peculiarities which throw much light on the subject of distribution.
In the Galapagos islands, between 500 and 600 miles from the west
coast of South America, there are thirty-two species of land-birds,
all but two or three being peculiar to the group. In Madeira,
about 400 miles from the coast of Morocco, there are nearly twice
as many land-birds as in the Galapagos, but only two of these are
peculiar to the island, the rest being South European or N. African
species. The Azores are 1,000 miles west of Portugal, and they
contain twenty-two species of land-birds, every one of which is
European except one bullfinch which is slightly different and forms
a peculiar species. This remarkable difference in the proportion of
peculiar species between the Galapagos and the Atlantic islands, is
well explained by the theory that land-birds rarely fly directly
out to sea, except when carried against their will by storms and
gales of wind. Now the Azores are situated in an especially stormy
zone, and it is an observed fact that after every severe gale of
wind some new bird or insect is seen on the islands. The Galapagos,
on the contrary, are in a very calm sea where violent storms are
almost unknown, and thus new birds from the mainland very rarely
visit these islands. Madeira is less stormy than the Azores, but
its comparative nearness makes up for this difference in the case
of birds. In insects, however, the species of Madeira are much
more peculiar (and more numerous) than those of the more distant
Azores; while those of the Galapagos are few, but all peculiar,
and belonging to groups many of which are widely spread over the
globe. All these facts are entirely in accordance with the view
that oceanic islands have been peopled from the nearest continents
by various accidental causes; while they are entirely opposed to
the theory that such islands are remnants of old continents and
have preserved some portion of their inhabitants.
It is a curious fact, that land reptiles, such as snakes and
lizards, are found in many islands where there are no mammalia or
frogs; and we therefore conclude that there must be some means by
which their ova can be safely carried across great widths of sea.
A single peculiar frog inhabits New Zealand, and some species are
found in the Pacific islands as far eastward as the Fijis, but they
are absent from all other oceanic islands. Snakes also extend to
the Fijis, and there are two species in the Galapagos, but none in
the other oceanic islands. Lizards, however, are found in Mauritius
and Bourbon; in New Zealand; in all the Pacific islands, and in
the Galapagos. It is clear then that next to Mammals, frogs and
toads are most completely shut out by an ocean barrier; then follow
snakes, but as these are only found in the Galapagos and are very
like South American species, they may possibly have been conveyed
in boats or by floating trees. Lizards, however, are so wide-spread
over almost all the warmer islands of the great oceans, that they
must have some natural way of passing over, but the exact mode in
which this is effected has not yet been discovered. Birds, as we
have seen, are liable to be carried by winds and storms over great
widths of sea, but this only applies to certain groups; and large
numbers which feed on the ground or which inhabit the depths of
the forests, are almost as strictly confined to their respective
countries by even a narrow arm of the sea as are the majority of
the mammalia.
This sketch of the mode in which the various kinds of islands
have been stocked with their animal inhabitants forms the best
introduction to the study of those changes in our continents which
have led to the existing distribution of animals. It demonstrates
the importance of the sea as a _barrier_ to the spread of all the
higher animals; and we are thus naturally led on to inquire, how
far and to what extent such barriers have in past time existed
between lands which are now united, and on the other hand what
existing oceanic barriers are of comparatively recent origin.
In pursuing this inquiry we shall have to take account of those
grand views of the course of nature associated with the names of
Lyell and Darwin--of the slow but never-ceasing changes in the
physical conditions, the outlines and the mutual relations of the
land-surfaces of the globe; and of the equally slow and equally
unceasing changes in the forms and structures of all organisms,
to a great extent correlated with, and perhaps dependent on, the
former set of changes. Combining these two great principles with
other ascertained causes of distribution, we shall be enabled to
deal adequately with the problem before us, and give a rational,
though often only an approximative and conjectural, solution of the
many strange anomalies we meet with in studying the distribution
of living things.
_Past and Present Distribution of Land and Sea._--Before proceeding
to give details as to the distribution of animals, it is necessary
to point out certain geographical features which have had great
influence in bringing about the existing state of things.
