalists, who were acquainted with the essential differences of
structure, it was a never-failing source of surprise, that under climates
and conditions so apparently identical, such strangely divergent forms
should be produced.
To the modern naturalist, on the other hand, the native country (or
"habitat" as it is technically termed) of an animal {51}or a group of
animals, is a matter of the first importance; and, as regards the general
history of life upon the globe, may be considered to be one of its
essential characters. The structure, affinities, and habits of a species,
now form only a part of its natural history. We require also to know its
exact range at the present day and in prehistoric times, and to have some
knowledge of its geological age, the place of its earliest appearance on
the globe, and of the various extinct forms most nearly allied to it. To
those who accept the theory of development as worked out by Mr. Darwin, and
the views as to the general permanence and immense antiquity of the great
continents and oceans so ably developed by Sir Charles Lyell, it ceases to
be a matter of surprise that the tropics of Africa, Asia, and America
should differ in their productions, but rather that they should have
anything in common. Their similarity, not their diversity, is the fact that
most frequently puzzles us.
The more accurate knowledge we have of late years obtained of the
productions of many remote regions, combined with the greater approaches
that have been made to a natural classification of the higher animals, has
shown, that every continent or well-marked division of a continent, every
archipelago and even every island, presents problems of more or less
complexity to the student of the geographical distribution of animals. If
we take up the subject from the zoological side, and study any family,
order, or even extensive genus, we are almost sure to meet with some
anomalies either in the present or past distribution of the various forms.
Let us adduce a few examples of these problems.
Deer have a wonderfully wide range, over the whole of Europe, Asia, and
North and South America; yet in Africa south of the great desert there are
none. Bears range over the whole of Europe, Asia, and North America, and
true pigs of the genus Sus, over all Europe and Asia and as far as New
Guinea; yet both bears and pigs, like deer, are absent from Tropical and
South Africa.
Again, the West Indian islands possess very few Mammalia, all of small size
and allied to those of America, except one {52}genus; and that belongs to
an Order, "Insectivora," entirely absent from South America, and to a
family, "Centetidæ," all the other species of which inhabit Madagascar
only. And as if to add force to this singular correspondence we have one
Madagascar species of a beautiful day-flying Moth, _Urania_, all the other
species of which inhabit tropical America. These insects are gorgeously
arrayed in green and gold, and are quite unlike any other Lepidoptera upon
the globe.
The island of Ceylon generally agrees in its productions with the Southern
part of India; yet it has several birds which are allied to Malayan and not
to Indian groups, and a fine butterfly of the genus _Hestia_, as well as
several genera of beetles, which are purely Malayan.
Various important groups of animals are distributed in a way not easy to
explain. The anthropoid apes in West Africa and Borneo; the tapirs in
Malaya and South America; the camel tribe in the deserts of Asia and the
Andes; the trogons in South America and Tropical Asia, with one species in
Africa; the marsupials in Australia and America, are examples.
The cases here adduced (and they might be greatly multiplied) are merely to
show the kind of problems with which the naturalist now has to deal; and in
order to do so he requires some system of geographical arrangement, which
shall serve the double purpose of affording a convenient subdivision of his
subject, and at the same time of giving expression to the main results at
which he has arrived. Hence the recent discussions on "Zoological Regions,"
or, what are the most natural primary divisions of the earth as regards its
forms of animal life.
The divisions in use till quite recently were of two kinds; either those
ready made by geographers, more especially the quarters or continents of
the globe; or those determined by climate and marked out by certain
parallels of latitude or by isothermal lines. Either of these methods was
better than none at all; but from the various considerations explained in
the preceding chapters, it will be evident, that such divisions must have
often been very unnatural, and have disguised many {53}of the most
important and interesting phenomena which a study of the distribution of
animals presents to us.
The merit of initiating a more natural system, that of determining
zoological regions, not by any arbitrary or _a priori_ consideration but by
studying the actual ranges of the more important groups of animals, is due
to Mr. Sclater, who, in 1857, established six primary zoological regions
from a detailed examination of the distribution of the chief genera and
families of Birds. Before stating what these regions are, what objections
have been made to them, what other divisions have been since proposed, and
what are those which we shall adopt in this work, it will be well to
consider the general principles which should guide us in the choice between
rival systems.
_Principles on which Zoological Regions should be formed._--It will be
evident in the first place that nothing like a perfect zoological division
of the earth is possible. The causes that have led to the present
distribution of animal life are so varied, their action and reaction have
been so complex, that anomalies and irregularities are sure to exist which
will mar the symmetry of any rigid system. On two main points every system
yet proposed, or that probably can be proposed, is open to objection; they
are,--1stly, that the several regions are not of equal rank;--2ndly, that
they are not equally applicable to all classes of animals. As to the first
objection, it will be found impossible to form any three or more regions,
each of which differs from the rest in an equal degree or in the same
manner. One will surpass all others in the possession of peculiar families;
another will have many characteristic genera; while a third will be mainly
distinguished by negative characters. There will also be found many
intermediate districts, which possess some of the characteristics of two
well-marked regions, with a few special features of their own, or perhaps
with none; and it will be a difficult question to decide in all cases which
region should possess this doubtful territory, or whether it should be
formed into a primary region itself. Again, two regions which have now
well-marked points of difference, may be shown to have been much more alike
at a comparatively recent geological epoch; {54}and this, it may be said,
proves their fundamental unity and that they ought to form but one primary
region. To obviate some of these difficulties a binary or dichotomous
division is sometimes proposed; that portion of the earth which differs
most from the rest being cut off as a region equal in rank to all that
remains, which is subjected again and again to the same process.
