SUMMARY AND CONCLUSION
The Present Volume is the Development and Application of a
Theory--Statement of the Biological and Physical Causes of
Dispersal--Investigation of the Facts of Dispersal--Of the Means of
Dispersal--Of Geographical Changes Affecting Dispersal--Of Climatal Changes
Affecting Dispersal--The Glacial Epoch and its Causes--Alleged Ancient
Glacial Epochs--Warm Polar Climates and their Causes--Conclusions as to
Geological Climates--How Far Different from those of Mr. Croll--Supposed
Limitations of Geological Time--Time Amply Sufficient both for Geological
and Biological Development--Insular Faunas and Floras--The North Atlantic
Islands--The Galapagos--St. Helena and the Sandwich Islands--Great Britain
as a Recent Continental Island--Borneo and Java--Japan and
Formosa--Madagascar as an Ancient Continental Island--Celebes and New
Zealand as Anomalous Islands--The Flora of New Zealand and its Origin--The
European Element in the South Temperate Floras--Concluding Remarks
_Pages_ 531-545
* * * * *
{xix}
MAPS AND ILLUSTRATIONS
PAGE
1. MAP SHOWING THE DISTRIBUTION OF THE TRUE JAYS _Frontispiece._
2. MAP SHOWING THE ZOOLOGICAL REGIONS _To face_ 31
3. MAP SHOWING THE DISTRIBUTION OF _PARUS PALUSTRIS_ _To face_ 66
4. A GLACIER WITH MORAINES (From Sir C. Lyell's _Principles
of Geology_) 109
5. MAP OF THE ANCIENT RHONE GLACIER (From Sir C. Lyell's
_Antiquity of Man_) 110
6. DIAGRAM SHOWING THE EFFECTS OF EXCENTRICITY AND PRECESSION
ON CLIMATE 127
7. DIAGRAM OF EXCENTRICITY AND PRECESSION 129
8. MAP SHOWING THE EXTENT OF THE NORTH AND SOUTH POLAR ICE 138
9. DIAGRAM SHOWING CHANGES OF EXCENTRICITY DURING THREE MILLION
YEARS 171
10. OUTLINE MAP OF THE AZORES 248
11. MAP OF BERMUDA AND THE AMERICAN COAST 263
12. SECTION OF BERMUDA AND ADJACENT SEA-BOTTOM 264
{xx}
13. MAP OF THE GALAPAGOS AND ADJACENT COASTS OF SOUTH AMERICA 276
14. MAP OF THE GALAPAGOS 277
15. MAP OF THE SOUTH ATLANTIC, SHOWING POSITION OF ST. HELENA 293
16. MAP OF THE SANDWICH ISLANDS 311
17. MAP OF THE NORTH PACIFIC, WITH ITS SUBMERGED BANKS 312
18. MAP SHOWING THE BANK CONNECTING BRITAIN WITH THE CONTINENT 333
19. MAP OF BORNEO AND JAVA, SHOWING THE GREAT SUBMARINE BANK OF
SOUTH-EASTERN ASIA 373
20. MAP OF JAPAN AND FORMOSA 392
21. PHYSICAL SKETCH MAP OF MADAGASCAR (From _Nature_) 413
22. MAP OF MADAGASCAR GROUP, SHOWING DEPTHS OF SEA 415
23. MAP OF THE INDIAN OCEAN 424
24. MAP OF CELEBES AND THE SURROUNDING ISLANDS 451
25. MAP SHOWING DEPTHS OF SEA AROUND AUSTRALIA AND NEW ZEALAND 471
26. MAP SHOWING THE PROBABLE CONDITION OF AUSTRALIA DURING THE
CRETACEOUS EPOCH 496
* * * * *
ISLAND LIFE
PART I
_THE DISPERSAL OF ORGANISMS_
_ITS PHENOMENA, LAWS, AND CAUSES_
{3}
CHAPTER I
INTRODUCTORY
Remarkable Contrasts in distribution of Animals--Britain and
Japan--Australia and New Zealand--Bali and Lombok--Florida and Bahama
Islands--Brazil and Africa--Borneo, Madagascar, and Celebes--Problems
in distribution to be found in every country--Can be solved only by the
combination of many distinct lines of inquiry, biological and
physical--Islands offer the best subjects for the study of
distribution--Outline of the subjects to be discussed in the present
volume.
When an Englishman travels by the nearest sea-route from Great Britain to
Northern Japan he passes by countries very unlike his own, both in aspect
and natural productions. The sunny isles of the Mediterranean, the sands
and date-palms of Egypt, the arid rocks of Aden, the cocoa groves of
Ceylon, the tiger-haunted jungles of Malacca and Singapore, the fertile
plains and volcanic peaks of Luzon, the forest-clad mountains of Formosa,
and the bare hills of China, pass successively in review; till after a
circuitous voyage of thirteen thousand miles he finds himself at Hakodadi
in Japan. He is now separated from his starting-point by the whole width of
Europe and Northern Asia, by an almost endless succession of plains and
mountains, arid deserts or icy plateaux, yet when he visits the interior of
the country he sees so many familiar natural objects that he can hardly
help fancying he is close to his home. He finds the woods and fields
tenanted by tits, hedge-sparrows, wrens, wagtails, larks, redbreasts, {4}
thrushes, buntings, and house-sparrows, some absolutely identical with our
own feathered friends, others so closely resembling them that it requires a
practised ornithologist to tell the difference. If he is fond of insects he
notices many butterflies and a host of beetles which, though on close
examination they are found to be distinct from ours, are yet of the same
general aspect, and seem just what might be expected in any part of Europe.
There are also of course many birds and insects which are quite new and
peculiar, but these are by no means so numerous or conspicuous as to remove
the general impression of a wonderful resemblance between the productions
of such remote islands as Britain and Yesso.
Now let an inhabitant of Australia sail to New Zealand, a distance of less
than thirteen hundred miles, and he will find himself in a country whose
productions are totally unlike those of his own. Kangaroos and wombats
there are none, the birds are almost all entirely new, insects are very
scarce and quite unlike the handsome or strange Australian forms, while
even the vegetation is all changed, and no gum-tree, or wattle, or
grass-tree meets the traveller's eye.
But there are some more striking cases even than this, of the diversity of
the productions of countries not far apart. In the Malay Archipelago there
are two islands, named Bali and Lombok, each about as large as Corsica, and
separated by a strait only fifteen miles wide at its narrowest part. Yet
these islands differ far more from each other in their birds and quadrupeds
than do England and Japan. The birds of the one are extremely _unlike_
those of the other, the difference being such as to strike even the most
ordinary observer. Bali has red and green woodpeckers, barbets,
weaver-birds, and black-and-white magpie-robins, none of which are found in
Lombok, where, however, we find screaming cockatoos and friar-birds, and
the strange mound-building megapodes, which are all equally unknown in
Bali. Many of the kingfishers, crow-shrikes, and other birds, though of the
same general form, are of very distinct species; and though a considerable
number of birds are the same in both islands the difference {5} is none the
less remarkable--as proving that mere distance is one of the least
important of the causes which have determined the likeness or unlikeness in
the animals of different countries.
In the western hemisphere we find equally striking examples. The Eastern
United States possess very peculiar and interesting plants and animals, the
vegetation becoming more luxuriant as we go south but not altering in
essential character, so that when we reach Alabama or Florida we still find
ourselves in the midst of pines, oaks, sumachs, magnolias, vines, and other
characteristic forms of the temperate flora; while the birds, insects, and
land-shells are of the same general character with those found further
north.[1] But if we now cross over the narrow strait, about fifty miles
wide, which separates Florida from the Bahama Islands, we find ourselves in
a totally different country, surrounded by a vegetation which is
essentially tropical and generally identical with that of Cuba. The change
is most striking, because there is little difference of climate, of soil,
or apparently of position, to account for it; and when we find that the
birds, the insects, and especially the land-shells of the Bahamas are
almost all West Indian, while the North American types of plants and
animals have almost all completely disappeared, we shall be convinced that
such differences and resemblances cannot be due to existing conditions, but
must depend upon laws and causes to which mere proximity of position offers
no clue.
Hardly less uncertain and irregular are the effects of climate. Hot
countries usually differ widely from cold ones in all their organic forms;
but the difference is by no means constant, nor does it bear any proportion
to difference of temperature. Between frigid Canada and sub-tropical
Florida there are less marked differences in the animal productions than
between Florida and Cuba or Yucatan, so much more alike in climate and so
much nearer together. So the differences between the birds and quadrupeds
of temperate Tasmania and tropical North {6} Australia are slight and
unimportant as compared with the enormous differences we find when we pass
from the latter country to equally tropical Java. If we compare
corresponding portions of different continents, we find no indication that
the almost perfect similarity of climate and general conditions has any
tendency to produce similarity in the animal world. The equatorial parts of
Brazil and of the West Coast of Africa are almost identical in climate and
in luxuriance of vegetation, but their animal life is totally diverse. In
the former we have tapirs, sloths, and prehensile-tailed monkeys; in the
latter elephants, antelopes, and man-like apes; while among birds, the
toucans, chatterers, and humming-birds of Brazil are replaced by the
plantain-eaters, bee-eaters, and sun-birds of Africa. Parts of
South-temperate America, South Africa, and South Australia, correspond
closely in climate; yet the birds and quadrupeds of these three districts
are as completely unlike each other as those of any parts of the world that
can be named.
If we visit the great islands of the globe, we find that they present
similar anomalies in their animal productions, for while some exactly
resemble the nearest continents others are widely different. Thus the
quadrupeds, birds and insects of Borneo correspond very closely to those of
the Asiatic continent, while those of Madagascar are extremely unlike
African forms, although the distance from the continent is less in the
latter case than in the former. And if we compare the three great islands
Sumatra, Borneo, and Celebes--lying as it were side by side in the same
ocean--we find that the two former, although furthest apart, have almost
identical productions, while the two latter, though closer together, are
more unlike than Britain and Japan situated in different oceans and
separated by the largest of the great continents.
These examples will illustrate the kind of questions it is the object of
the present work to deal with. Every continent, every country, and every
island on the globe, offers similar problems of greater or less complexity
and interest, and the time has now arrived when their solution can be
attempted with some prospect of success. Many {7} years study of this class
of subjects has convinced me that there is no short and easy method of
dealing with them; because they are, in their very nature, the visible
outcome and residual product of the whole past history of the earth. If we
take the organic productions of a small island, or of any very limited
tract of country, such as a moderate-sized country parish, we have, in
their relations and affinities--in the fact that they are _there_ and
others are _not_ there, a problem which involves all the migrations of
these species and their ancestral forms--all the vicissitudes of climate
and all the changes of sea and land which have affected those
migrations--the whole series of actions and reactions which have determined
the preservation of some forms and the extinction of others,--in fact the
whole history of the earth, inorganic and organic, throughout a large
portion of geological time.
We shall perhaps better exhibit the scope and complexity of the subject,
and show that any intelligent study of it was almost impossible till quite
recently, if we concisely enumerate the great mass of facts and the number
of scientific theories or principles which are necessary for its
elucidation.
We require then in the first place an adequate knowledge of the fauna and
flora of the whole world, and even a detailed knowledge of many parts of
it, including the islands of more special interest and their adjacent
continents. This kind of knowledge is of very slow growth, and is still
very imperfect;[2] and in many cases it can {8} never now be obtained owing
to the reckless destruction of forests and with them of countless species
of plants and animals. In the next place we require a true and natural
classification of animals and plants, so that we may know their real
affinities; and it is only now that this is being generally arrived at. We
further have to make use of the theory of "descent with modification" as
the only possible key to the interpretation of the facts of distribution,
and this theory has only been generally accepted within the last twenty
years. It is evident that, so long as the belief in "special creations" of
each species prevailed, no explanation of the complex facts of distribution
_could_ be arrived at or even conceived; for if each species was created
where it is now found no further inquiry can take us beyond that fact, and
there is an end of the whole matter. Another important factor in our
interpretation of the phenomena of distribution, is a knowledge of the
extinct forms that have inhabited each country during the tertiary and
secondary periods of geology. New facts of this kind are daily coming to
light, but except as regards Europe, North America, and parts of India,
they are extremely scanty; and even in the best-known countries the record
itself is often very defective and fragmentary. Yet we have already
obtained remarkable evidence of the migrations of many animals and plants
in past ages, throwing an often unexpected light on the actual distribution
of many groups.[3] By this means alone can we obtain positive evidence of
the past migrations of organisms; and when, as too frequently is the case,
this is altogether wanting, we {9} have to trust to collateral evidence and
more or less probable hypothetical explanations. Hardly less valuable is
the evidence of stratigraphical geology; for this often shows us what parts
of a country have been submerged at certain epochs, and thus enables us to
prove that certain areas have been long isolated and the fauna and flora
allowed time for special development. Here, too, our knowledge is
exceedingly imperfect, though the blanks upon the geological map of the
world are yearly diminishing in extent. Lastly, as a most valuable
supplement to geology, we require to know approximately, the depth and
contour of the ocean-bed, since this affords an important clue to the
former existence of now-submerged lands, uniting islands to continents, or
affording intermediate stations which have aided the migrations of many
organisms. This kind of information has only been partially obtained during
the last few years; and it will be seen in the latter part of this volume,
that some of the most recent deep-sea soundings have afforded a basis for
an explanation of one of the most difficult and interesting questions in
geographical biology--the origin of the fauna and flora of New Zealand.