The extreme inequality with which land and water is distributed
has often been remarked, but what is less frequently noted is the
singular way in which all the great masses of land are linked
together. Notwithstanding the small proportion of land to water,
the vast difference in the quantity of land in the northern and
southern hemispheres, and the apparently hap-hazard manner in which
it is spread over the globe, we yet find that no important area
is completely isolated from the rest. We may even travel from the
extreme north of Asia to the three great southern promontories--Cape
Horn, the Cape of Good Hope, and Tasmania--without ever going out of
sight of land; and, if we examine a terrestrial globe, we find that
the continents in their totality may be likened to a huge creeping
plant, whose roots are at or around the North Pole, whose matted
stems and branches cover a large part of the northern hemisphere,
while it sends out in three directions great offshoots towards
the South Pole. This singular arrangement of the land surface into
what is practically one huge mass with diverging arms, offers great
facilities for the transmission of the varied forms of animal life
over the whole earth, and is no doubt one of the chief causes of
the essential unity of type which everywhere characterises the
existing animal and vegetable productions of the globe.
There is, moreover, good reason to believe that the general features
of this arrangement are of vast antiquity; and that throughout much
of the Tertiary period, at all events, the relative positions of
our continents and oceans have remained the same, although they
have certainly undergone some changes in their extent, and in the
degree of their connection with each other. This is proved by
two kinds of evidence. In the first place, it is now ascertained
by actual measurement that the depths of the great oceans are so
vast over wide areas, while the highest elevations of the land
are limited to comparatively narrow ridges, that the mass of land
(above the sea-level) is not more than ¹⁄₃₆th part of the mass
of the ocean. Now we have reason to believe that subsidence and
elevation bear some kind of proportion to each other, whence it
follows that although several mountain ranges have risen to great
heights during the Tertiary period, this amount of elevation bears
no proportion to the amount of subsidence required to have changed
any considerable area of what was once land into such profound
depths as those of the Atlantic or Pacific Oceans. In the second
place, we find over a considerable area of all the great continents
fresh-water deposits containing the remains of land animals and
plants; which deposits must have been formed in lakes or estuaries,
and which therefore, speaking generally, imply the existence in
their immediate vicinity of land areas comparable to those which
still exist. The Miocene deposits of Central and Western Europe,
of Greece, of India, and of China, as well as those of various
parts of North America, strikingly prove this; while the Eocene
deposits of London and Paris, of Belgium, and of various parts of
North and South America, though often marine, yet by their abundant
remains of land-animals and plants, equally indicate the vicinity
of extensive continents. For our purpose it is not necessary to go
further back than this, but there is much evidence to show that
throughout the Secondary, and even some portion of the Palæozoic
periods, the land-areas coincided to a considerable extent with our
existing continents. Professor Ramsay has shown[64] that not only
the Wealden formation, and considerable portions of the Upper and
Lower Oolite, but also much of the Trias, and the larger part of
the Permian, Carboniferous, and Old Red Sandstone formations, were
almost certainly deposited either in lakes, inland seas, or extensive
estuaries. This would prove that, throughout the whole of the vast
epochs extending back to the time of the Devonian formation, our
present continents have been substantially in existence, subject,
no doubt, to vast fluctuations by extension or contraction, and by
various degrees of union or separation, but never so completely
submerged as to be replaced by oceans comparable in depth with our
Atlantic or Pacific.
[64] _Nature_, 1873, p. 333; _Quarterly Journal of the Geological
Society_, 1871, pp. 189 and 241.
This general conclusion is of great importance in the study of the
geographical distribution of animals, because it bids us avoid the
too hasty assumption that the countless anomalies we meet with are to
be explained by great changes in the distribution of land and sea,
and leads us to rely more on the inherent powers of dispersal which
all organisms possess, and on the union or disruption, extension or
diminution, of existing lands--but always in such directions and
to such a limited extent as not to involve the elevation of what
are now the profoundest depths of the great oceans.