To decide these various points it seems advisable that convenience,
intelligibility, and custom, should largely guide us. The first essential
is, a broadly marked and easily remembered set of regions; which
correspond, as nearly as truth to nature will allow, with the distribution
of the most important groups of animals. What these groups are we shall
presently explain. In determining the number, extent, and boundaries of
these regions, we must be guided by a variety of indications, since the
application of fixed rules is impossible. They should evidently be of a
moderate number, corresponding as far as practicable with the great natural
divisions of the globe marked out by nature, and which have always been
recognized by geographers. There should be some approximation to equality
of size, since there is reason to believe that a tolerably extensive area
has been an essential condition for the development of most animal forms;
and it is found that, other things being equal, the numbers, variety and
importance of the forms of animal and vegetable life, do bear some
approximate relation to extent of area. Although the possession of peculiar
families or genera is the main character of a primary zoological region,
yet the negative character of the absence of certain families or genera is
of equal importance, _when this absence does not manifestly depend on
unsuitability to the support of the group_, and especially _when there is
now no physical barrier preventing their entrance_. This will become
evident when we consider that the importance of the possession of a group
by one region depends on its absence from the adjoining regions; and if
there is now no barrier to its entrance, we may be sure that there has once
been one; and that the possession of the area by a distinct and well
balanced set of organisms, which must have been slowly {55}developed and
adjusted, is the living barrier that now keeps out intruders.
When it is ascertained that the chief differences which now obtain between
two areas did not exist in Miocene or Pliocene times, the fact is one of
great interest, and enables us to speculate with some degree of probability
as to the causes that have brought about the present state of things; but
it is not a reason for uniting these two areas into one region. Our object
is to represent as nearly as possible the main features of the distribution
of existing animals, not those of any or all past geological epochs. Should
we ever obtain sufficient information as to the geography and biology of
the earth at past epochs, we might indeed determine approximately what were
the Pliocene or Miocene or Eocene zoological regions; but any attempt to
exhibit all these in combination with those of our own period, must lead to
confusion.
The binary or dichotomous system, although it brings out the fundamental
differences of the respective regions, is an inconvenient one in its
application, and rather increases than obviates the difficulty as to
equality or inequality of regions; for although _a_, _b_, _c_, and _d_, may
be areas of unequal zoological rank, _a_ being the most important, and _d_
the least, yet this inequality will probably be still greater if we first
divide them into _a_, on one side, and _b_, _c_, and _d_, on the other, and
then, by another division, make _b_, an area of the second, and _c_, and
_d_, of the third rank only.
Coming to the second objection, the often incompatible distribution of
different groups of animals, affords ground for opposition to any proposed
scheme of zoological regions. There is first the radical difference between
land and sea animals; the most complete barriers to the dispersal of the
one, sometimes offering the greatest facilities for the emigration of the
other, and _vice versa_. A large number of marine animals, however,
frequent shallow water only; and these, keeping near the coasts, will agree
generally in their distribution with those inhabiting the land. But among
land animals themselves there are very great differences of distribution,
due to certain specialities {56}in their organization or mode of life.
These act mainly in two ways,--1stly, by affecting the facilities with
which they can be dispersed, either voluntarily or involuntarily;--2ndly,
by the conditions which enable them to multiply and establish themselves in
certain areas and not in others. When both these means of diffusion are at
a maximum, the dispersal of a group becomes universal, and ceases to have
much interest for us. This is the case with certain groups of fungi and
lichens, as well as with some of the lower animals; and in a less degree,
as has been shown by Mr. Darwin, with many fresh-water plants and animals.
At the other extreme we may place certain arboreal vertebrata such as
sloths and lemurs, which have no means of passing such barriers as narrow
straits or moderately high mountains, and whose survival in any new country
they might reach, would be dependent on the presence of suitable forests
and the absence of dangerous enemies. Almost equally, or perhaps even more
restricted, are the means of permanent diffusion of terrestrial molluscs;
since these are without any but very rare and accidental means of being
safely transported across the sea; their individual powers of locomotion
are highly restricted; they are especially subject to the attacks of
enemies; and they often depend not only on a peculiar vegetation, but on
the geological character of the country, their abundance being almost in
direct proportion to the presence of some form of calcareous rocks. Between
these extremes we find animals possessed of an infinite gradation of powers
to disperse and to maintain themselves; and it will evidently be impossible
that the limits which best define the distribution of one group, should be
equally true for all others.