Such are the various classes of evidence that bear directly on the question
of the distribution of organisms; but there are others of even a more
fundamental character, and the importance of which is only now beginning to
be recognised by students of nature. These are, firstly, the wonderful
alterations of climate which have occurred in the temperate and polar
zones, as proved by the evidences of glaciation in the one and of luxuriant
vegetation in the other; and, secondly, the theory of the permanence of
existing continents and oceans. If glacial epochs in temperate lands and
mild climates near the poles have, as now believed by men of eminence,
occurred several times over in the past history of the earth, the effects
of such great and repeated changes, both on the migration, modification,
and extinction, of species, must have been of overwhelming importance--of
more importance perhaps than even the geological changes of sea and land.
It is therefore necessary to consider the evidence for these climatal
changes; {10} and then, by a critical examination of their possible causes,
to ascertain whether they were isolated phenomena, were due to recurrent
cosmical actions, or were the result of a great system of terrestrial
development. The latter is the conclusion we arrive at; and this conclusion
brings with it the conviction, that in the theory which accounts for both
glacial epochs and warm polar climates, we have the key to explain and
harmonize many of the most anomalous biological and geological phenomena,
and one which is especially valuable for the light it throws on the
dispersal and existing distribution of organisms. The other important
theory, or rather corollary from the preceding theory--that of the
permanence of oceans and the general stability of continents throughout all
geological time, is as yet very imperfectly understood, and seems, in fact,
to many persons in the nature of a paradox. The evidence for it, however,
appears to me to be conclusive; and it is certainly the most fundamental
question in regard to the subject we have to deal with: since, if we once
admit that continents and oceans may have changed places over and over
again (as many writers maintain), we lose all power of reasoning on the
migrations of ancestral forms of life, and are at the mercy of every wild
theorist who chooses to imagine the former existence of a now-submerged
continent to explain the existing distribution of a group of frogs or a
genus of beetles.
As already shown by the illustrative examples adduced in this chapter, some
of the most remarkable and interesting facts in the distribution and
affinities of organic forms are presented by islands in relation to each
other and to the surrounding continents. The study of the productions of
the Galapagos--so peculiar, and yet so decidedly related to the American
continent--appears to have had a powerful influence in determining the
direction of Mr. Darwin's researches into the origin of species; and every
naturalist who studies them has always been struck by the unexpected
relations or singular anomalies which are so often found to characterize
the fauna and flora of islands. Yet their full importance in connection
with the history of the earth and its inhabitants has hardly yet {11} been
recognised; and it is in order to direct the attention of naturalists to
this most promising field of research, that I restrict myself in this
volume to an elucidation of some of the problems they present to us. By far
the larger part of the islands of the globe are but portions of continents
undergoing some of the various changes to which they are ever subject; and
the correlative proposition, that every portion of our continents has again
and again passed through insular conditions, has not been sufficiently
considered, but is, I believe, the statement of a great and most suggestive
truth, and one which lies at the foundation of all accurate conception of
the physical and organic changes which have resulted in the present state
of the earth.
The indications now given of the scope and purpose of the present volume
renders it evident that, before we can proceed to the discussion of the
remarkable phenomena presented by insular faunas and floras, and the
complex causes which have produced them, we must go through a series of
preliminary studies, adapted to give us a command of the more important
facts and principles on which the solution of such problems depends. The
succeeding eight chapters will therefore be devoted to the explanation of
the mode of distribution, variation, modification, and dispersal, of
species and groups, illustrated by facts and examples; of the true nature
of geological change as affecting continents and islands; of changes of
climate, their nature, causes, and effects; of the duration of geological
time and the rate of organic development.
* * * * *
{12}
CHAPTER II
THE ELEMENTARY FACTS OF DISTRIBUTION
Importance of Locality as an essential character of Species--Areas of
Distribution--Extent and Limitations of Specific Areas--Specific range
of Birds--Generic Areas--Separate and overlapping areas--The species of
Tits as illustrating Areas of Distribution--The distribution of the
species of Jays--Discontinuous generic areas--Peculiarities of generic
and family distribution--General features of overlapping and
discontinuous areas--Restricted areas of Families--The distribution of
Orders.
So long as it was believed that the several species of animals and plants
were "special creations," and had been formed expressly to inhabit the
countries in which they are now found, their habitat was an ultimate fact
which required no explanation. It was assumed that every animal was
_exactly_ adapted to the climate and surroundings amid which it lived, and
that the only, or, at all events, the chief reason why it did not inhabit
another country was, that the climate or general conditions of that country
were not suitable to it, but in what the unsuitability consisted we could
rarely hope to discover. Hence the exact locality of any species was not
thought of much importance from a scientific point of view, and the idea
that anything could be learnt by a comparative study of different floras
and faunas never entered the minds of the older naturalists.
But so soon as the theory of evolution came to be generally adopted, and it
was seen that each animal could only have come into existence in some area
where ancestral {13} forms closely allied to it already lived, a real and
important relation was established between an animal and its native
country, and a new set of problems at once sprang into existence. From the
old point of view the _diversities_ of animal life in the separate
continents, even where physical conditions were almost identical, was the
fact that excited astonishment; but seen by the light of the evolution
theory, it is the _resemblances_ rather than the diversities in these
distant continents and islands that are most difficult to explain. It thus
comes to be admitted that a knowledge of the exact area occupied by a
species or a group is a real portion of its natural history, of as much
importance as its habits, its structure, or its affinities; and that we can
never arrive at any trustworthy conclusions as to how the present state of
the organic world was brought about, until we have ascertained with some
accuracy the general laws of the distribution of living things over the
earth's surface.
_Areas of Distribution._--Every species of animal has a certain area of
distribution to which, as a rule, it is permanently confined, although, no
doubt, the limits of its range fluctuate somewhat from year to year, and in
some exceptional cases may be considerably altered in a few years or
centuries. Each species is moreover usually limited to one continuous area,
over the whole of which it is more or less frequently to be met with, but
there are many apparent and some real exceptions to this rule. Some animals
are so adapted to certain kinds of country--as to forests or marshes,
mountains or deserts--that they cannot, permanently, live elsewhere. These
may be found scattered over a wide area in suitable spots only, but can
hardly on that account be said to have several distinct areas of
distribution. As an example we may name the chamois, which lives only on
high mountains, but is found in the Pyrenees, the Alps, the Carpathians, in
some of the Greek mountains and the Caucasus. The variable hare is another
and more remarkable case, being found all over Northern Europe and Asia
beyond lat. 55deg, and also in Scotland and Ireland. In central Europe it
is unknown till we come to the Alps, the Pyrenees, and the Caucasus, where
it again appears. This is one of the best cases known of the {14}
discontinuous distribution of a _species_, there being a gap of about a
thousand miles between its southern limits in Russia, and its reappearance
in the Alps. There are of course numerous instances in which species occur
in two or more islands, or in an island and continent, and are thus
rendered discontinuous by the sea, but these involve questions of changes
in sea and land which we shall have to consider further on. Other cases are
believed to exist of still wider separation of a species, as with the marsh
titmice and the reed buntings of Europe and Japan, where similar forms are
found in the extreme localities, while distinct varieties or sub-species,
inhabit the intervening districts.
_Extent and Limitations of Specific Areas._--Leaving for the present these
cases of want of continuity in a species, we find the most wide difference
between the extent of country occupied, varying in fact from a few square
miles to almost the entire land surface of the globe. Among the mammalia,
however, the same species seldom inhabits both the old and new worlds,
unless they are strictly arctic animals, as the reindeer, the elk, the
arctic fox, the glutton, the ermine, and some others. The common wolf of
Europe and Northern Asia is thought by many naturalists to be identical
with the variously coloured wolves of North America extending from the
Arctic Ocean to Mexico, in which case this will have perhaps the widest
range of any species of mammal. Little doubt exists as to the identity of
the brown bears and the beavers of Europe and North America; but all these
species range up to the arctic circle, and there is no example of a mammal
universally admitted to be identical yet confined to the temperate zones of
the two hemispheres. Among the undisputed species of mammalia the leopard
has an enormous range, extending all over Africa and South Asia to Borneo
and the east of China, and thus having probably the widest range of any
known mammal. The winged mammalia have not usually very wide ranges, there
being only one bat common to the Old and New Worlds. This is a British
species, _Vesperugo serotinus_, which is found over the larger part of
North America, Europe and Asia, as far {15} as Pekin, and even extends into
tropical Africa, thus rivalling the leopard and the wolf in the extent of
country it occupies.
Of very restricted ranges there are many examples, but some of these are
subject to doubts as to the distinctness of the species or as to its
geographical limits being really known. In Europe we have a distinct
species of ibex (_Capra Pyrenaica_) confined to the Pyrenean mountains,
while the true marmot is restricted to the Alpine range. More remarkable is
the Pyrenean water-mole (_Mygale Pyrenaica_), a curious small insectivorous
animal found only in a few places in the northern valleys of the Pyrenees.
In islands there are many cases of undoubted restriction of species to a
small area, but these involve a different question from the range of
species on continents where there is no _apparent_ obstacle to their wider
extension.
_Specific range of Birds._--Among birds we find instances of much wider
range of species, which is only what might be expected considering their
powers of flight; but, what is very curious, we also find more striking
(though perhaps not more frequent) examples of extreme limitation of range
among birds than among mammals. Of the former phenomenon perhaps the most
remarkable case is that afforded by the osprey or fishing-hawk, which
ranges over the greater portion of all the continents, as far as Brazil,
South Africa, the Malay Islands, and Tasmania. The barn owl (_Strix
flammea_) has nearly as wide a range, but in this case there is more
diversity of opinion as to the specific difference of many of the forms
inhabiting remote countries, some of which seem undoubtedly to be distinct.
Among passerine birds the raven has probably the widest range, extending
from the arctic regions to Texas and New Mexico in America, and to North
India and Lake Baikal in Asia; while the little northern willow-wren
(_Phylloscopus borealis_) ranges from arctic Norway across Asia to Alaska,
and southward to Ceylon, China, Borneo, and Timor.
Of very restricted continental ranges the best examples in Europe are, the
little blue magpie (_Cyanopica cooki_) confined to the central portions of
the Spanish peninsula; and the Italian sparrow found only in Italy and
Corsica. {16} In Asia, Palestine affords some examples of birds of very
restricted range--a beautiful sun-bird (_Nectarinea osea_) a peculiar
starling (_Amydrus tristramii_) and some others, being almost or quite
confined to the warmer portions of the valley of the Jordan. In the
Himalayas there are numbers of birds which have very restricted ranges, but
those of the Neilgherries are perhaps better known, several species of
laughing thrushes and some other birds being found only on the summits of
these mountains. The most wonderfully restricted ranges are, however, to be
found among the humming-birds of tropical America. The great volcanic peaks
of Chimborazo and Pichincha have each a peculiar species of humming-bird
confined to a belt just below the limits of perpetual snow, while the
extinct volcano of Chiriqui in Veragua has a species confined to its wooded
crater. One of the most strange and beautiful of the humming-birds
(_Loddigesia mirabilis_) was obtained once only, more than forty years ago,
near Chachapoyas in the Andes of northern Peru; and though Mr. Gould sent
many drawings of the bird to people visiting the district and for many
years offered a high reward for a specimen, no other has ever been seen![4]
The above details will sufficiently explain what is meant by the "specific
area" or range of a species. The very wide and very narrow ranges are
exceptional, the great majority of species both of mammals and birds
ranging over moderately wide areas, which present no striking contrasts in
climate and physical conditions. Thus a large proportion of European birds
range over the whole continent in an east and west direction, but
considerable numbers are restricted either to the northern or the southern
half. In Africa some species range over all the continent south of the
desert, while large numbers are restricted to the equatorial forests, or to
the upland plains. In North America, if we exclude the tropical and the
arctic portions, a considerable number of species range over all the
temperate parts of the continent, while still {17} more are restricted to
the east, the centre, or the west, respectively.
_Generic Areas._--Having thus obtained a tolerably clear idea of the main
facts as to the distribution of isolated species, let us now consider those
collections of closely-allied species termed genera. What a genus is will
be sufficiently understood by a few illustrations. All the different kinds
of dogs, jackals, and wolves belong to the dog genus, Canis; the tiger,
lion, leopard, jaguar, and the wild cats, to the cat genus, Felis; the
blackbird, song-thrush, missel-thrush, fieldfare, and many others to the
thrush genus, Turdus; the crow, rook, raven, and jackdaw, to the crow
genus, Corvus; but the magpie belongs to another, though closely-allied
genus, Pica, distinguished by the different form and proportions of its
wings and tail from all the species of the crow genus. The number of
species in a genus varies greatly, from one up to several hundreds. The
giraffe, the glutton, the walrus, the bearded reedling, the secretary-bird,
and many others, have no close allies, and each forms a genus by itself.