_Zoological Regions._--We will now proceed to sketch out the
zoological features of the six great biological regions; and will
afterwards discuss their probable changes during the more recent
geographical periods, in accordance with the principles here laid
down.
_The Palæarctic Region._
The Palæarctic, or North Temperate region of the Old World, is not
only by far the most extensive of the zoological regions, but is the
one which agrees least with our ordinary geographical divisions.
It includes the whole of Europe, by far the largest part of Asia,
and a considerable tract of North Africa; yet over the whole of
this vast area there prevails a unity of the forms of animal life
which renders any primary subdivision of it impossible, and even
secondary divisions difficult. But besides being the largest of the
great zoological regions, there are good reasons for believing this
to represent the most ancient, and therefore the most important
centre of the development of the higher forms of animal life,--and
it is therefore well to consider it first in order.
In enumerating the most important animal groups characteristic of
this and other regions, it must be clearly understood that such
groups are not always absolutely confined to one region. Here and
there they will often overlap the boundaries, while in other cases
single species may have a wide distribution in one or more of the
adjacent regions; but this does not at all affect the main fact,
that the group as a whole is very abundant and very widely spread
over the region in question, while it is very rare, or confined to
a very limited area in adjacent regions, and is therefore specially
characteristic of the one as compared with other parts of the world.
Bearing this in mind, we shall find that the Palæarctic region is
well characterized by a considerable number of typical groups,
although, as we shall presently see, it has in recent geological
times lost much of its ancient richness and variety of animal life.
Among Mammalia the groups most characteristic of this region are
the moles (Talpidæ), a family consisting of eight distinct genera
which range over the whole region, but beyond it barely enter
the Oriental region in North India, and the Nearctic region in
North-West America; camels, confined to the deserts of North Africa
and Asia; sheep and goats (Capra), only found beyond the region in
the Nilgherries and Rocky Mountains; several groups of antelopes,
and many peculiar forms of deer; hamsters (Cricetus), sand rats
(Psammomys), mole rats (Spalax), and pikas (Lagomys), with several
other forms of rodents. Wolves, foxes, and bears, are also very
characteristic, though by no means confined to the region.
Among birds the most important group is certainly the small-sized,
but highly-organized warblers (Sylviidæ), which, although almost
universally distributed, are more numerous, and have more peculiar
and characteristic genera here than in any other region. Most of our
song-birds, and many of the commonest tenants of our fields, woods,
and gardens, belong to this family; and identical or representative
species are often found ranging from Spain to China, and from
Ireland to Japan. The reedlings (Panuridæ), the tits (Paridæ),
and the magpies (Pica), are also very characteristic; while among
the finches (Fringillidæ), a considerable number of genera are
peculiar. A large number of peculiar groups of grouse (Tetraonidæ),
and pheasants (Phasianidæ) are also characteristic of this region.
Although the reptiles and fresh-water fishes are comparatively
few, yet many of them are peculiar. Thus, no less than 2 genera
of snakes, 7 of lizards, and 16 of batrachia, are confined to the
Palæarctic region, as well as 20 genera of fresh-water fishes.
The insects and land-shells offer their full proportion of peculiar
types; but it would lead us beyond our special object to enter into
details with regard to these less known groups of animals.[65]
[65] Details will be found in the Author’s work on _The Geographical
Distribution of Animals_.
_The Ethiopian Region._
The Ethiopian region, consisting of Africa south of the Tropic of
Cancer with Madagascar, is of very small area compared with the
Palæarctic region; yet owing to the absence of extreme climates, and
the tropical luxuriance of a considerable portion of its surface,
it supports a greater number and variety of large animals than any
other part of the globe of equal extent. Much of the speciality
of the region is, however, due to the rich and isolated fauna of
Madagascar, the peculiarities of which may be set aside till we
come to discuss the past history of the Ethiopian region.