_Which class of Animals is of most importance in determining Zoological
Regions._--To decide this question we have to consider which groups of
animals are best adapted to exhibit, by their existing distribution, the
past changes and present physical condition of the earth's surface; and at
the same time, by the abundance of their remains in the various tertiary
formations will best enable us to trace out the more recent of the series
of changes, both of the earth's surface and {57}of its inhabitants, by
which the present state of things has been brought about. For this purpose
we require a group which shall be dependent for its means of dispersal on
the distribution of land and water, and on the presence or absence of lofty
mountains, desert plains or plateaux, and great forests; since these are
the chief physical features of the earth's surface whose modifications at
successive periods we wish to discover. It is also essential that they
should not be subject to dispersal by many accidental causes; as this would
inevitably in time tend to obliterate the effect of natural barriers, and
produce a scattered distribution, the causes of which we could only guess
at. Again, it is necessary that they should be so highly organized as not
to be absolutely dependent on other groups of animals, and with so much
power of adaptation as to be able to exist in one form or another over the
whole globe. And lastly, it is highly important that the whole group should
be pretty well known, and that a fairly natural classification, especially
of its minor divisions such as families and genera, should have been
arrived at; the reason for which last proviso is explained in our next
chapter, on classification.
Now in every one of these points the mammalia are preeminent; and they
possess the additional advantage of being the most highly developed class
of organized beings, and that to which we ourselves belong. We should
therefore construct our typical or standard Zoological Regions in the first
place, from a consideration of the distribution of mammalia, only bringing
to our aid the distribution of other groups to determine doubtful points.
Regions so established will be most closely in accordance with those
long-enduring features of physical geography, on which the distribution of
all forms of life fundamentally depend; and all discrepancies in the
distribution of other classes of animals must be capable of being
explained, either by their exceptional means of dispersion or by special
conditions affecting their perpetuation and increase in each locality.
If these considerations are well founded, the objections of those who study
insects or molluscs, for example,--that our regions are not true for their
departments of nature--cannot be {58}maintained. For they will find, that a
careful consideration of the exceptional means of dispersal and conditions
of existence of each group, will explain most of the divergences from the
normal distribution of higher animals.
We shall thus be led to an intelligent comprehension of the phenomena of
distribution in all groups, which would not be the case if every specialist
formed regions for his own particular study. In many cases we should find
that no satisfactory division of the earth could be made to correspond with
the distribution even of an entire class; but we should have the
coleopterist and the lepidopterist each with his own Geography. And even
this would probably not suffice, for it is very doubtful if the detailed
distribution of the Longicornes, so closely dependent on woody vegetation,
could be made to agree with that of the Staphylinidæ or the Carabidæ which
abound in many of the most barren regions, or with that of the Scarabeidæ,
largely dependent on the presence of herbivorous mammalia. And when each of
these enquirers had settled a division of the earth into "regions" which
exhibited with tolerable accuracy the phenomena of distribution of his own
group, we should have gained nothing whatever but a very complex mode of
exhibiting the bare facts of distribution. We should then have to begin to
work out the causes of the divergence of one group from another in this
respect; but as each worker would refer to his own set of regions as the
type, the whole subject would become involved in inextricable confusion.
These considerations seem to make it imperative that one set of "regions"
should be established as typical for Zoology; and it is hoped the reasons
here advanced will satisfy most naturalists that these regions can be best
determined, in the first place, by a study of the distribution of the
mammalia, supplemented in doubtful cases by that of the other vertebrates.
We will now proceed to a discussion of what these regions are.
_Various Zoological Regions proposed since 1857._--It has already been
pointed out that a very large number of birds are limited by the same kind
of barriers as mammalia; it will therefore not be surprising that a system
of regions formed to suit the {59}one, should very nearly represent the
distribution of the other. Mr. Sclater's regions are as follows:--
1. The Palæarctic Region; including Europe, Temperate Asia, and N. Africa
to the Atlas mountains.
2. The Ethiopian Region; Africa south of the Atlas, Madagascar, and the
Mascarene Islands, with Southern Arabia.
3. The Indian Region; including India south of the Himalayas, to South
China, and to Borneo and Java.
4. The Australian Region; including Celebes and Lombock, eastward to
Australia and the Pacific Islands.
5. The Nearctic Region; including Greenland, and N. America, to Northern
Mexico.
6. The Neotropical Region; including South America, the Antilles, and
Southern Mexico.
This division of the earth received great support from Dr. Günther, who, in
the _Proceedings of the Zoological Society_ for 1858, showed that the
geographical distribution of Reptiles agreed with it very closely, the
principal difference being that the reptiles of Japan have a more Indian
character than the birds, this being especially the case with the snakes.
In the volume for 1868 of the same work, Professor Huxley discusses at
considerable length the primary and secondary zoological divisions of the
earth. He gives reasons for thinking that the most radical primary
division, both as regards birds and mammals, is into a Northern and
Southern hemisphere (Arctogæa and Notogæa), the former, however, embracing
all Africa, while the latter includes only Australasia and the Neotropical
or Austro-Columbian region. Mr. Sclater had grouped his regions primarily
into Palæogæa and Neogæa, the Old and New Worlds of geographers; a division
which strikingly accords with the distribution of the passerine birds, but
not so well with that of mammalia or reptiles. Professor Huxley points out
that the Nearctic, Palæarctic, Indian, and Ethiopian regions of Mr. Sclater
have a much greater resemblance to each other than any one of them has to
Australia or to South America; and he further suggests that New Zealand
alone has peculiarities which might entitle it to rank as a primary region
{60}along with Australasia and South America; and that a Circumpolar
Province might be conveniently recognised as of equal rank with the
Palæarctic and Nearctic provinces.