The beaver genus, Castor, and the camel genus, Camelus, each consist of two
species. On the other hand, the deer genus, Cervus has forty species; the
mouse and rat genus, Mus more than a hundred species; and there is about
the same number of the thrush genus; while among the lower classes of
animals genera are often very extensive, the fine genus Papilio, or
swallow-tailed butterflies, containing more than four hundred species; and
Cicindela, which includes our native tiger beetles, has about the same
number. Many genera of shells are very extensive, and one of them--the
genus Helix, including the commonest snails, and ranging all over the
world--is probably the most extensive in the animal kingdom, numbering
about two thousand described species.[5]
_Separate and Overlapping Areas._--The species of a genus are distributed
in two ways. Either they occupy distinct areas which do not touch each
other and are sometimes widely separated, or they touch and occasionally
overlap {18} each other, each species occupying an area of its own which
rarely coincides exactly with that of any other species of the same genus.
In some cases, when a river, a mountain-chain, or a change of conditions as
from pasture to desert or forest, determines the range of species, the
areas of two species of the same genus may just meet, one beginning where
the other ends; but this is comparatively rare. It occurs, however, in the
Amazon valley, where several species of monkeys, birds, and insects come up
to the south bank of the river but do not pass it, while allied species
come to the north bank, which in like manner forms their boundary. As
examples we may mention that one of the Saki monkeys (_Pithecia monachus?_)
comes up to the south bank of the Upper Amazon, while immediately we cross
over to the north bank we find another species (_Pithecia rufibarbata?_).
Among birds we have the green jacamar (_Galbula viridis_), abundant on the
north bank of the Lower Amazon, while on the south bank we have two allied
species (_Galbula rufoviridis_ and _G. cyaneicollis_); and among insects we
have at Santarem on the south bank of the Amazon, the beautiful blue
butterfly, _Callithea sapphira_, while almost opposite to it, at
Monte-alegre, an allied species, _Callithea Leprieuri_ is alone found.
Perhaps the most interesting and best known case of a series of allied
species, whose ranges are separate but conterminous, is that of the
beautiful South American wading birds, called trumpeters, and forming the
genus Psophia. There are five species, all found in the Amazon valley, but
each limited to a well-marked district bounded by great rivers. On the
north bank of the Amazon there are two species, one in its lower valley
extending up to the Rio Negro; and the other in the central part of the
valley beyond that river; while to the south of the Amazon there are three,
one above the Madeira, one below it, and a third near Para, probably
separated from the last by the Tocantins river.
Overlapping areas among the species of a genus is a more common phenomenon,
and is almost universal where these species are numerous in the same
continent. It is, however, exceedingly irregular, so that we often find one
{19} species extending over a considerable portion of the area occupied by
the genus and including the entire areas of some of the other species. So
little has been done to work out accurately the limits of species that it
is very difficult to give examples. One of the best is to be found in the
genus _Dendroeca_, a group of American wood-warblers. These little birds
all migrate in the winter into the tropical regions, but in the summer they
come north, each having its particular range. Thus, _D. dominica_ comes as
far as the middle Eastern States, _D. coerulea_ keeps west of the
Alleghanies, _D. discolor_ comes to Michigan and New England; four other
species go farther north in Canada, while several extend to the borders of
the Arctic zone.
_The Species of Tits as Illustrating Areas of Distribution._--In our own
hemisphere the overlapping of allied species may be well illustrated by the
various kinds of titmice, constituting the genus Parus, several of which
are among our best known English birds. The great titmouse (_Parus major_)
has the widest range of all, extending from the Arctic circle to Algeria,
Palestine, and Persia, and from Ireland right across Siberia to the Ochotsk
sea, probably following the great northern forest belt. It does not extend
into China and Japan, where distinct species are found. Next in extent of
range is the coal tit (_Parus ater_) which inhabits all Europe from the
Mediterranean to about 64deg N. latitude, in Asia Minor to the Lebanon and
Caucasus, and across Siberia to Amoorland and Japan. The marsh tit (_Parus
palustris_) inhabits temperate and south Europe from 61deg N. latitude in
Norway to Poland and South-west Russia, and in the south from Spain to Asia
Minor. Closely allied to this--of which it is probably only a variety or
sub-species--is the northern marsh tit (_Parus borealis_), which overlaps
the last in Norway and Sweden, and also in South Russia and the Alps, but
extends further north into Lapland and North Russia, and thence probably in
a south-easterly direction across Central Asia to North China. Yet another
closely-allied species (_Parus camtschatkensis_) ranges from North-eastern
Russia across Northern Siberia to Lake Baikal and to Hakodadi in Japan,
thus overlapping _Parus borealis_ in the {20} western portion of its area.
Our little favourite, the blue tit (_Parus coeruleus_) ranges over all
Europe from the Arctic circle to the Mediterranean, and on to Asia Minor
and Persia, but does not seem to pass beyond the Ural mountains. Its lovely
eastern ally the azure tit (_Parus cyaneus_) overlaps the range of _P.
coeruleus_ in Western Europe as far as St. Petersburg and Austria, rarely
straggling to Denmark, while it stretches all across Central Asia between
the latitudes 35deg and 56deg N. as far as the Amoor valley. Besides these
wide-ranging species there are several others which are more restricted.
_Parus teneriffae_, a beautiful dark blue form of our blue tit, inhabits
North-west Africa and the Canaries; _Parus ledouci_, closely allied to our
coal tit, is found only in Algeria; _Parus lugubris_, allied to the marsh
tit, is confined to South-east Europe and Asia Minor, from Hungary and
South Russia to Palestine; and _Parus cinctus_, another allied form, is
confined to the extreme north in Lapland, Finland, and perhaps Northern
Russia and Siberia. Another beautiful little bird, the crested titmouse
(_Parus cristatus_) is sometimes placed in a separate genus. It inhabits
nearly all Central and South Europe, wherever there are pine forests, from
64deg N. latitude to Austria and North Italy, and in the west to Spain and
Gibraltar, while in the east it does not pass the Urals and the Caucasus
range. Its nearest allies are in the high Himalayas.
These are all the European tits, but there are many others inhabiting Asia,
Africa, and North America; so that the genus Parus has a very wide range,
in Asia to Ceylon and the Malay Islands, in Africa to the Cape, and in
North America to the highlands of Mexico.
_The Distribution of the Species of Jays._--Owing to the very wide range of
several of the tits, the uncertainty of the specific distinction of others,
and the difficulty in many cases of ascertaining their actual distribution,
it has not been found practicable to illustrate this genus by means of a
map. For this purpose we have chosen the genus Garrulus or the jays, in
which the species are less numerous, the specific areas less extensive, and
the species generally better defined; while being large and handsome {21}
birds they are sure to have been collected, or at least noticed, wherever
they occur. There are, so far as yet known, twelve species of true jays,
occupying an area extending from Western Europe to Eastern Asia and Japan,
and nowhere passing the Arctic circle to the north, or the tropic of Cancer
to the south, so that they constitute one of the most typical of the
Palaearctic[6] genera. The following are the species, beginning with the
most westerly and proceeding towards the east. The numbers prefixed to each
species correspond to those on the coloured map which forms the
frontispiece to this volume.
1. _Garrulus glandarius._--The common jay, inhabits the British Isles and
all Europe except the extreme north, extending also into North Africa,
where it has been observed in many parts of Algeria. It occurs near
Constantinople, but apparently not in Asia Minor; and in Russia, up to, but
not beyond, the Urals. The jays being woodland birds are not found in open
plains or barren uplands, and their distribution is hence by no means
uniform within the area they actually occupy.
2. _Garrulus cervicalis._--The Algerian jay, is a very distinct species
inhabiting a limited area in North Africa, and found in some places along
with the common species.
3. _Garrulus krynicki._--The black-headed jay, is closely allied to the
common species, but quite distinct, inhabiting a comparatively small area
in South-eastern Europe, and Western Asia.
4. _Garrulus atricapillus._--The Syrian jay, is very closely allied to the
last, and inhabits an adjoining area in Syria, Palestine, and Southern
Persia.
5. _Garrulus hyrcanus._--The Persian jay, is a small species allied to our
jay and only known from the Elburz Mountains in the north of Persia.
6. _Garrulus brandti._--Brandt's jay, is a very distinct species, having an
extensive range across Asia from the Ural Mountains to North China,
Mandchuria, and the northern island of Japan, and also crossing the Urals
into {22} Russia where it has been found as far west as Kazan in districts
where the common jay also occurs.
7. _Garrulus lanceolatus._--The black-throated jay, is a very distinct form
known only from the North-western Himalayas and Nepal, common about Simla,
and extending into Cashmere beyond the range of the next species.
8. _Garrulus bispecularis._--The Himalayan jay is also very distinct,
having the head coloured like the back, and not striped as in all the
western species. It inhabits the Himalayas east of Cashmere, but is more
abundant in the western than the eastern division, though according to the
Abbe David it reaches Moupin in East Thibet.
9. _Garrulus sinensis._--The Chinese jay, is very closely allied to the
Himalayan, of which it is sometimes classed as a sub-species. It seems to
be found in all the southern mountains of China, from Foochow on the east
to Sze-chuen and East Thibet on the west, as it is recorded from Moupin by
the Abbe David as well as the Himalayan bird--a tolerable proof that it is
a distinct form.
10. _Garrulus taivanus._--The Formosan jay is a very close ally of the
preceding, confined to the island of Formosa.
11. _Garrulus japonicus._--The Japanese jay is nearly allied to our common
British species, being somewhat smaller and less brightly coloured, and
with black orbits; yet these are the most widely separated species of the
genus. According to Mr. Seebohm this species is equally allied to the
Chinese and Siberian jays.
In the accompanying map (see frontispiece) we have laid down the
distribution of each species so far as it can be ascertained from the works
of Sharpe and Dresser for Europe, Jerdon for India, Swinhoe for China, and
Mr. Seebohm's recent work for Japan. There is, however, much uncertainty in
many places, and gaps have to be filled up conjecturally, while such a
large part of Asia is still very imperfectly explored, that considerable
modifications may have to be made when the country becomes more accurately
known. But though details may be modified we can hardly suppose that the
great features of the several specific areas, or their relations to each
other {23} will be much affected; and these are what we have chiefly to
consider as bearing on the questions here discussed.
The first thing that strikes us on looking at the map, is, the small amount
of overlapping of the several areas, and the isolation of many of the
species; while the next most striking feature is the manner in which the
Asiatic species almost surround a vast area in which no jays are found. The
only species with large areas, are the European _G. glandarius_ and the
Asiatic _G. Brandti_. The former has three species overlapping it--in
Algeria, in South-eastern and North-eastern Europe respectively. The Syrian
jay (No. 4), is not known to occur anywhere with the black-headed jay (No.
3), and perhaps the two areas do not meet. The Persian jay (No. 5), is
quite isolated. The Himalayan and Chinese jays (Nos. 7, 8, and 9) form a
group which are isolated from the rest of the genus; while the Japanese jay
(No. 11), is also completely isolated as regards the European jays to which
it is nearly allied. These peculiarities of distribution are no doubt in
part dependent on the habits of the jays, which live only in well-wooded
districts, among deciduous trees, and are essentially non-migratory in
their habits, though sometimes moving southwards in winter. This will
explain their absence from the vast desert area of Central Asia, but it
will not account for the gap between the North and South Chinese species,
nor for the absence of jays from the wooded hills of Turkestan, where Mr.
N. A. Severtzoff collected assiduously, obtaining 384 species of birds but
no jay. These peculiarities, and the fact that jays are never very abundant
anywhere, seem to indicate that the genus is now a decaying one, and that
it has at no very distant epoch occupied a larger and more continuous area,
such as that of the genus Parus at the present day.