Considering then, first, the zoological features of tropical and
southern Africa alone, we find a number of very peculiar forms
of mammalia. Such are the golden moles, the Potamogale, and the
elephant-shrews among Insectivora; the hippopotami and the giraffes,
among Ungulata; the hyæna-like Proteles (Aard-wolf), and Lycaon
(hyæna-dog), among Carnivora; and the Aard-varks (Orycteropus) among
Edentata. These are all peculiar; but among highly characteristic
forms are the baboons, and several genera of monkeys and apes;
several peculiar Lemurs; a great variety of the civet-family
(Viverridæ), and of rodents; peculiar genera of swine (Potamochærus
and Phacochærus), and a greater abundance and variety of antelopes
than are to be found in all the other regions combined. But the
Ethiopian region is strikingly distinguished from all others, not
only by possessing many peculiar forms, but by the absence of a
number of common and widely distributed groups of mammalia. Such
are--the bears, which range over the whole northern hemisphere, and
as far south as Sumatra in the eastern and Chili in the western
hemisphere, yet they are totally wanting in Tropical and South
Africa;--the deer, which are still more widely distributed, ranging
all over North and South America, and over all Asia to Celebes and
the Moluccas, yet they are totally absent from the Ethiopian region;
goats and sheep, true oxen (Bos), and true pigs (Sus), are also
absent; though as to the last there is some doubt, certain wild pigs
having been observed, though rarely, in various parts of Tropical
Africa, but it is not yet determined whether they are indigenous,
or escaped from domestication. The absence of such wide-spread
families as the bears and deer is, however, most important, and must
be taken into account when we come to consider the geographical
changes needed to explain the actual state of the Ethiopian fauna.
The birds are not proportionately so peculiar, yet there are
many remarkable forms. Most important are the plantain-eaters,
the ground-hornbills, the colies, and the anomalous
secretary-bird;--while among characteristic families there are
numbers of peculiar genera of flycatchers, shrikes, crows, sun-birds,
weaver-birds, starlings, larks, francolins, and the remarkable
subfamily of the Guinea-fowls. There are not such striking
deficiencies among birds as among mammals, yet there are some of
importance. Thus, there are no wrens, creepers, or nut-hatches, and
none of the wide-spread group comprising the true pheasants and
jungle fowl--a deficiency almost comparable with that of the bears
or the deer. Among the lower vertebrates there are 3 peculiar
families of snakes and 1 of lizards, as well as 1 of toads and 3
of fresh-water fishes.
_The Oriental Region._
The Oriental region comprises all tropical Asia east of the Indus,
with the Malay Islands as far as Java, Borneo, and the Philippines.
In its actual land-area it is the smallest region except the
Australian; but if we take into account the wide extent of shallow
sea connecting Indo-China with the Malay Islands, and which has,
doubtless, at no distant epoch, formed an extension of the Asiatic
Continent, it will not be much smaller than the Ethiopian region.
Here we find all the conditions favourable to the development of a
rich and varied fauna. The land is broken up into great peninsulas
and extensive islands; lofty mountains and large rivers everywhere
intersect it; while along its northern boundary stretches the
highest mountain-range upon the globe. Much of this region lies
within the equatorial belt, where the equability of temperature and
abundance of moisture produce a tropical vegetation of unsurpassed
luxuriance. We find here, as might be expected, that the variety
and beauty of the birds and insects is somewhat greater than in
the Ethiopian region; although, as regards mammalia, the latter is
the most prolific, both in genera, species, and individuals.
The families of Mammalia actually peculiar to this region are few
in number, and of limited extent. They are,--the Galeopithecidæ,
or flying lemurs; the Tarsiidæ, consisting of the curious little
tarsier, allied to the lemurs; and the Tupaiidæ, a remarkable group
of squirrel-like Insectivora. There are, however, a considerable
number of peculiar genera, forming highly characteristic groups
of animals--such as the various apes, monkeys, and lemurs, almost
all the genera of which are peculiar; a large number of civets
and weasels; the beautiful deer-like Chevrotains, often called
mouse-deer; and a few peculiar antelopes and rodents. It must be
remarked that we find here none of those deficiencies of wide-spread
families which were so conspicuous a feature of the Ethiopian
region--the only one worth notice being the dormice (Myoxidæ), a
small family spread over the Palæarctic and Ethiopian regions, but
not found in the Oriental.