In 1866, Mr. Andrew Murray published a large and copiously illustrated
volume on the _Geographical Distribution of Mammals_, in which he maintains
that the great and primary mammalian regions are only four: 1st. The
Palæarctic region of Mr. Sclater, extended to include the Sahara and Nubia;
2nd. the Indo-African region, including the Indian and Ethiopian regions of
Mr. Sclater; 3rd. the Australian region (unaltered); 4th. the American
region, including both North and South America. These are the regions as
_described_ by Mr. Murray, but his coloured map of "Great Mammalian
Regions" shows all Arctic America to a little south of the Isothermal of
32° Fahr. as forming with Europe and North Asia one great region.
At the meeting of the British Association at Exeter in 1869, Mr. W. T.
Blanford read a paper on the Fauna of British India, in which he maintained
that a large portion of the peninsula of India had derived its Fauna mainly
from Africa; and that the term "Indian region" of Mr. Sclater was
misleading, because India proper, if it belongs to it at all, is the least
typical portion of it. He therefore proposes to call it the "Malayan
region," because in the Malay countries it is most highly developed. Ceylon
and the mountain ranges of Southern India have marked Malay affinities.
In 1871 Mr. E. Blyth published in _Nature_ "A suggested new Division of the
Earth into Zoological Regions," in which he indicates seven primary
divisions or regions, subdivided into twenty-six sub-regions. The seven
regions are defined as follows: 1. The Boreal region; including the whole
of the Palæarctic and Nearctic regions of Mr. Sclater along with the West
Indies, Central America, the whole chain of the Andes, with Chili and
Patagonia. 2. The Columbian region; consisting of the remaining part of
South America. 3. The Ethiopian region; comprising besides that region of
Mr. Sclater, the valley of the Jordan, Arabia, and the desert country
towards India, with all the plains and table lands of India and the
northern {61}half of Ceylon. 4. The Lemurian region; consisting of
Madagascar and its adjacent islands. 5. The Austral-Asian region; which is
the Indian region of Mr. Sclater without the portion taken to be added to
the Ethiopian region. 6. The Melanesian region; which is the Australian
region of Mr. Sclater without New Zealand and the Pacific Islands, which
form 7. the Polynesian region. Mr. Blyth thinks this is "a true
classification of zoological regions as regards mammalia and birds."
In an elaborate paper on the birds of Eastern North America, their
distribution and migrations (_Bulletin of Museum of Comparative Zoology,
Cambridge, Massachusetts_, Vol. 2), Mr. J. A. Allen proposes a division of
the earth in accordance with what he terms, "the law of circumpolar
distribution of life in zones," as follows: 1. Arctic realm. 2. North
temperate realm. 3. American tropical realm. 4. Indo-African tropical
realm. 5. South American tropical realm. 6. African temperate realm. 7.
Antarctic realm. 8. Australian realm. Some of these are subdivided into
regions; (2) consisting of the American and the Europæo-Asiatic regions;
(4) into the African and Indian regions; (8) into the tropical Australian
region, and one comprising the southern part of Australia and New Zealand.
The other realms each form a single region.
_Discussion of proposed Regions._--Before proceeding to define the regions
adopted in this work, it may be as well to make a few remarks on some of
the preceding classifications, and to give the reasons which seem to render
it advisable to adopt very few of the suggested improvements on Mr.
Sclater's original proposal. Mr. Blyth's scheme is one of the least
natural, and also the most inconvenient. There can be little use in the
knowledge that a group of animals is found in the Boreal Region, if their
habitat might still be either Patagonia, the West Indies, or Japan; and it
is difficult to see on what principle the Madagascar group of islands is
made of equal rank with this enormous region, seeing that its forms of life
have marked African affinities. Neither does it seem advisable to adopt the
Polynesian Region, or that comprising New Zealand alone (as hinted at by
Professor Huxley and since adopted by {62}Mr. Sclater in his Lectures on
Geographical Distribution at the Zoological Gardens in May 1874), because
it is absolutely without indigenous mammalia and very poor in all forms of
life, and therefore by no means prominent or important enough to form a
primary region of the earth.
It may be as well here to notice what appears to be a serious objection to
making New Zealand, or any similar isolated district, one of the great
zoological regions, comparable to South America, Australia, or Ethiopia;
which is, that its claim to that distinction rests on grounds which are
liable to fail. It is because New Zealand, in addition to its negative
merits, possesses three families of birds (Apterygidæ living, Dinornithidæ
and Palapterygidæ extinct), and a peculiar lizard-like reptile, _Hatteria_,
which has to be classed in a distinct order, Rhynchocephalina, that the
rank of a Region is claimed for it. But supposing, what is not at all
improbable, that other Rhynchocephalina should be discovered in the
interior of Australia or in New Guinea, and that Apterygidæ or
Palapterygidæ should be found to have inhabited Australia in Post-Pliocene
times, (as Dinornithidæ have already been proved to have done) the claims
of New Zealand would entirely fail, and it would be universally
acknowledged to be a part of the great Australian region. No such reversal
can take place in the case of the other regions; because they rest, not
upon one or two, but upon a large number of peculiarities, of such a nature
that there is no room upon the globe for discoveries that can seriously
modify them. Even if one or two peculiar types, like Apterygidæ or
_Hatteria_, should permanently remain characteristic of New Zealand alone,
we can account for these by the extreme isolation of the country, and the
absence of enemies, which have enabled these defenceless birds and reptiles
to continue their existence; just as the isolation and protection of the
caverns of Carniola have enabled the _Proteus_ to survive in Europe. But
supposing that the _Proteus_ was the sole representative of an order of
Batrachia, and that two or three other equally curious and isolated forms
occurred with it, no one would propose that these caverns or the district
containing them, should form one of the {63}primary divisions of the earth.