_Discontinuous generic Areas._--It is not very easy to find good examples
of genera whose species occupy two or more quite disconnected areas, for
though such cases may not be rare, we are seldom in a position to mark out
the limits of the several species with sufficient accuracy. The best and
most remarkable case among European birds is {24} that of the blue magpies,
forming the genus Cyanopica. One species (_C. cooki_) is confined (as
already stated) to the wooded and mountainous districts of Spain and
Portugal, while the only other species of the genus (_C. cyanus_) is found
far away in North-eastern Asia and Japan, so that the two species are
separated by about 5,000 miles of continuous land. Another case is that of
the curious little water-moles forming the genus Mygale, one species _M.
muscovitica_, being found only on the banks of the Volga and Don in
South-eastern Russia, while the other, _M. pyrenaica_, is confined to
streams on the northern side of the Pyrenees. In tropical America there are
four different kinds of bell-birds belonging to the genus Chasmorhynchus,
each of which appears to inhabit a restricted area completely separated
from the others. The most northerly is _C. tricarunculatus_ of Costa Rica
and Veragua, a brown bird with a white head and three long caruncles
growing upwards at the base of the beak. Next comes _C. variegatus_, in
Venezuela, a white bird with a brown head and numerous caruncles on the
throat, perhaps conterminous with the last; in Guiana, extending to near
the mouth of the Rio Negro, we have _C. niveus_, the bell-bird described by
Waterton, which is pure white, with a single long fleshy caruncle at the
base of the beak; the last species, _C. nudicollis_, inhabits South-east
Brazil, and is also white, but with black stripes over the eyes, and with a
naked throat. These birds are about the size of thrushes, and are all
remarkable for their loud, ringing notes, like a bell or a blow on an
anvil, as well as for their peculiar colours. They are therefore known to
the native Indians wherever they exist, and we may be the more sure that
they do not spread over the intervening areas where they have never been
found, and where the natives know nothing of them.
A good example of isolated species of a group nearer home, is afforded by
the snow-partridges of the genus Tetraogallus. One species inhabits the
Caucasus range and nowhere else, keeping to the higher slopes from 6,000 to
11,000 feet above the sea, and accompanying the ibex in its wanderings, as
both feed on the same plants. Another {25} has a wider range in Asia Minor
and Persia, from the Taurus mountains to the South-east corner of the
Caspian Sea; a third species inhabits the Western Himalayas, between the
forests and perpetual snow, extending eastwards to Nepal; while a fourth is
found on the north side of the mountains in Thibet, and the ranges of these
two perhaps overlap; the last species inhabit the Altai mountains, and like
the two first appears to be completely separated from all its allies.
There are some few still more extraordinary cases in which the species of
one genus are separated in remote continents or islands. The most striking
of these is that of the tapirs, forming the genus Tapirus, of which there
are two or three species in South America, and one very distinct species in
Malacca and Borneo, separated by nearly half the circumference of the
globe. Another example among quadrupeds is a peculiar genus of moles named
Urotrichus, of which one species inhabits Japan and the other British
Columbia. The cuckoo-like honey-guides, forming the genus Indicator, are
tolerably abundant in tropical Africa, but there are two outlying species,
one in the Eastern Himalaya mountains, the other in Borneo, both very rare,
and recently an allied species has been found in the Malay peninsula. The
beautiful blue and green thrush-tits forming the genus Cochoa, have two
species in the Eastern Himalayas and Eastern China, while the third is
confined to Java; the curious genus Eupetes, supposed to be allied to the
dippers, has one species in Sumatra and Malacca, while four other species
are found two thousand miles distant in New Guinea; lastly, the lovely
ground-thrushes of the genus Pitta, range from Hindostan to Australia,
while a single species, far removed from all its near allies, inhabits West
Africa.
_Peculiarities of Generic, and Family Distribution._--The examples now
given sufficiently illustrate the mode in which the several species of a
genus are distributed. We have next to consider genera as the component
parts of families, and families of orders, from the same point of view.
{26}
All the phenomena presented by the species of a genus are reproduced by the
genera of a family, and often in a more marked degree. Owing, however, to
the extreme restriction of genera by modern naturalists, there are not many
among the higher animals that have a world-wide distribution. Among the
mammalia there is no such thing as a truly cosmopolitan genus. This is
owing to the absence of all the higher orders except the mice from
Australia, while the genus Mus, which occurs there, is represented by a
distinct group, Hesperomys, in America. If, however, we consider the
Australian dingo as a native animal we might class the genus Canis as
cosmopolite, but the wild dogs of South America are now formed into
separate genera by some naturalists. Many genera, however, range over three
or more continents, as Felis (the cat genus) absent only from Australia;
Ursus (the bear genus) absent from Australia and tropical Africa; Cervus
(the deer genus) with nearly the same range; and Sciurus (the squirrel
genus) found in all the continents but Australia. Among birds Turdus, the
thrush, and Hirundo, the swallow genus, are the only perching birds which
are truly cosmopolites; but there are many genera of hawks, owls, wading
and swimming birds, which have a world-wide range.
As a great many genera consist of single species there is no lack of cases
of great restriction, such as the curious lemur called the "potto," which
is found only at Sierra Leone, and forms the genus Perodicticus; the true
chinchillas found only in the Andes of Peru and Chili south of 9deg S. lat.
and between 8,000 and 12,000 feet elevation; several genera of finches each
confined to limited portions of the higher Himalayas, the blood-pheasants
(Ithaginis) found only above 10,000 feet from Nepal to East Thibet; the
bald-headed starling of the Philippine islands, the lyre-birds of East
Australia, and a host of others.
It is among the different genera of the same family that we meet with the
most striking examples of discontinuity, although these genera are often as
unmistakably allied as are the species of a genus; and it is these cases
that furnish the most interesting problems to the student of distribution.
{27} We must therefore consider them somewhat more fully.
Among mammalia the most remarkable of these divided families is that of the
camels, of which one genus Camelus, the true camels, comprising the camel
and dromedary, is confined to Asia, while the other Auchenia, comprising
the llamas and alpacas, is found only in the high Andes and in the plains
of temperate South America. Not only are these two genera separated by the
Atlantic and by the greater part of the land of two continents, but one is
confined to the Northern and the other to the Southern hemisphere. The next
case, though not so well known, is equally remarkable; it is that of the
Centetidae, a family of small insectivorous animals, which are wholly
confined to Madagascar and the large West Indian islands Cuba and Hayti,
the former containing five genera and the latter a single genus with a
species in each island. Here again we have the whole continent of Africa as
well as the Atlantic ocean separating allied genera. Two families (or
subfamilies) of rat-like animals, Octodontidae and Echimyidae, are also
divided by the Atlantic. Both are mainly South American, but the former has
two genera in North and East Africa, and the latter also two in South and
West Africa. Two other families of mammalia, though confined to the Eastern
hemisphere, are yet markedly discontinuous. The Tragulidae are small
deer-like animals, known as chevrotains or mouse-deer, abundant in India
and the larger Malay islands and forming the genus Tragulus; while another
genus, Hyomoschus, is confined to West Africa. The other family is the
Simiidae or anthropoid apes, in which we have the gorilla and chimpanzee
confined to West and Central Africa, while the allied orangs are found only
in the islands of Sumatra and Borneo, the two groups being separated by a
greater space than the Echimyidae and other rodents of Africa and South
America.
Among birds and reptiles we have several families, which, from being found
only within the tropics of Asia, Africa, and America, have been termed
tropicopolitan groups. The Megalaemidae or barbets are gaily coloured {28}
fruit-eating birds, almost equally abundant in tropical Asia and Africa,
but less plentiful in America, where they probably suffer from the
competition of the larger sized toucans. The genera of each country are
distinct, but all are closely allied, the family being a very natural one.
The trogons form a family of very gorgeously coloured and remarkable
insect-eating birds very abundant in tropical America, less so in Asia, and
with a single genus of two species in Africa.
Among reptiles we have two families of snakes--the Dendrophidae or
tree-snakes, and the Dryiophidae or green whip-snakes--which are also found
in the three tropical regions of Asia, Africa, and America, but in these
cases even some of the genera are common to Asia and Africa, or to Africa
and America. The lizards forming the family Amphisbaenidae are divided
between tropical Africa and America, a few species only occurring in the
southern portion of the adjacent temperate regions; while even the
peculiarly American family of the iguanas is represented by two genera in
Madagascar, and one in the Fiji and Friendly Islands. Passing on to the
Amphibians the worm-like Caeciliadae are tropicopolitan, as are also the
toads of the family Engystomatidae. Insects also furnish some analogous
cases, three genera of Cicindelidae, (Pogonostoma, Ctenostoma, and
Peridexia) showing a decided connection between this family in South
America and Madagascar; while the beautiful family of diurnal moths,
Uraniidae, is confined to the same two countries. A somewhat similar but
better known illustration is afforded by the two genera of ostriches, one
confined to Africa and Arabia, the other to the plains of temperate South
America.
_General features of Overlapping and Discontinuous Areas._--These numerous
examples of discontinuous genera and families form an important section of
the facts of animal dispersal which any true theory must satisfactorily
account for. In greater or less prominence they are to be found all over
the world, and in every group of animals, and they grade imperceptibly into
those cases of conterminous and overlapping areas which we have seen to
{29} prevail in most extensive groups of species, and which are perhaps
even more common in those large families which consist of many closely
allied genera. A sufficient proof of the overlapping of generic areas is
the occurrence of a number of genera of the same family together. Thus in
France or Italy about twenty genera of warblers (Sylviadae) are found, and
as each of the thirty-three genera of this family inhabiting temperate
Europe and Asia has a different area, a great number must here overlap. So,
in most parts of Africa, at least ten or twelve genera of antelopes may be
found, and in South America a large proportion of the genera of monkeys of
the family Cebidae occur in many districts; and still more is this the case
with the larger bird families, such as the tanagers, the tyrant shrikes, or
the tree-creepers, so that there is in all these extensive families no
genus whose area does not overlap that of many others. Then among the
moderately extensive families we find a few instances of one or two genera
isolated from the rest, as the spectacled bear, Tremarctos, found only in
Chili, while the remainder of the family extends from Europe and Asia over
North America to the Mountains of Mexico, but no further south; the
Bovidae, or hollow-horned ruminants, which have a few isolated genera in
the Rocky Mountains and the islands of Sumatra and Celebes; and from these
we pass on to the cases of wide separation already given.
_Restricted Areas of Families._--As families sometimes consist of single
genera and even single species, they often present examples of very
restricted range; but what is perhaps more interesting are those cases in
which a family contains numerous species and sometimes even several genera,
and yet is confined to a narrow area. Such are the golden moles
(Chrysochloridae) consisting of two genera and three species, confined to
extratropical South Africa; the hill-tits (Liotrichidae), a family of
numerous genera and species mainly confined to the Himalayas, but with a
few straggling species in the Malay countries and the mountains of China;
the Pteroptochidae, large wren-like birds, consisting of eight genera and
nineteen species, almost entirely confined to temperate South America and
{30} the Andes; and the birds-of-paradise, consisting of nineteen or twenty
genera and about thirty-five species, almost all inhabitants of New Guinea
and the immediately surrounding islands, while a few, doubtfully belonging
to the family, extend to East Australia. Among reptiles the most striking
case of restriction is that of the rough-tailed burrowing snakes
(Uropeltidae), the five genera and eighteen species being strictly confined
to Ceylon and the southern parts of the Indian Peninsula.
_The Distribution of Orders._--When we pass to the larger groups, termed
orders, comprising several families, we find comparatively few cases of
restriction and many of worldwide distribution; and the families of which
they are composed are strictly comparable to the genera of which families
are composed, inasmuch as they present examples of overlapping, or
conterminous, or isolated areas, though the latter are comparatively rare.
Among mammalia the Insectivora offer the best example of an order, several
of whose families inhabit areas more or less isolated from the rest; while
the Marsupialia have six families in Australia, and one, the opossums, far
off in America.
Perhaps, more important is the limitation of some entire orders to certain
well-defined portions of the globe. Thus the Proboscidea, comprising the
single family and genus of the elephants, and the Hyracoidea, that of the
Hyrax or Syrian coney, are confined to parts of Africa and Asia; the
Marsupials to Australia and America; and the Monotremata, the lowest of all
mammals--comprising the duck-billed Platypus and the spiny Echidna, to
Australia and New Guinea. Among birds the Struthiones or ostrich tribe are
almost confined to the three Southern continents, South America, Africa and
Australia; and among Amphibia the tailed Batrachia--the newts and
salamanders--are similarly restricted to the northern hemisphere.
These various facts will receive their explanation in a future chapter.
* * * * *
[Illustration]
{31}
CHAPTER III
CLASSIFICATION OF THE FACTS OF DISTRIBUTION.--ZOOLOGICAL REGIONS
The Geographical Divisions of the Globe do not correspond to Zoological
divisions--The range of British Mammals as indicating a Zoological
Region--Range of East Asian and North African Mammals--The Range of
British Birds--Range of East Asian Birds--The limits of the Palaearctic
Region--Characteristic features of the Palaearctic Region--Definition
and characteristic groups of the Ethiopian Region--Of the Oriental
Region--Of the Australian Region--Of the Nearctic Region--Of the
Neotropical Region--Comparison of Zoological Regions with the
Geographical Divisions of the Globe.
Having now obtained some notion of how animals are dispersed over the
earth's surface, whether as single species or as collected in those groups
termed genera, families, and orders, it will be well, before proceeding
further, to understand something of the classification of the facts we have
been considering, and some of the simpler conclusions these facts lead to.