The birds of the Oriental region are exceedingly numerous and varied,
there being representatives of about 350 genera of land-birds,
of which nearly half are peculiar. Three families are confined
to the region--the hill-tits (Liotrichidæ), the green bulbuls
(Phyllornithidæ), and the gapers (Eurylæmidæ); while four other
families are more abundant here than elsewhere, and are so widely
distributed throughout the region as to be especially characteristic
of it. These are--the elegant pittas, or ground-thrushes (Pittidæ),
the trogons (Trogonidæ), the hornbills (Bucerotidæ), and the
pheasants (Phasianidæ); represented by such magnificent birds as the
fire-backed pheasants, the ocellated pheasants, the Argus-pheasant,
the pea-fowl, and the jungle-fowl.
Reptiles are very abundant, but only 3 small families of snakes
are peculiar. There are also 3 peculiar families of fresh-water
fishes.
_Past Changes of the Great Eastern Continent._--Having thus briefly
sketched the main features of the existing faunas of Europe, Asia,
and Africa, it will be well, while their resemblances and differences
are fresh in our memory, to consider what evidence we have of
the changes which may have resulted in their present condition.
All these countries are so intimately connected, that their past
history is greatly elucidated by the knowledge we possess of the
tertiary fauna of Europe and India; and we shall find that when we
once obtain clear ideas of their mutual relations, we shall be in
a better position to study the history of the remaining continents.
Let us therefore go back to the Miocene or middle tertiary epoch, and
see what was then the distribution of the higher animals in these
countries. Extensive deposits, rich in animal remains of the Miocene
age, occur in France, Switzerland, Germany, Hungary, Greece; and
also in North-Western India at the Siwalik Hills, in Central India
in the Nerbudda Valley, in Burmah, and in North China; and over the
whole of this immense area we find a general agreement in the fossil
mammalia, indicating that this great continent was probably then,
as now, one continuous land. The next important geographical fact
that meets us, is, that many of the largest and most characteristic
animals, now confined to the tropics of the Oriental and Ethiopian
regions, were then abundant over much of the Palæarctic region.
Elephants, rhinoceroses, tapirs, horses, giraffes, antelopes, hyænas,
lions, as well as numerous apes and monkeys, ranged all over Central
Europe, and were often represented by a greater variety of species
than exist now. Antelopes were abundant in Greece, and several
of these appear to have been the ancestors of those now living in
Africa; while two species of giraffes also inhabited Greece and
North-West India. Equally suggestive is the occurrence in Europe of
such birds as trogons and jungle-fowl characteristic of tropical
Asia, along with parrots and plantain-eaters allied to forms now
living in West Africa.
Let us now inquire what information Geology affords us of changes in
land and sea at this period. From the prevalence of early tertiary
deposits over the Sahara and over parts of Arabia, Persia, and
Northern India, geologists are of opinion that a continuous sea
or strait extended from the Bay of Bengal to the Atlantic Ocean,
thus cutting off the Peninsula of India with Ceylon, as well as all
tropical and South Africa from the great northern continent.[66]
At the same time, and down to a comparatively recent period, it
is almost certain that Northern Africa was united to Spain and to
Italy, while Asia Minor was united to Greece, thus reducing the
Mediterranean to the condition of two inland seas. We also know that
the north-western Himalayas and some of the high lands of Central
Asia were at such a moderate elevation as to enjoy a climate as
mild as that which prevailed in Central Europe during the Miocene
epoch,[67] and was therefore perhaps equally productive in animal
and vegetable life.
[66] Mr. Searles V. Wood, “On the Form and Distribution of the
Land-tracts during the Secondary and Tertiary Periods respectively,”
_Philosophical Magazine_, 1862.
[67] This part of the Himalayas was elevated during the Eocene
period, and remains of a fossil _Rhinoceros_ have been found at
16,000 feet elevation in Thibet.