Neither can much stress be laid on the negative characteristics of New
Zealand, since they are found to an almost equal extent in every oceanic
island.
Again, it is both inconvenient and misleading to pick out certain tracts
from the midst of one region or sub-region and to place them in another, on
account of certain isolated affinities which may often be accounted for by
local peculiarities. Even if the resemblance of the fauna of Chili and
Patagonia to that of the Palæarctic and Nearctic regions was much greater
than it is, this mode of dealing with it would be objectionable; but it is
still more so, when we find that these countries have a strongly marked
South American character, and that the northern affinities are altogether
exceptional. The Rodentia, which comprise a large portion of the mammalia
of these countries, are wholly South American in type, and the birds are
almost all allied to forms characteristic of tropical America.
For analogous reasons the Ethiopian must not be made to include any part of
India or Ceylon; for although the Fauna of Central India has some African
affinities, these do not preponderate; and it will not be difficult to show
that to follow Mr. Andrew Murray in uniting bodily the Ethiopian and Indian
regions of Mr. Sclater, is both unnatural and inconvenient. The
resemblances between them are of the same character as those which would
unite them both with the Palæarctic and Nearctic regions; and although it
may be admitted, that, as Professor Huxley maintains, this group forms one
of the great primary divisions of the globe, it is far too extensive and
too heterogeneous to subserve the practical uses for which we require a
division of the world into zoological regions.
_Reasons for adopting the six Regions first proposed by Mr. Sclater._--So
that we do not violate any clear affinities or produce any glaring
irregularities, it is a positive, and by no means an unimportant, advantage
to have our named regions approximately equal in size, and with easily
defined, and therefore easily remembered, boundaries. All elaborate
definitions of interpenetrating frontiers, as well as regions extending
over three-fourths of the land surface of the globe, and including places
which are {64}the antipodes of each other, would be most inconvenient, even
if there were not such difference of opinion about them. There can be
little doubt, for example, that the most radical zoological division of the
earth is made by separating the Australian region from the rest; but
although it is something useful and definite to know that a group of
animals is peculiar to Australia, it is exceedingly vague and
unsatisfactory to say of any other group merely that it is
extra-Australian. Neither can it be said that, from any point of view,
these two divisions are of equal importance. The next great natural
division that can be made is the separation of the Neotropical Region of
Mr. Sclater from the rest of the world. We thus have three primary
divisions, which Professor Huxley seems inclined to consider as of
tolerably equal zoological importance. But a consideration of all the
facts, zoological and palæontological, indicates, that the great northern
division (Arctogæa) is fully as much more important than either Australia
or South America, as its four component parts are less important; and if
so, convenience requires us to adopt the smaller rather than the larger
divisions.
This question, of comparative importance or equivalence of value, is very
difficult to determine. It may be considered from the point of view of
speciality or isolation, or from that of richness and variety of animal
forms. In isolation and speciality, determined by what they want as well as
what they possess, the Australian and Neotropical regions are undoubtedly
each comparable with the rest of the earth (Arctogæa). But in richness and
variety of forms, they are both very much inferior, and are much more
nearly comparable with the separate regions which compose it. Taking the
families of mammalia as established by the best authors, and leaving out
the Cetacea and the Bats, which are almost universally distributed, and
about whose classification there is much uncertainty, the number of
families represented in each of Mr. Sclater's regions is as follows:
I. Palæarctic region has 31 families of terrestrial mammalia.
II. Ethiopian " " 40 " " "
III. Indian " " 31 " " "
IV. Australian " " 14 " " "
V. Neotropical " " 26 " " "
VI. Nearctic " " 23 " " "
{65}We see, then, that even the exceedingly rich and isolated Neotropical
region is less rich and diversified in its forms of mammalian life than the
very much smaller area of the Indian region, or the temperate Palæarctic,
and very much less so than the Ethiopian region; while even the
comparatively poor Nearctic region, is nearly equal to it in the number of
its family types. If these were united they would possess fifty-five
families, a number very disproportionate to those of the remaining two.