We have hitherto described the distribution of species and groups of
animals by means of the great geographical divisions of the globe in common
use; but it will have been observed that in hardly any case do these define
the limits of anything beyond species, and very seldom, or perhaps never,
even those accurately. Thus the term "Europe" will not give, with any
approach to accuracy, the range of any one genus of mammals or birds, and
{32} perhaps not that of half-a-dozen species. Either they range into
Siberia, or Asia Minor, or Palestine, or North Africa; and this seems to be
always the case when their area of distribution occupies a large portion of
Europe. There are, indeed, a few species limited to Central or Western or
Southern Europe, and these are almost the only cases in which we can use
the word for zoological purposes without having to add to it some portion
of another continent. Still less useful is the term Asia for this purpose,
since there is probably no single animal or group confined to Asia which is
not also more or less nearly confined to the tropical or the temperate
portion of it. The only exception is perhaps the tiger, which may really be
called an Asiatic animal, as it occupies nearly two-thirds of the
continent; but this is an unique example, while the cases in which Asiatic
animals and groups are strictly limited to a portion of Asia, or extend
also into Europe or into Africa or to the Malay Islands, are exceedingly
numerous. So, in Africa, very few groups of animals range over the whole of
it without going beyond either into Europe or Asia Minor or Arabia, while
those which are purely African are generally confined to the portion south
of the tropic of Cancer. Australia and America are terms which better serve
the purpose of the zoologist. The former defines the limit of many
important groups of animals; and the same may be said of the latter, but
the division into North and South America introduces difficulties, for
almost all the groups especially characteristic of South America are found
also beyond the isthmus of Panama, in what is geographically part of the
northern continent.
It being thus clear that the old and popular divisions of the globe are
very inconvenient when used to describe the range of animals, we are
naturally led to ask whether any other division can be made which will be
more useful, and will serve to group together a considerable number of the
facts we have to deal with. Such a division was made by Mr. P. L. Sclater
more than twenty years ago, and it has, with some slight modifications,
come into pretty general use in this country, and to some extent also {33}
abroad; we shall therefore proceed to explain its nature and the principles
on which it is established, as it will have to be often referred to in
future chapters of this work, and will take the place of the old
geographical divisions whose inconvenience has already been pointed out.
The primary zoological divisions of the globe are called "regions," and we
will begin by ascertaining the limits of the region of which our own
country forms a part.
_The Range of British Mammals as indicating a Zoological Region._--We will
first take our commonest wild mammalia and see how far they extend, and
especially whether they are confined to Europe or range over parts of other
continents:
1. Wild Cat | Europe | N. Africa | Siberia, Afghanistan.
2. Fox | Europe | N. Africa | Central Asia to Amoor.
3. Weasel | Europe | N. Africa | Central Asia to Amoor.
4. Otter | Europe | N. Africa | Siberia.
5. Badger | Europe | N. Africa | Central Asia to Amoor.
6. Stag | Europe | N. Africa | Central Asia to Amoor.
7. Hedgehog | Europe | -- | Central Asia to Amoor.
8. Mole | Europe | -- | Central Asia.
9. Squirrel | Europe | -- | Central Asia to Amoor.
10. Dormouse | Europe | -- | --
11. Water-rat | Europe | -- | Central Asia to Amoor.
12. Hare | Europe | -- | W. Siberia, Persia.
13. Rabbit | Europe | N. Africa | --
We thus see that out of thirteen of our commonest quadrupeds only one is
confined to Europe, while seven are found also in Northern Africa, and
eleven range into Siberia, most of them stretching quite across Asia to the
valley of the Amoor on the extreme eastern side of that continent. Two of
the above-named British species, the fox and weasel, are also inhabitants
of the New World, being as common in the northern parts of North America as
they are with us; but with these exceptions the entire range of our
commoner species is given, and they clearly show that all Northern Asia and
Northern Africa must be added to Europe in order to form the region which
they collectively inhabit. If now we go into Central Europe and take, for
example, the quadrupeds of Germany, we shall find that these too, although
much more numerous, are confined to the same limits, except that some of
the {34} more arctic kinds, as already stated, extend into the colder
regions of North America.
_Range of East Asian and North African Mammals._--Let us now pass to the
other side of the great northern continent, and examine the list of the
quadrupeds of Amoorland, in the same latitude as Germany. We find that
there are forty-four terrestrial species (omitting the bats, the seals, and
other marine animals), and of these no less than twenty-six are identical
with European species, and twelve or thirteen more are closely allied
representatives, leaving only five or six which are peculiarly Asiatic. We
can hardly have a more convincing proof of the essential oneness of the
mammalia of Europe and Northern Asia.
In Northern Africa we do not find so many European species (though even
here they are very numerous) because a considerable number of West Asiatic
and desert forms occur. Having, however, shown that Europe and Western Asia
have almost identical animals, we may treat all these as really European,
and we shall then be able to compare the quadrupeds of North Africa with
those of Europe and West Asia. Taking those of Algeria as the best known,
we find that there are thirty-three species identical with those of Europe
and West Asia, while twenty-four more, though distinct, are closely allied,
belonging to the same genera; thus making a total of fifty-seven of
European type. On the other hand, we have seven species which are either
identical with species of tropical Africa or allied to them, and six more
which are especially characteristic of the African and Asiatic deserts
which form a kind of neutral zone between the temperate and tropical
regions. If now we consider that Algeria and the adjacent countries
bordering the Mediterranean form part of Africa, while they are separated
from Europe by a wide sea and are only connected with Asia by a narrow
isthmus, we cannot but feel surprised at the wonderful preponderance of the
European and West Asiatic elements in the mammalia which inhabit the
district.
_The Range of British Birds._--As it is very important that no doubt should
exist as to the limits of the zoological {35} region of which Europe forms
a part, we will now examine the birds, in order to see how far they agree
in their distribution with the mammalia. Of late years great attention has
been paid to the distribution of European and Asiatic birds, many
ornithologists having travelled in North Africa, in Palestine, in Asia
Minor, in Persia, in Siberia, in Mongolia, and in China; so that we are now
able to determine the exact ranges of many species in a manner that would
have been impossible a few years ago. These ranges are given for all
British species in the new edition of Yarrell's _History of British Birds_
edited by Professor Newton, while those of all European birds are given in
still more detail in Mr. Dresser's beautiful work on the birds of Europe.
In order to confine our examination within reasonable limits, and at the
same time give it the interest attaching to familiar objects, we will take
the whole series of British Passeres or perching birds given in Professor
Newton's work (118 in number) and arrange them in series according to the
extent of their range. These include not only the permanent residents and
regular migrants to our country, but also those which occasionally straggle
here, so that it really comprises a large proportion of all European birds.
I. BRITISH BIRDS WHICH EXTEND TO NORTH AFRICA AND CENTRAL OR NORTH-EAST
ASIA.
1. _Lanius collurio_ Red backed Shrike (also all Africa).
2. _Oriolus Galbula_ Golden Oriole (also all Africa).
3. _Turdus musicus_ Song-Thrush.
4. ,, _iliacus_ Red-wing.
5. ,, _pilaris_ Fieldfare.
6. _Monticola saxatilis_ Blue rock Thrush.
7. _Ruticilla suecica_ Bluethroat (also India in winter).
8. _Saxicola rubicola_ Stonechat (also India in winter).
9. ,, _oenanthe_ Wheatear (also N. America).
10. _Acrocephalus arundinaceus_ Great Reed-Warbler.
11. _Sylvia curruca_ Lesser Whitethroat.
12. _Parus major_ Great Titmouse.
13. _Motacilla sulphurea_ Grey Wagtail (also China and Malaya).
14. ,, _raii_ Yellow Wagtail.
15. _Anthus trivialis_ Tree Pipit.
16. ,, _spiloletta_ Water Pipit.
17. ,, _campestris_ Tawny Pipit.
18. _Alauda arvensis_ Skylark.
19. ,, _cristata_ Crested Lark.
{36}
20. _Emberiza schoeniclus_ Reed Bunting.
21. ,, _citrinella_ Yellow-hammer.
22. _Fringilla montifringilla_ Brambling.
23. _Passer montanus_ Tree Sparrow (also S. Asia).
24. ,, _domesticus_ House Sparrow.
25. _Coccothraustes vulgaris_ Hawfinch.
26. _Carduelis spinus_ Siskin (also China).
27. _Loxia curvirostra_ Crossbill.
28. _Sturnus vulgaris_ Starling.
29. _Pyrrhocorax graculus_ Chough.
30. _Corvus corone_ Crow.
31. _Hirundo rustica_ Swallow (all Africa and Asia).
32. _Cotyle riparia_ Sand Martin (also India and N. America).
II. BRITISH BIRDS WHICH RANGE TO CENTRAL OR NORTH-EAST ASIA.
1. _Lanius excubitor_ Great Grey Shrike.
2. _Turdus varius_ White's Thrush (also to Japan).
3. ,, _atrigularis_ Black-throated Thrush.
4. _Acrocephalus naevius_ Grasshopper Warbler.
5. _Phylloscopus superciliosus_ Yellow-browed Warbler.
6. _Certhia familiaris_ Tree-creeper.
7. _Parus coeruleus_ Blue Titmouse.
8. ,, _ater_ Coal Titmouse.
9. ,, _palustris_ Marsh Titmouse.
10. _Acredula caudata_ Long-tailed Titmouse.
11. _Ampelis garrulus_ Wax-wing.
12. _Anthus richardi_ Richard's Pipit.
13. _Alauda alpestris_ Shore Lark (also N. America).
14. _Plectrophanes nivalis_ Snow-Bunting (also N. America).
15. ,, _lapponicus_ Lapland Bunting.
16. _Emberiza rustica_ Rustic Bunting (also China).
17. ,, _pusilla_ Little Bunting.
18. _Linota linaria_ Mealy Redpole (also N. America).
19. _Pyrrhula erythrina_ Scarlet Grosbeak (also N. India, China).
20. ,, _enucleator_ Pine Grosbeak (also N. America).
21. _Loxia bifasciata_ Two-barred Crossbill.
22. _Pastor roseus_ Rose-coloured Starling (also India).
23. _Corvus corax_ Raven (also N. America).
24. _Pica rustica_ Magpie.
25. _Nucifraga caryocatactes_ Nutcracker.
III. BRITISH BIRDS RANGING INTO N. AFRICA AND W. ASIA.
1. _Lanius minor_ Lesser Grey Shrike.
2. ,, _auriculatus_ Woodchat (also Tropical Africa).
3. _Muscicapa grisola_ Spotted Flycatcher (also E. and S.
Africa).
4. ,, _atricapilla_ Pied Flycatcher (also Central Africa).
5. Turdus _viscivorus_ Mistletoe-Thrush (N. India in winter).
6. ,, _merula_ Blackbird.
7. ,, _torquatus_ Ring Ouzel.
8. _Accentor modularis_ Hedge Sparrow.
9. _Erithacus rubecula_ Redbreast.
10. _Daulias luscinia_ Nightingale.
{37}
11. _Ruticilla phaenicurus_ Redstart.
12. ,, _tithys_ Black Redstart.
13. _Saxicola rubetra_ Whinchat.
14. _Aedon galactodes_ Rufous Warbler.
15. _Acrocephalus streperus_ Reed Warbler.
16. ,, _schaenobenus_ Sedge Warbler.
17. _Melizophilus undatus_ Dartford Warbler.
18. _Sylvia rufa_ Greater Whitethroat.
19. ,, _salicaria_ Garden Warbler.
20. ,, _atricapilla_ Blackcap.
21. ,, _orphea_ Orphean Warbler.
22. _Phylloscopus sibilatrix_ Wood Wren.
23. ,, _trochilus_ Willow Wren.
24. ,, _collybita_ Chiffchaff.
25. _Regulus cristatus_ Golden-crested Wren.
26. ,, _ignicapillus_ Fire-crested Wren.
27. _Troglodytes parvulus_ Wren.
28. _Sitta caesia_ Nuthatch.
29. _Motacilla alba_ White Wagtail (also W. Africa).
30. ,, _flava_ Blue-headed Wagtail.
31. _Anthus pratensis_ Meadow-Pipit.
32. _Alauda arborea_ Woodlark.
33. _Calandrella brachydactyla_ Short-toed Lark.
34. _Emberiza miliaria_ Common Bunting.
35. ,, _cirlus_ Cirl Bunting.
36. ,, _hortulana_ Ortolan.
37. _Fringilla coelebs_ Chaffinch.
38. _Coccothraustes chloris_ Greenfinch.
39. _Serinus hortulanus_ Serin.
40. _Carduelis elegans_ Goldfinch.
41. _Linota cannabina_ Linnet.
42. _Corvus monedula_ Jackdaw.
43. _Chelidon urbica_ House-Martin.
IV. BRITISH BIRDS RANGING TO NORTH AFRICA.
1. _Hypolais icterina_ Icterine Warbler.
2. _Acrocephalus aquaticus_ Aquatic Warbler.
3. ,, _luscinioides_ Savi's Warbler.
4. _Motacilla lugubris_ Pied Wagtail.
5. _Pyrrhula europaea_ Bullfinch.
6. _Garrulus glandarius_ Jay.
V. BRITISH BIRDS RANGING TO WEST ASIA ONLY.
1. _Accentor collaris_ Alpine Accentor.
2. _Muscicapa parva_ Red-breasted Flycatcher (to N. W.
India).
3. _Panurus biarmicus_ Bearded Titmouse.
4. _Melanocorypha sibirica_ White-winged Lark.
5. _Euspiza melanocephala_ Black-headed Bunting.
6. _Linota flavirostris_ Twite.
7. _Corvus frugilegus_ Rook.
VI. BRITISH BIRDS CONFINED TO EUROPE.
1. _Cinclus aquaticus_ Dipper (closely allied races inhabit
other parts of the Palaearctic Region).
2. _Parus cristatus_ Crested Titmouse.
{38}
3. _Anthus obscurus_ Rock Pipit.
4. _Linota rufescens_ Lesser Redpoll (closely allied races in
N. Asia and N. America).
5. _Loxia pityopsittacus_ Parrot Crossbill (a closely allied form
in N. Asia).
We find, that out of a total of 118 British Passeres there are:
32 species which range to North Africa and Central or East Asia.
25 species which range to Central or East Asia, but not to North
Africa.
43 species which range to North Africa and Western Asia.
6 species which range to North Africa, but not at all into Asia.
7 species which range to West Asia, but not to North Africa.
5 species which do not range out of Europe.
These figures agree essentially with those furnished by the mammalia, and
complete the demonstration that all the temperate portions of Asia and
North Africa must be added to Europe to form a natural zoological division
of the earth. We must also note how comparatively few of these overpass the
limits thus indicated; only seven species extending their range
occasionally into tropical or South Africa, eight into some parts of
tropical Asia, and six into arctic or temperate North America.
_Range of East Asian Birds._--To complete the evidence we only require to
know that the East Asiatic birds are as much like those of Europe, as we
have already shown to be the case when we take the point of departure from
our end of the continent. This does not follow necessarily, because it is
possible that a totally distinct North Asiatic fauna might there prevail;
and, although our birds go eastward to the remotest parts of Asia, their
birds might not come westward to Europe. The birds of Eastern Siberia have
been carefully studied by Russian naturalists and afford us the means of
making the required comparison. There are 151 species belonging to the
orders Passeres and Picariae (the perching and climbing birds), and of
these no less than 77, or more than half, are absolutely identical {39}
with European species; 63 are peculiar to North Asia, but all except five
or six of these are allied to European forms; the remaining 11 species are
migrants from South-eastern Asia. The resemblance is therefore equally
close whichever extremity of the Euro-Asiatic continent we take as our
starting point, and is equally remarkable in birds as in mammalia. We have
now only to determine the limits of this, our first zoological region,
which has been termed the "Palaearctic" by Mr. Sclater, meaning the
"northern old-world" region--a name now well known to naturalists.
_The Limits of the Palaearctic Region._--The boundaries of this region, as
nearly as they can be ascertained, are shown on our general map at the
beginning of this chapter, but it will be evident on consideration, that,
except in a few places, its limits can only be approximately defined. On
the north, east, and west it extends to the ocean, and includes a number of
islands whose peculiarities will be pointed out in a subsequent chapter; so
that the southern boundary alone remains, but as this runs across the
entire continent from the Atlantic to the Pacific ocean, often traversing
little-known regions, we may perhaps never be able to determine it
accurately, even if it admits of such determination. In drawing the
boundary line across Africa we meet with our first difficulty. The
Euro-Asiatic animals undoubtedly extend to the northern borders of the
Sahara, while those of tropical Africa come up to its southern margin, the
desert itself forming a kind of sandy ocean between them. Some of the
species on either side penetrate and even cross the desert, but it is
impossible to balance these with any accuracy, and it has therefore been
thought best, as a mere matter of convenience, to consider the geographical
line of the tropic of Cancer to form the boundary. We are thus enabled to
define the Palaearctic region as including all north temperate Africa; and,
a similar intermingling of animal types occurring in Arabia, the same
boundary line is continued to the southern shore of the Persian Gulf.
Persia and Afghanistan undoubtedly belong to the Palaearctic region, and
Baluchistan should probably go with these. The boundary in the
north-western part of India is again difficult to determine, but it {40}
cannot be far one way or the other from the river Indus as far up as
Attock, opposite the mouth of the Cabool river. Here it will bend to the
south-east, passing a little south of Cashmeer, and along the southern
slopes of the Himalayas into East Thibet and China, at heights varying from
9,000 to 11,000 feet according to soil, aspect, and shelter. It may,
perhaps, be defined as extending to the upper belt of forests as far as
coniferous trees prevail; but the temperate and tropical faunas are here so
intermingled that to draw any exact parting line is impossible. The two
faunas are, however, very distinct. In and above the pine woods there are
abundance of warblers of northern genera, with wrens, numerous titmice, and
a great variety of buntings, grosbeaks, bullfinches and rosefinches, all
more or less nearly allied to the birds of Europe and Northern Asia; while
a little lower down we meet with a host of peculiar birds allied to those
of tropical Asia and the Malay Islands, but often of distinct genera. There
can be no doubt, therefore, of the existence here of a pretty sharp line of
demarkation between the temperate and tropical faunas, though this line
will be so irregular, owing to the complex system of valleys and ridges,
that in our present ignorance of much of the country it cannot be marked in
detail on any map.
Further east in China it is still more difficult to determine the limits of
the region, owing to the great intermixture of migrating birds; tropical
forms passing northwards in summer as far as the Amoor river, while the
northern forms visit every part of China in winter. From what we know,
however, of the distribution of some of the more typical northern and
southern species, we are able to fix the limits of the Palaearctic region a
little south of Shanghai on the east coast. Several tropical genera come as
far north as Ningpo or even Shanghai, but rarely beyond; while in Formosa
and Amoy tropical forms predominate. Such decidedly northern forms as
bullfinches and hawfinches are found at Shanghai; hence we may commence the
boundary line on the coast between Shanghai and Ningpo, but inland it
probably bends a little southward, and then northward to the mountains and
valleys of West {41} China and East Thibet in about 32deg N. latitude;
where, at Moupin, a French missionary, Pere David, made extensive
collections showing this district to be at the junction of the tropical and
temperate faunas. Japan, as a whole, is decidedly Palaearctic, although its
extreme southern portion, owing to its mild insular climate and evergreen
vegetation, gives shelter to a number of tropical forms.
_Characteristic Features of the Palaearctic Region._--Having thus
demonstrated the unity of the Palaearctic region by tracing out the
distribution of a large proportion of its mammalia and birds, it only
remains to show how far it is characterised by peculiar groups such as
genera and families, and to say a few words on the lower forms of life
which prevail in it.
Taking first the mammalia, we find this region distinguished by possessing
two peculiar genera of Talpidae or moles, the family being confined to the
Palaearctic and Nearctic regions. The true hedgehogs (Erinaceus) are also
characteristic, being only found elsewhere in South Africa and in the
northern part of the Oriental region. Among Carnivora, the racoon-dog
(Nyctereutes) of North-eastern Asia, and the true badgers of the genus
Meles are peculiar, most other parts of the world possessing distinct
genera of badgers. It has six peculiar genera, or subgenera, of deer; seven
peculiar genera of Bovidae, chiefly antelopes; while the entire group of
goats and sheep, comprising twenty-two species, is almost confined to it,
one species only occurring in the Rocky mountains of North America and
another in the Nilgiris of Southern India. Among the rodents there are nine
genera with twenty-seven species wholly confined to it, while several
others, as the hamsters, the dormice, and the pikas, have only a few
species elsewhere.
In birds there are a large number of peculiar genera of which we need
mention only a few of the more important, as the grass-hopper warblers
(Locustella) with seven species, the Accentors with twelve species, and
about a dozen other genera of warblers, including the robins; the bearded
titmouse and several allied genera; the long-tailed titmice forming the
genus Acredula; the magpies, choughs, and nut-crackers; a host of finches,
among which the bullfinches (Pyrrhula) and the buntings (Emberiza) are the
{42} most important. The true pheasants (Phasianus) are wholly Palaearctic,
except one species in Formosa, as are several genera of wading birds.
Though the reptiles of cold countries are few as compared with those of the
tropics, the Palaearctic region in its warmer portions has a considerable
number, and among these are many which are peculiar to it. Such are four
genera of snakes, seven of lizards, five of frogs and toads, and twelve of
newts and salamanders; while of fresh-water fishes there are about twenty
peculiar genera.[7] Among insects we may mention the elegant Apollo
butterflies of the Alps as forming a peculiar genus (Parnassius), only
found elsewhere in the Rocky Mountains of North America, while the
beautiful genus Thais of the south of Europe and Sericinus of North China
are equally remarkable. Among other insects we can only now refer to the
great family of Carabidae, or predaceous ground-beetles, which are
immensely numerous in this region, there being about fifty peculiar genera;
while the large and handsome genus Carabus, with its allies Procerus and
Procrustes, containing nearly 300 species, is almost wholly confined to
this region, and would alone serve to distinguish it zoologically from all
other parts of the globe.
{43}
Having given so full an exposition of the facts which determine the extent
and boundaries of the Palaearctic region, there is less need of entering
into much detail as regards the other regions of the Eastern Hemisphere;
their boundaries being easily defined, while their forms of animal life are
well marked and strongly contrasted.
_Definition and Characteristic Groups of the Ethiopian Region._--The
Ethiopian region consists of all tropical and south Africa, to which are
appended the large island of Madagascar and the Mascarene Islands to the
east and north of it, though these differ materially from the continent,
and will have to be discussed in a separate chapter. For the present, then,
we will take Africa south of the tropic of Cancer, and consider how far its
animals are distinct from those of the Palaearctic region.
Taking first the mammalia, we find the following remarkable animals at once
separating it from the Palaearctic and every other region. The gorilla and
chimpanzee, the baboons, numerous lemurs, the spotted hyaena, the aard-wolf
and hyaena-dog, zebras, the hippopotamus, giraffe, and more than seventy
peculiar antelopes. Here we have a wonderful collection of large and
peculiar quadrupeds, but the Ethiopian region is also characterised by the
absence of others which are not only abundant in the Palaearctic region but
in many tropical regions as well. The most remarkable of these deficiencies
are the bears the deer and the wild oxen, all of which abound in the
tropical parts of Asia while bears and deer extend into both North and
South America. Besides the large and conspicuous animals mentioned above,
Africa possesses a number of completely isolated groups; such are the
potamogale, a curious otter-like water-shrew, discovered by Du Chaillu in
West Africa, so distinct as to constitute a new family, Potamogalidae; the
goldenmoles, also forming a peculiar family, Chrysochloridae; as do the
elephant-shrews, Macroscelididae; the singular aard-varks, or earth-pigs,
forming a peculiar family of Edentata called Orycteropodidae; while there
are numerous peculiar genera of monkeys, swine, civets, and rodents.
Among birds the most conspicuous and remarkable are, the great-billed
vulture-crows (Corvultur), the long-tailed {44} whydah finches (Vidua), the
curious ox-peckers (Buphaga), the splendid metallic starlings
(Lamprocolius), the handsome plantain-eaters (Musophaga), the
ground-hornbills (Bucorvus), the numerous guinea-fowls belonging to four
distinct genera, the serpent-eating secretary-bird (Serpentarius), the huge
boat-billed heron (Balaeniceps), and the true ostriches. There are also
three quite peculiar African families, the Musophagidae or plantain-eaters,
including the elegant crested touracos; the curious little finch-like
colies (Coliidae), and the Irrisoridae, insect-eating birds allied to the
hoopoes but with glossy metallic plumage and arboreal habits.
In reptiles, fishes, insects, and land-shells, Africa is very rich, and
possesses an immense number of peculiar forms. These are not sufficiently
familiar to require notice in a work of this character, but we may mention
a few as mere illustrations: the puff-adders, the most hideous of poisonous
snakes; the chameleons, the most remarkable of lizards; the
goliath-beetles, the largest and handsomest of the Cetoniidae; and some of
the Achatinae, which are the largest of all known land-shells.
_Definition and Characteristic Groups of the Oriental Region._--The
Oriental region comprises all Asia south of the Palaearctic limits, and
along with this the Malay Islands as far as the Philippines, Borneo, and
Java. It was called the Indian region by Mr. Sclater, but this term has
been objected to because the Indo-Chinese and Malayan districts are the
richest and most characteristic, while the peninsula of India is the
poorest portion of it. The name "Oriental" has therefore been adopted in my
work on _The Geographical Distribution of Animals_ as preferable to either
Malayan or Indo-Australian, both of which have been proposed, but are
objectionable, as being already in use in a different sense.