Another consideration is, that although the absence of certain forms of
life makes a region more isolated, it does not make it zoologically more
important; for we have only to suppose some five or six families, now
common to both, to become extinct either in the Ethiopian or the Indian
regions, and they would become as strongly differentiated from all other
regions as South America, while still remaining as rich in family types. In
birds exactly the same phenomenon recurs, the family types being less
numerous in South America than in either of the other tropical regions of
the earth, but a larger proportion of them are restricted to it. It will be
shown further on, that the Ethiopian and Indian, (or, as I propose to call
it in this work, Oriental) regions, are sufficiently differentiated by very
important groups of animals peculiar to each; and that, on strict
zoological principles they are entitled to rank as regions of equal value
with the Neotropical and Australian. It is perhaps less clear whether the
Palæarctic should be separated from the Oriental region, with which it has
undoubtedly much in common; but there are many and powerful reasons for
keeping it distinct. There is an unmistakably different facies in the
animal forms of the two regions; and although no families of mammalia or
birds, and not many genera, are wholly confined to the Palæarctic region, a
very considerable number of both have their metropolis in it, and are very
richly represented. The distinction between the characteristic forms of
life in tropical and cold countries is, on the whole, very strongly marked
in the northern hemisphere; and to refuse to recognise this in a
subdivision of the earth which is established for the very purpose of
expressing such contrasts more clearly and concisely than by ordinary
geographical terminology, would be both illogical and {66}inconvenient. The
one question then remains, whether the Nearctic region should be kept
separate, or whether it should form part of the Palæarctic or of the
Neotropical regions. Professor Huxley and Mr. Blyth advocate the former
course; Mr. Andrew Murray (for mammalia) and Professor Newton (for birds)
think the latter would be more natural. No doubt much is to be said for
both views, but both cannot be right; and it will be shown in the latter
part of this chapter that the Nearctic region is, on the whole, fully as
well defined as the Palæarctic, by positive characters which differentiate
it from both the adjacent regions. More evidence in the same direction will
be found in the Second Part of this work, in which the extinct faunas of
the several regions are discussed.
A confirmation of the general views here set forth, as to the distinctness
and approximate equivalence of the six regions, is to be found in the fact,
that if any two or more of them are combined they themselves become
divisions of the next lower rank, or "sub-regions;"--and these will be very
much more important, both zoologically and geographically, than the
subdivisions of the remaining regions. It is admitted then that these six
regions are by no means of precisely equal rank, and that some of them are
far more isolated and better characterized than others; but it is
maintained that, looked at from every point of view, they are more equal in
rank than any others that can be formed; while in geographical equality,
compactness of area, and facility of definition, they are beyond all
comparison better than any others that have yet been proposed for the
purpose of facilitating the study of geographical distribution. They may be
arranged and grouped as follows, so as to exhibit their various relations
and affinities.
Regions.
{ NEOTROPICAL Austral zone Notogæa.
Neogæa {
{ NEARCTIC } }
} Boreal zone }
{ PALÆARCTIC } }
{ } Arctogæa.
{ ETHIOPIAN } }
Palæogæa { } Palæotropical zone }
{ ORIENTAL } }
{
{ AUSTRALIAN Austral zone Notogæa.
The above table shows the regions placed in the order followed in the
Fourth Part of this work, and the reasons for which are {67}explained in
Chapter IX. As a matter of convenience, and for other reasons adduced in
the same chapter, the detailed exposition of the geographical distribution
of the animals of the several regions in Part III. commences with the
Palæarctic and terminates with the Nearctic region.
_Objections to the system of Circumpolar Zones._--Mr. Allen's system of
"realms" founded on climatic zones (given at p. 61), having recently
appeared in an ornithological work of considerable detail and research,
calls for a few remarks. The author continually refers to the "_law of the
distribution of life in circumpolar zones_," as if it were one generally
accepted and that admits of no dispute. But this supposed "law" only
applies to the smallest details of distribution--to the range and
increasing or decreasing numbers of _species_ as we pass from north to
south, or the reverse; while it has little bearing on the great features of
zoological geography--the limitation of groups of _genera_ and _families_
to certain areas. It is analogous to the "_law of adaptation_" in the
organisation of animals, by which members of various groups are suited for
an aerial, an aquatic, a desert, or an arboreal life; are herbivorous,
carnivorous, or insectivorous; are fitted to live underground, or in fresh
waters, or on polar ice. It was once thought that these adaptive
peculiarities were suitable foundations for a classification,--that whales
were fishes, and bats birds; and even to this day there are naturalists who
cannot recognise the essential diversity of structure in such groups as
swifts and swallows, sun-birds and humming-birds, under the superficial
disguise caused by adaptation to a similar mode of life. The application of
Mr. Allen's principle leads to equally erroneous results, as may be well
seen by considering his separation of "the southern third of Australia" to
unite it with New Zealand as one of his secondary zoological divisions. If
there is one country in the world whose fauna is strictly homogeneous, that
country is Australia; while New Guinea on the one hand, and New Zealand on
the other, are as sharply differentiated from Australia as any adjacent
parts of the same primary zoological division can possibly be. Yet the
"_law of circumpolar distribution_" leads to the division of {68}Australia
by an arbitrary east and west line, and a union of the northern two-thirds
with New Guinea, the southern third with New Zealand. Hardly less unnatural
is the supposed equivalence of South Africa (the African temperate realm)
to all tropical Africa and Asia, including Madagascar (the Indo-African
tropical realm). South Africa has, it is true, some striking peculiarities;
but they are absolutely unimportant as compared with the great and radical
differences between tropical Africa and tropical Asia. On these examples we
may fairly rest our rejection of Mr. Allen's scheme.
We must however say a few words on the zoo-geographical nomenclature
proposed in the same paper, which seems also very objectionable. The
following terms are proposed: _realm_, _region_, _province_, _district_,
_fauna and flora_; the first being the highest, the last the lowest and
smallest sub-division. Considering that most of these terms have been used
in very different senses already, and that no means of settling their
equivalence in different parts of the globe has been even suggested, such a
complex system must lead to endless confusion. Until the whole subject is
far better known and its first principles agreed upon, the simpler and the
fewer the terms employed the better; and as "region" was employed for the
primary divisions by Mr. Sclater, eighteen years ago, and again by Mr.