The great features of the mammals of the Oriental region are, the
long-armed apes, the orang-utans, the tiger, the sun-bears and honey-bears,
the tapir, the chevrotains or mouse-deer, and the Indian elephant. Its most
conspicuous birds are the immense number and variety of babbling-thrushes
(Timaliidae), its beautiful little hill-tits (Liotrichidae), its green
bulbuls (Phyllornithidae), its many varieties {45} of the crow-family, its
beautiful gapers and pittas adorned with the most delicate colours, its
great variety of hornbills, and its magnificent Phasianidae, comprising the
peacocks, argus-pheasants, fire-backed pheasants, and jungle-fowl. Many of
these are, it is true, absent from the peninsula of Hindostan, but
sufficient remain there to ally it with the other parts of the region.
Among the remarkable but less conspicuous forms of mammalia which are
peculiar to this region are, monkeys of the genus Presbyter, extending to
every part of it; lemurs of three peculiar genera--Nycticebus and Loris
(slow lemurs) and Tarsius (spectre lemurs); the flying lemur
(Galeopithecus), now classed as a peculiar family of Insectivora and found
only in the Malay Islands; the family of the Tupaias, or squirrel-shrews,
curious little arboreal Insectivora somewhat resembling squirrels; no less
than twelve peculiar genera of the civet family, three peculiar antelopes,
five species of rhinoceros, and the round-tailed flying squirrels forming
the genus Pteromys.
Of the peculiar groups of birds we can only mention a few. The curious
little tailor-birds of the genus Orthotomus are found over the whole region
and almost alone serve to characterise it, as do the fine
laughing-thrushes, forming the genus Garrulax; while the beautiful
grass-green fruit-thrushes (Phyllornis), and the brilliant little minivets
(Pericrocotus), are almost equally universal. Woodpeckers are abundant,
belonging to a dozen peculiar genera; while gaudy barbets and strange forms
of cuckoos and hornbills are also to be met with everywhere. Among game
birds, the only genus that is universally distributed, and which may be
said to characterise the region, is Gallus, comprising the true
jungle-fowl, one of which, Gallus bankiva, is found from the Himalayas and
Central India to Malacca, Java, and even eastward to Timor, and is the
undoubted origin of almost all our domestic poultry. Southern India and
Ceylon each possesses distinct species of jungle-fowl, and a third very
handsome green bird (Gallus aeneus inhabits Java.)
Reptiles are as abundant as in Africa, but they present no well-known
groups which can be considered as specially characteristic. Among insects
we may notice the {46} magnificent golden and green Papilionidae of various
genera as being unequalled in the world; while the great Atlas moth is
probably the most gigantic of Lepidoptera, being sometimes ten inches
across the wings, which are also very broad. Among the beetles the strange
flat-bodied Malayan mormolyce is the largest of all the Carabidae, while
the catoxantha is equally a giant among the Buprestidae. On the whole, the
insects of this region probably surpass those of any other part of the
world, except South America, in size, variety, and beauty.
_Definition and Characteristic Groups of the Australian Region._--The
Australian region is so well marked off from the Oriental, as well as from
all other parts of the world, by zoological peculiarities, that we need not
take up much time in describing it, especially as some of its component
islands will come under review at a subsequent stage of our work. Its most
important portions are Australia and New Guinea, but it also includes all
the Malayan and Pacific Islands to the east of Borneo, Java, and Bali, the
Oriental region terminating with the submarine bank on which those islands
are situated. The island of Celebes is included in this region from a
balance of considerations, but it almost equally well belongs to the
Oriental, and must be left out of the account in our general sketch of the
zoological features of the Australian region.
The great feature of the Australian region is the almost total absence of
all the forms of terrestrial mammalia which abound in the rest of the
world, their place being supplied by a great variety of Marsupials. In
Australia and New Guinea there are no Insectivora, Carnivora, nor Ungulata,
while even the rodents are only represented by a few small rats and mice.
In the remoter Pacific Islands mammals are altogether absent (except
perhaps in New Zealand), but in the Moluccas and other islands bordering on
the Oriental region the higher mammals are represented by a few deer,
civets, and pigs, though it is doubtful whether the two former may not have
been introduced by man, as was almost certainly the case with the
semi-domesticated dingo of Australia.[8] These peculiarities in the
mammalia {47} are so great that every naturalist agrees that Australia must
be made a separate region, the only difference of opinion being as to its
extent, some thinking that New Zealand should form another separate region;
but this question need not now delay us.
In birds Australia is by no means so isolated from the rest of the world,
as it contains great numbers of warblers, thrushes, flycatchers, shrikes,
crows, and other familiar types of the Eastern Hemisphere; yet a
considerable number of the most characteristic Oriental families are
absent. Thus there are no vultures, woodpeckers, pheasants, bulbuls, or
barbets in the Australian region; and the absence of these is almost as
marked a feature as that of cats, deer, or monkeys, among mammalia. The
most conspicuous and characteristic birds of the Australian region are, the
piping crows; the honey-suckers (Meliphagidae), a family quite peculiar to
the region; the lyre-birds; the great terrestrial kingfishers (Dacelo); the
great goat-suckers called more-porks in Australia and forming the genus
Podargus; the wonderful abundance of parrots, including such remarkable
forms as the white and black cockatoos, and the gorgeously coloured
brush-tongued lories; the almost equal abundance of fine pigeons more gaily
coloured than any others on the globe; the strange brush-turkeys and
mound-builders, the only birds that {48} never sit upon their eggs, but
allow them to be hatched, reptile-like, by the heat of the sand or of
fermenting vegetable matter; and lastly, the emus and cassowaries, in which
the wings are far more rudimentary than in the ostriches of Africa and
South America. New Guinea and the surrounding islands are remarkable for
their tree-kangaroos, their birds-of-paradise, their raquet-tailed
kingfishers, their great crown-pigeons, their crimson lories, and many
other remarkable birds. This brief outline being sufficient to show the
distinctness and isolation of the Australian region, we will now pass to
the consideration of the Western Hemisphere.
_Definition and Characteristic Groups of the Nearctic Region._--The
Nearctic region comprises all temperate and arctic North America, including
Greenland, the only doubt being as to its southern boundary, many northern
types penetrating into the tropical zone by means of the highlands and
volcanic peaks of Mexico and Guatemala, while a few which are
characteristic of the tropics extend northward into Texas and California.
There is, however, considerable evidence showing that on the east coast the
Rio Grande del Norte, and on the west a point nearly opposite Cape St.
Lucas, form the most natural boundary; but instead of being drawn straight
across, the line bends to the south-east as soon as it rises on the flanks
of the table-land, forming a deep loop which extends some distance beyond
the city of Mexico, and perhaps ought to be continued along the higher
ridges of Guatemala.
The Nearctic region is so similar to the Palaearctic in position and
climate, and the two so closely approach each other at Behring Straits,
that we cannot wonder at there being a certain amount of similarity between
them--a similarity which some naturalists have so far over-estimated as to
think that the two regions ought to be united. Let us therefore carefully
examine the special zoological features of this region, and see how far it
resembles, and how far differs from, the Palaearctic.
At first sight the mammalia of North America do not seem to differ much
from those of Europe or Northern Asia. There are cats, lynxes, wolves and
foxes, weasels, bears, elk and deer, voles, beavers, squirrels, marmots,
and {49} hares, all very similar to those of the Eastern Hemisphere, and
several hardly distinguishable. Even the bison or "buffalo" of the
prairies, once so abundant and characteristic, is a close ally of the now
almost extinct "aurochs" of Lithuania. Here, then, we undoubtedly find a
very close resemblance between the two regions, and if this were all, we
should have great difficulty in separating them. But along with these, we
find another set of mammals, not quite so conspicuous but nevertheless very
important. We have first, three peculiar genera of moles, one of which, the
star-nosed mole, is a most extraordinary creature, quite unlike anything
else. Then there are three genera of the weasel family, including the
well-known skunk (Mephitis), all quite different from Eastern forms. Then
we come to a peculiar family of carnivora, the racoons, very distinct from
anything in Europe or Asia; and in the Rocky Mountains we find the
prong-horn antelope (Antilocapra) and the mountain goat of the trappers
(Aplocerus), both peculiar genera. Coming to the rodents we find that the
mice of America differ in some dental peculiarities from those of the rest
of the world, and thus form several distinct genera; the jumping mouse
(Xapus) is a peculiar form of the jerboa family, and then we come to the
pouched rats (Geomyidae), a very curious family consisting of four genera
and nineteen species, peculiar to North America, though not confined to the
Nearctic region. The prairie dogs (Cynomys), the tree porcupine
(Erethizon), the curious sewellel (Haploodon), and the opossum (Didelphys)
complete the list of peculiar mammalia which distinguish the northern
region of the new world from that of the old. We must add to these
peculiarities some remarkable deficiencies. The Nearctic region has no
hedgehogs, nor wild pigs, nor dormice, and only one wild sheep in the Rocky
Mountains as against twenty species of sheep and goats in the Palaearctic
region.
In birds also the similarities to our own familiar songsters first strike
us, though the differences are perhaps really greater than in the
quadrupeds. We see thrushes and wrens, tits and finches, and what seem to
be warblers and flycatchers and starlings in abundance; but a closer
examination shows the ornithologist that what he took for the {50} latter
are really quite distinct, and that there is not a single true flycatcher
of the family Muscicapidae, or a single starling of the family Sturnidae in
the whole continent, while there are very few true warblers (Sylviidae),
their place being taken by the quite distinct families Mniotiltidae or
wood-warblers, and Vireonidae or greenlets. In like manner the flycatchers
of America belong to the totally distinct family of tyrant-birds,
Tyrannidae, and those that look like starlings to the hang-nests,
Icteridae; and these four peculiar families comprise about a hundred and
twenty species, and give a special character to the ornithology of the
country. Add to these such peculiar birds as the mocking thrushes (Mimus),
the blue jays (Cyanocitta), the tanagers, the peculiar genera of cuckoos
(Coccygus and Crotophaga), the humming-birds, the wild turkeys (Meleagris),
and the turkey-buzzards (Cathartes), and we see that if there is any doubt
as to the mammals of North America being sufficiently distinct to justify
the creation of a separate region, the evidence of the birds would alone
settle the question.
The reptiles, and some others of the lower animals, add still more to this
weight of evidence. The true rattlesnakes are highly characteristic, and
among the lizards are several genera of the peculiar American family, the
Iguanidae. Nowhere in the world are the tailed batrachians so largely
developed as in this region, the Sirens and the Amphiumidae forming two
peculiar families, while there are nine peculiar genera of salamanders, and
two others allied respectively to the Proteus of Europe and the Sieboldia
or giant salamander of Japan. There are seven peculiar families and about
thirty peculiar genera of fresh-water fishes; while the fresh-water
molluscs are more numerous than in any other region, more than thirteen
hundred species and varieties having been described.
Combining the evidence derived from all these classes of animals, we find
the Nearctic region to be exceedingly well characterised, and to be amply
distinct from the Palaearctic. The few species that are common to the two
are almost all arctic, or, at least, northern types, and may be compared
with those desert forms which occupy the debatable ground between the
Palaearctic, Ethiopian, and Oriental regions. {51} If, however, we compare
the number of species, which are common to the Nearctic and Palaearctic
regions with the number common to the western and eastern extremities of
the latter region, we shall find a wonderful difference between the two
cases; and if we further call to mind the number of important groups
characteristic of the one region but absent from the other, we shall be
obliged to admit that the relation that undoubtedly exists between the
faunas of North America and Europe is of a very distinct nature from that
which connects together Western Europe and North-eastern Asia in the bonds
of zoological unity.
_Definition and Characteristic Groups of the Neotropical Region._--The
Neotropical region requires very little definition, since it comprises the
whole of America south of the Nearctic region, with the addition of the
Antilles or West Indian Islands. Its zoological peculiarities are almost as
marked as those of Australia, which, however, it far exceeds in the extreme
richness and variety of all its forms of life. To show how distinct it is
from all the other regions of the globe, we need only enumerate some of the
best known and more conspicuous of the animal forms which are peculiar to
it. Such are, among mammalia--the prehensile-tailed monkeys and the
marmosets, the blood-sucking bats, the coati-mundis, the peccaries, the
llamas and alpacas, the chinchillas, the agoutis, the sloths, the
armadillos, and the ant-eaters; a series of types more varied, and more
distinct from those of the rest of the world than any other continent can
boast of. Among birds we have the charming sugar-birds, forming the family
Coerebidae; the immense and wonderfully varied group of tanagers; the
exquisite little manakins, and the gorgeously-coloured chatterers; the host
of tree-creepers of the family Dendrocolaptidae; the wonderful toucans; the
puff-birds, jacamars, todies and motmots; the marvellous assemblage of four
hundred distinct kinds of humming-birds; the gorgeous macaws; the
curassows, the trumpeters, and the sun-bitterns. Here again there is no
other continent or region that can produce such an assemblage of remarkable
and perfectly distinct groups of birds; and no less wonderful is its
richness in species, since these fully equal, if they do not surpass, those
of the {52} two great tropical regions of the Eastern Hemisphere (the
Ethiopian and the Oriental) combined.
As an additional indication of the distinctness and isolation of the
Neotropical region from all others, and especially from the whole Eastern
Hemisphere, we must say something of the otherwise widely distributed
groups which are absent. Among mammalia we have first the order
Insectivora, entirely absent from South America, though a few species are
found in Central America and the West Indies; the Viverridae or civet
family is wholly wanting, as are every form of sheep, oxen, or antelopes;
while the swine, the elephants, and the rhinoceroses of the old world are
represented by the diminutive peccaries and tapirs.
Among birds we have to notice the absence of tits, true flycatchers,
shrikes, sunbirds, starlings, larks (except a solitary species in the
Andes), rollers, bee-eaters, and pheasants, while warblers are very scarce,
and the almost cosmopolitan wagtails are represented by a single species of
pipit.
We must also notice the preponderance of low or archaic types among the
animals of South America. Edentates, marsupials, and rodents form the
majority of the terrestrial mammalia; while such higher groups as the
carnivora and hoofed animals are exceedingly deficient. Among birds a low
type of Passeres, characterised by the absence of the singing muscles, is
excessively prevalent, the enormous groups of the ant-thrushes, tyrants,
tree-creepers, manakins, and chatterers belonging to it. The Picariae (a
lower group) also prevail to a far greater extent than in any other
regions, both in variety of forms and number of species; and the chief
representatives of the gallinaceous birds--the curassows and tinamous, are
believed to be allied, the former to the brush-turkeys of Australia, the
latter (very remotely) to the ostriches, two of the least developed types
of birds.
Whether, therefore, we consider its richness in peculiar forms of animal
life, its enormous variety of species, its numerous deficiencies as
compared with other parts of the world, or the prevalence of a low type of
organisation among its higher animals, the Neotropical region stands out as
undoubtedly the most remarkable of the great zoological divisions of the
earth.
In reptiles, amphibia, fresh-water fishes, and insects, {53} this region is
equally peculiar, but we need not refer to these here, our only object now
being to establish by a sufficient number of well-known and easily
remembered examples, the distinctness of each region from all others, and
its unity as a whole. The former has now been sufficiently demonstrated,
but it may be well to say a few words as to the latter point.
The only outlying portions of the region about which there can be any doubt
are--Central America, or that part of the region north of the Isthmus of
Panama, the Antilles or West Indian Islands, and the temperate portion of
South America including Chili and Patagonia.
In Central America, and especially in Mexico, we have an intermixture of
South American and North American animals, but the former undoubtedly
predominate, and a large proportion of the peculiar Neotropical groups
extend as far as Costa Rica. Even in Guatemala and Mexico we have howling
and spider-monkeys, coati-mundis, tapirs, and armadillos; while chatterers,
manakins, ant-thrushes, and other peculiarly Neotropical groups of birds
are abundant. There is therefore no doubt as to Mexico forming part of this
region, although it is comparatively poor, and exhibits the intermingling
of temperate and tropical forms.
The West Indies are less clearly Neotropical, their poverty in mammals as
well as in most other groups being extreme, while great numbers of North
American birds migrate there in winter. The resident birds, however,
comprise trogons, sugar-birds, chatterers, with many humming-birds and
parrots, representing eighteen peculiar Neotropical genera; a fact which
decides the region to which the islands belong.
South temperate America is also very poor as compared with the tropical
parts of the region, and its insects contain a considerable proportion of
north temperate forms. But it contains armadillos, cavies and opossums; and
its birds all belong to American groups, though, owing to the inferior
climate and deficiency of forests, a number of the families of birds
peculiar to tropical America are wanting. Thus there are no manakins,
chatterers, toucans, trogons, or motmots; but there are abundance of
hang-nests, tyrant-birds, ant-thrushes, tree-creepers, and a fair {54}
proportion of humming-birds, tanagers and parrots. The zoology is therefore
thoroughly Neotropical, although somewhat poor; and it has a number of
peculiar forms of strictly Neotropical types--as the chinchillas, alpacas,
&c., which are not found in the tropical regions except in the high Andes.
_Comparison of Zoological Regions with the Geographical Divisions of the
Globe._--Having now completed our survey of the great zoological regions of
the globe, we find that they do not differ so much from the old
geographical divisions as our first example might have led us to suppose.
Europe, Asia, Africa, Australia, North America, and South America, really
correspond, each to a zoological region, but their boundaries require to be
modified more or less considerably; and if we remember this, and keep their
extensions or limitations always in our mind, we may use the terms "South
American" or "North American," as being equivalent to Neotropical and
Nearctic, without much inconvenience, while "African" and "Australian"
equally well serve to express the zoological type of the Ethiopian and
Australian regions. Europe and Asia require more important modifications.
The European fauna does indeed well represent the Palaearctic in all its
main features, and if instead of Asia we say tropical Asia we have the
Oriental region very fairly defined; so that the relation of the
geographical with the zoological primary divisions of the earth is
sufficiently clear. In order to make these relations visible to the eye and
more easily remembered, we will put them into a tabular form:
Regions. Geographical Equivalent.
Palaearctic EUROPE, with north temperate Africa and Asia.
Ethiopian AFRICA (south of the Sahara) with Madagascar.
Oriental TROPICAL ASIA, to Philippines and Java.
Australian AUSTRALIA, with Pacific Islands, Moluccas, &c.
Nearctic NORTH AMERICA, to North Mexico.
Neotropical SOUTH AMERICA, with tropical N. America and W. Indies.
The following arrangement of the regions will indicate their geographical
position, and to a considerable extent their relation to each other.
N E A R C T I C--P A L A E A R C T I C
| | |
| | ORIENTAL
| ETHIOPIAN |
NEO- |
TROPICAL AUSTRALIAN
May 4th. Diameter of spot 31deg 24'
June 4th. ,, ,, 28deg 0'
,, 17th. ,, ,, 22deg 54'
July 4th. ,, ,, 18deg 24'
,, 12th. ,, ,, 15deg 20'
,, 20th. ,, ,, 18deg 0'
We thus see that Mars has two permanent snow-caps, of nearly equal size in
winter but diminishing very unequally {55} in summer, when the southern cap
is reduced to nearly one third the size of the northern; and this fact is
held by Mr. Carpenter, as it was by the late Mr. Belt, to be opposed to the
view of the hemisphere which has winter in _aphelion_ (as the southern now
has both in the Earth and Mars), having been alone glaciated during periods
of high excentricity.[9]
Before, however, we can draw any conclusion from the case of Mars, we must
carefully scrutinise the facts, and the conditions they imply. In the first
place, there is evidently this radical difference between the state of Mars
now and of the Earth during a glacial period--that Mars has no great
ice-sheets spreading over its temperate zone, as the Earth undoubtedly had.
This we know from the fact of the _rapid_ disappearance of the white
patches over a belt three degrees wide in a fortnight (equal to a width of
about 100 miles of our measure), and in the northern hemisphere of eight
degrees wide (about 280 miles) between May 4th and July 12th. Even with our
much more powerful sun, which gives us more than twice as much heat as Mars
receives, no such diminution of an ice-sheet, or of glaciers of even
moderate thickness, could possibly occur; but the phenomenon is on the
contrary exactly analogous to what actually takes place on the plains of
Siberia in summer. These, as I am informed by Mr. Seebohm, are covered with
snow during winter and spring to a depth of six or eight feet, which
diminishes very little even under the hot suns of May, till warm winds
combine with the sun in June, when in about a fortnight the whole of it
disappears, and a little later the whole of northern Asia is free from its
winter covering. As, however, the sun of Mars is so much less powerful than
ours, we may be {56} sure that the snow (if it is real snow) is much less
thick--a mere surface-coating in fact, such as occurs in parts of Russia
where the precipitation is less, and the snow accordingly does not exceed
two or three feet in thickness.
We now see the reason why the _southern_ pole of Mars parts with its white
covering so much more quickly and to so much greater an extent than the
_northern_, for the south pole during summer is nearest the sun, and, owing
to the great excentricity of Mars, would have about one-third more heat
than during the summer of the northern hemisphere; and this greater heat
would cause the winds from the equator to be both warmer and more powerful,
and able to produce the same effects on the scanty Martian snows as they
produce on our northern snow-plains. The reason why both poles of Mars are
almost equally snow-covered in winter is not difficult to understand. Owing
to the greater obliquity of the ecliptic, and the much greater length of
the year, the polar regions will be subject to winter darkness fully twice
as long as with us, and the fact that one pole is nearer the sun during
this period than the other at a corresponding period, will therefore make
no perceptible difference. It is also probable that the two poles of Mars
are approximately alike as regards their geographical features, and that
neither of them is surrounded by very high land on which ice may
accumulate. With us at the present time, on the other hand, geographical
conditions completely mask and even reverse the influence of excentricity,
and that of winter in _perihelion_ in the northern, and summer in
_perihelion_ in the southern, hemisphere. In the north we have a
preponderance of sea within the Arctic circle, and of lowlands in the
temperate zone. In the south exactly opposite conditions prevail, for there
we have a preponderance of land (and much of it high land) within the
Antarctic circle, and of sea in the temperate zone. Ice, therefore,
accumulates in the south, while a thin coating of snow, easily melted in
summer, is the prevalent feature in the north; and these contrasts react
upon climate to such an extent, that in the southern ocean, islands in the
latitude of Ireland have glaciers descending to the level of the sea, and
constant snowstorms {57} in the height of summer, although the sun is then
actually nearer the earth than it is during our northern summer!
It is evident, therefore, that the phenomena presented by the varying polar
snows of Mars are in no way opposed to that modification of Dr. Croll's
theory of the conditions which brought about the glacial epochs of our
northern hemisphere, which is here advocated; but are perfectly explicable
on the same general principles, if we keep in mind the distinction between
an ice-sheet--which a summer's sun cannot materially diminish, but may even
increase by bringing vapour to be condensed into snow--and a thin snowy
covering which may be annually melted and annually renewed, with great
rapidity and over large areas. Except within the small circles of perpetual
polar snow there can at the present time be no ice-sheets in Mars; and the
reason why this permanent snowy area is more extensive around the northern
than around the southern pole may be partly due to higher land at the
north, but is perhaps sufficiently explained by the diminished power of the
summer sun, owing to its greatly increased distance at that season in the
northern hemisphere, so that it is not able to melt so much of the snow
which has accumulated during the long night of winter.
* * * * *
{58}
CHAPTER IX
ANCIENT GLACIAL EPOCHS, AND MILD CLIMATES IN THE ARCTIC REGIONS
Dr. Croll's Views on Ancient Glacial Epochs--Effects of Denudation in
Destroying the Evidence of Remote Glacial Epochs--Rise of Sea-level
Connected with Glacial Epochs a Cause of Further Denudation--What
Evidence of Early Glacial Epochs may be Expected--Evidences of
Ice-action During the Tertiary Period--The Weight of the Negative
Evidence--Temperate Climates in the Arctic Regions--The Miocene Arctic
Flora--Mild Arctic Climates of the Cretaceous Period--Stratigraphical
Evidence of Long-continued Mild Arctic Conditions--The Causes of Mild
Arctic Climates--Geographical Conditions Favouring Mild Northern
Climates in Tertiary Times--The Indian Ocean as a Source of Heat in
Tertiary Times--Condition of North America During the Tertiary
Period--Effect of High Excentricity on Warm Polar Climates--Evidences
as to Climate in the Secondary and Palaeozoic Epochs--Warm Arctic
Climates in Early Secondary and Palaeozoic Times--Conclusions as to the
Climates of Secondary and Tertiary Periods--General View of Geological
Climates as Dependent on the Physical Features of the Earth's
Surface--Estimate of the Comparative Effects of Geographical and
Physical Causes in Producing Changes of Climate.
If we adopt the view set forth in the preceding chapter as to the character
of the glacial epoch and of the accompanying alternations of climate, it
must have been a very important agent in producing changes in the
distribution of animal and vegetable life. The intervening mild periods,
which almost certainly occurred during its earlier and later phases, may
have been sometimes more equable than even our present insular climate, and
severe frosts were probably then unknown. During the four or five {59}
thousand years that each specially mild period may have lasted, some
portions of the north temperate zone, which had been buried in snow or ice,
would become again clothed with vegetation and stocked with animal life,
both of which, as the cold again came on, would be driven southward, or
perhaps partially exterminated. Forms usually separated would thus be
crowded together, and a struggle for existence would follow, which must
have led to the modification or the extinction of many species. When the
survivors in the struggle had reached a state of equilibrium, a fresh field
would be opened to them by the later ameliorations of climate; the more
successful of the survivors would spread and multiply; and after this had
gone on for thousands of generations, another change of climate, another
southward migration, another struggle of northern and southern forms would
take place.