Andrew Murray, in his Geographical Distribution of Mammals; nothing but
obscurity can result from each writer using some new, and doubtfully
better, term. For the sub-divisions of the regions no advantage is gained
by the use of a distinct term--"province"--which has been used (by
Swainson) for the primary divisions, and which does not itself tell you
what rank it holds; whereas the term "sub-region" speaks for itself as
being unmistakably next in subordination to region, and this clearness of
meaning gives it the preference over any independent term. As to minor
named sub-divisions, they seem at present uncalled for; and till the
greater divisions are themselves generally agreed on, it seems better to
adopt no technical names for what must, for a long time to come, be
indeterminate.
_Does the Arctic Fauna characterize an independent {69}Region._--The
proposal to consider the Arctic regions as constituting one of the primary
zoological divisions of the globe, has been advocated by many naturalists.
Professor Huxley seems to consider it advisable, and Mr. Allen
unhesitatingly adopts it, as well as an "antarctic" region to balance it in
the southern hemisphere. The reason why an "Arctic Region" finds no place
in this work may therefore be here stated.
No species or group of animals can properly be classed as "arctic," which
does not exclusively inhabit or greatly preponderate in arctic lands. For
the purpose of establishing the need of an "arctic" zoological region, we
should consider chiefly such groups as are circumpolar as well as arctic;
because, if they are confined to, or greatly preponderate in, either the
eastern or western hemispheres, they can be at once allocated to the
Nearctic or Palæarctic regions, and can therefore afford no justification
for establishing a new primary division of the globe.
Thus restricted, only three genera of land mammalia are truly arctic:
_Gulo_, _Myodes_, and _Rangifer_. Two species of widely dispersed genera
are also exclusively arctic, _Ursus maritimus_ and _Vulpes lagopus_.
Exclusively arctic birds are not much more numerous. Of land birds there
are only three genera (each consisting of but a single species),
_Pinicola_, _Nyctea_, and _Surnia_. _Lagopus_ is circumpolar, but the genus
has too wide an extension in the temperate zone to be considered arctic.
Among aquatic birds we have the genus of ducks, _Somateria_; three genera
of Uriidæ, _Uria_, _Catarractes_, and _Mergulus_; and the small family
Alcidæ, consisting of the genera _Alca_ and _Fratercula_. Our total then
is, three genera of mammalia, three of land, and six of aquatic birds,
including one peculiar family.
In the southern hemisphere there is only the single genus _Aptenodytes_
that can be classed as antarctic; and even that is more properly south
temperate.
In dealing with this arctic fauna we have two courses open to us; we must
either group them with the other species and genera which are common to the
two northern regions, or we {70}must form a separate primary region for
them. As a matter of convenience the former plan seems the best; and it is
that which is in accordance with our treatment of other intermediate tracts
which contain special forms of life. The great desert zone, extending from
the Atlantic shores of the Sahara across Arabia to Central Asia, is a
connecting link between the Palæarctic, Ethiopian, and Oriental regions,
and contains a number of "desert" forms wholly or almost wholly restricted
to it; but the attempt to define it as a separate region would introduce
difficulty and confusion. Neither to the "desert" nor to the "arctic"
regions could any defined limits, either geographical or zoological, be
placed; and the attempt to determine what species or genera should be
allotted to them would prove an insoluble problem. The reason perhaps is,
that both are essentially unstable, to a much greater extent than those
great masses of land with more or less defined barriers, which constitute
our six regions. The Arctic Zone has been, within a recent geological
period, both vastly more extensive and vastly less extensive than it is at
present. At a not distant epoch it extended over half of Europe and of
North America. At an earlier date it appears to have vanished altogether;
since a luxuriant vegetation of tall deciduous trees and broad-leaved
evergreens flourished within ten degrees of the Pole! The great deserts
have not improbably been equally fluctuating; hence neither the one nor the
other can present that marked individuality in their forms of life, which
seems to have arisen only when extensive tracts of land have retained some
considerable stability both of surface and climatal conditions, during
periods sufficient for the development and co-adaptation of their several
assemblages of plants and animals.
We must also consider that there is no geographical difficulty in dividing
the Arctic Zone between the two northern regions. The only debateable
lands, Greenland and Iceland, are generally admitted to belong respectively
to America and Europe. Neither is there any zoological difficulty; for the
land mammalia and birds are on the whole wonderfully restricted to their
respective regions even in high latitudes; and the aquatic forms {71}are,
for our present purpose, of much less importance. As a primary division the
"Arctic region" would be out of all proportion to the other six, whether as
regards its few peculiar types or the limited number of forms and species
actually inhabiting it; but it comes in well as a connecting link between
two regions, where the peculiar forms of both are specially modified; and
is in this respect quite analogous to the great desert zone above referred
to.
I now proceed to characterize briefly the six regions adopted in the
present work, together with the sub-regions into which they may be most
conveniently and naturally divided, as shown in our general map.
_Palæarctic Region._--This very extensive region comprises all temperate
Europe and Asia, from Iceland to Behring's Straits and from the Azores to
Japan. Its southern boundary is somewhat indefinite, but it seems advisable
to comprise in it all the extra-tropical part of the Sahara and Arabia, and
all Persia, Cabul, and Beloochistan to the Indus. It comes down to a little
below the upper limit of forests in the Himalayas, and includes the larger
northern half of China, not quite so far down the coast as Amoy. It has
been said that this region differs from the Oriental by negative characters
only; a host of tropical families and genera being absent, while there is
little or nothing but peculiar species to characterize it absolutely. This
however is not true. The Palæarctic region is well characterized by
possessing 3 families of vertebrata peculiar to it, as well as 35 peculiar
genera of mammalia, and 57 of birds, constituting about one-third of the
total number it possesses. These are amply sufficient to characterize a
region positively; but we must also consider the absence of many important
groups of the Oriental, Ethiopian, and Nearctic regions; and we shall then
find, that taking positive and negative characters together, and making
some allowance for the necessary poverty of a temperate as compared with
tropical regions, the Palæarctic is almost as strongly marked and well
defined as any other.
_Sub-divisions of the Palæarctic Region._--These are by no means {72}so
clearly indicated as in some of the other regions, and they are adopted
more for convenience than because they are very natural or strongly marked.
The first, or European sub-region, comprises Central and Northern Europe as
far South as the Pyrenees, the Maritime and Dinaric Alps, the Balkan
mountains, the Black Sea, and the Caucasus. On the east the Caspian sea and
the Ural mountains seem the most obvious limit; but it is doubtful if they
form the actual boundary, which is perhaps better marked by the valley of
the Irtish, where a pre-glacial sea almost certainly connected the Aral and
Caspian seas with the Arctic ocean, and formed an effective barrier which
must still, to some extent, influence the distribution of animals.
The next, or Mediterranean sub-region, comprises South Europe, North Africa
with the extra-tropical portion of the Sahara, and Egypt to about the first
or second cataracts; and eastward through Asia Minor, Persia, and Cabul, to
the deserts of the Indus.
The third, or Siberian sub-region, consists of all north and central Asia
north of Herat, as far as the eastern limits of the great desert plateau of
Mongolia, and southward to about the upper limit of trees on the Himalayas.
The fourth, or Manchurian sub-region, consists of Japan and North China
with the lower valley of the Amoor; and it should probably be extended
westward in a narrow strip along the Himalayas, embracing about 1,000 or
2,000 feet of vertical distance below the upper limit of trees, till it
meets an eastern extension of the Mediterranean sub-region a little beyond
Simla. These extensions are necessary to avoid passing from the Oriental
region, which is essentially tropical, directly to the Siberian sub-region,
which has an extreme northern character; whereas the Mediterranean and
Manchurian sub-regions are more temperate in climate. It will be found that
between the upper limit of most of the typical Oriental groups and the
Thibetan or Siberian fauna, there is a zone in which many forms occur
common to temperate China. This is especially the case among the pheasants
and finches.
{73}_Ethiopian Region._--The limits of this region have been indicated by
the definition of the Palæarctic region. Besides Africa south of the tropic
of Cancer, and its islands, it comprises the southern half of Arabia.
This region has been said to be identical in the main characters of its
mammalian fauna with the Oriental region, and has therefore been united
with it by Mr. A. Murray. Most important differences have however been
overlooked, as the following summary of the peculiarities of the Ethiopian
region will, I think, show.
It possesses 22 peculiar families of vertebrates; 90 peculiar genera of
mammalia, being two-thirds of its whole number; and 179 peculiar genera of
birds, being three-fifths of all it possesses. It is further characterized
by the absence of several families and genera which range over the whole
northern hemisphere, details of which will be found in the chapter treating
of the region. There are, it is true, many points of resemblance, not to be
wondered at between two tropical regions in the same hemisphere, and which
have evidently been at one time more nearly connected, both by intervening
lands and by a different condition of the lands that even now connect them.
But these resemblances only render the differences more remarkable; since
they show that there has been an ancient and long-continued separation of
the two regions, developing a distinct fauna in each, and establishing
marked specialities which the temporary intercommunication and immigration
has not sufficed to remove. The entire absence of such wide-spread groups
as bears and deer, from a country many parts of which are well adapted to
them, and in close proximity to regions where they abound, would alone mark
out the Ethiopian region as one of the primary divisions of the earth, even
if it possessed a less number than it actually does of peculiar family and
generic groups.
_Sub-divisions of the Ethiopian Region._--The African continent south of
the tropic of Cancer is more homogeneous in its prominent and superficial
zoological features than most of the other regions, but there are
nevertheless important and {74}deep-seated local peculiarities. Two
portions can be marked off as possessing many peculiar forms; the luxuriant
forest district of equatorial West Africa, and the southern extremity or
Cape district. The remaining portion has no well-marked divisions, and a
large proportion of its animal forms range over it from Nubia and
Abyssinia, to Senegal on the one side and to the Zambesi on the other; this
forms our first or East-African sub-region.
The second, or West African sub-region extends along the coast from Senegal
to Angola, and inland to the sources of the Shary and the Congo.
The third, or South African sub-region, comprises the Cape Colony and
Natal, and is roughly limited by a line from Delagoa Bay to Walvish Bay.