with low and ancient types, but the latter largely predominate;
and the most probable explanation seems to be that the same
concurrence of favourable conditions which allowed the megatherium
and megalonyx to enter North America also led to an immigration of
horses, deer, mastodons, and many of the Felidæ into South America.
These inter-migrations appear to have taken place at several remote
intervals, the northern and southern continents being for the most
part quite separated, and each developing its own peculiar forms of
life. This view is supported by the curious fact of a large number
of the marine fishes of the two sides of Central America being
absolutely identical--implying a recent union of the two oceans and
separation of the continents--while the mollusca of the Pacific
coast of America bear so close a relation to those of the Caribbean
Sea and the Atlantic coasts, as to indicate a somewhat more remote
but longer continued sea-passage. The straits connecting the two
oceans were probably situated in Nicaragua and to the south of
Panama, leaving the highlands of Mexico and Guatemala united to
North America.
Around the Gulf of Mexico and the Caribbean Sea there is a wide
belt of rather shallow water, and during the alternate elevations
and subsidences to which this region has been subjected, the newly
raised land would afford a route for the passage of immigrants
between North and South America. The great depression of the ocean,
believed to have occurred during the Glacial period (caused by
the locking-up of the water in the two polar masses of ice), may
perhaps have afforded the opportunity for those latest immigrations
which gave so striking a character to the North American fauna in
Post-Pliocene times.
Among the changes which South America itself has undergone, perhaps
the most important has been its separation into a group of large
islands. Such a change is clearly indicated by the immense area and
low elevation of the great alluvial plains of the Orinoko, Amazon,
and La Plata, as well as by certain features in the distribution
of the existing Neotropical fauna. A subsidence of less than 2,000
feet would convert the highlands of Guiana and Brazil into islands
separated by a shallow strait from the chain of the Andes. When
this occurred the balance of the land was probably restored by an
elevation of the extensive submerged banks on the east coast of
South America, which in South Brazil and Patagonia are several
hundred miles wide, embracing the Falkland Islands, and reaching
far to the south of Cape Horn.
Looking, then, at the whole of the evidence at our command, we
seem justified in concluding that the past histories of North and
South America have been different, and in some respects strongly
contrasted. North America was evidently in very early times so
far connected with Europe and Asia as to interchange with those
continents the higher types of animal life as they were successively
developed in either hemisphere. These more perfectly organised beings
rapidly gained the ascendency, and led to the extinction of most
of the lower forms which had preceded them. The Nearctic has thus
run a course parallel to that of the Palæarctic region, although
its fauna is, and perhaps always has been, less diversified and
more subject to incursions of lower types from adjacent lands in
the southern hemisphere.
South America, on the other hand, has had a history in many respects
parallel to that of Africa. Both have long existed either as
continents or groups of large islands in the southern hemisphere,
and for the most part completely separated from the northern
continents; and each accordingly developed its peculiar types from
those ancestral and lowly-organised forms which first entered it.
South America, however, seems to have had a larger area and more
favourable conditions, and it remained almost completely isolated
till a later period. It was therefore able to develop a more-varied
and extensive fauna of its own peculiar types, and its union
with the northern continent has been so recent, and is even now
maintained by so narrow an isthmus, that it has never been overrun
with the more perfect mammalia to anything like the extent that has
occurred in Africa. South America, therefore, almost as completely
as Australia, has preserved for us examples of a number of low and
early types of mammalian life, which, had not the entire country
been isolated from the northern continent during middle and late
Tertiary times, would long since have become extinct.
_The Australian Region._--There only remains for us now to consider
the relation of the island-continent of Australia to Asia and
South America, with both which countries it has a certain amount
of zoological connection.
Australia, including New Guinea (which has in recent times been
united with it), differs from all the other continents by the extreme
uniformity and lowly organisation of its mammalia which almost
all belong to one of the lowest orders--the marsupials. Monkeys,
carnivora, insectivora, and the great and almost ubiquitous class
of hoofed-animals, are all alike wanting; the only mammals besides
marsupials being a few species of a still lower type--the monotremes,
and a few of the very smallest forms of rodents--the mice. The
marsupials, however, are very numerous and varied, constituting 6
families and 33 genera, of which there are about 120 known species.
None of these families is represented in any other continent; and
this fact alone is sufficient to prove that Australia must have
remained almost or quite isolated during the whole of the Tertiary
period.
In birds there is, as we might expect, less complete isolation;
yet there are a number of very peculiar types. About 15 families
are confined to the Australian region, among which are the
paradise-birds, the honey-suckers, the lyre-birds, the brush-tongued
lories, the mound-makers, and the cassowaries.
Our knowledge of the former mammalian inhabitants of Australia is
imperfect, as all yet discovered are from Post-Tertiary or very
late Tertiary deposits. It is interesting to find, however, that
all belong to the marsupial type, although several are quite unlike
any living animals, and some are of enormous size, almost rivalling
the mastodons and megatheriums of the northern continents. In the
earliest Tertiary formation of Europe remains of marsupials have been
found, but they all belong to the opossum type, which is unknown
in Australia; and this supports the view that no communication
existed between the Palæarctic and Australian regions even at
this early period. Much farther back, however, in the Oolite and
Trias formations, remains of a number of small mammalia have been
found which are almost certainly marsupial, and bear a very close
resemblance to the Myrmecobius, a small and very rare mammal still
living in Australia. An animal of somewhat similar type has been
discovered in rocks of the same age in North America; and we have,
therefore, every reason to believe, that it was at or near this
remote epoch when Australia, or some land which has been since
in connection with it, received a stock of mammalian immigrants
from the great northern continent; since which time it has almost
certainly remained completely isolated.
The occurrence of the marsupial opossums in America has been thought
by some writers to imply an early connection between that continent
and Australia; but the fact that opossums existed in Europe in
Eocene and Miocene times, and that no trace of them has been found
in North or South America before the Post-Pliocene period, renders
it almost certain that they entered America from Europe or North
Asia in middle or late Tertiary times, and have flourished there
in consequence of a less severe competition with highly-developed
forms of life.
The birds of Australia and South America only exhibit a few cases
of very remote and general affinity, which are best explained by
the preservation in each country of once wide-spread types, but is
quite inconsistent with the theory of a direct union between the
two countries during Tertiary times.
Reptiles are even more destitute of proofs of any such connection
than even mammalia or birds; but in amphibia, fresh-water fishes,
and insects the case is different, all these classes furnishing
examples of the same families or genera inhabiting the temperate
parts of both continents. But the fact that such cases are confined
to these three groups and to plants, is the strongest possible proof
that they are not due to land-connection; for all these organisms
may be transmitted across the ocean in various ways. Violent storms
of wind, floating ice, drift-wood, and aquatic birds, are all known
to be effective means for the distribution of these animals or their
ova, and the seeds of plants. All of them too, it must be noted,
are to a considerable degree patient of cold; the reverse being
the case with true reptiles and land-birds, which are essentially
heat-loving; so that the whole body of facts seems to point rather to
an extension of the Antarctic lands and islands reducing the width
of open sea, than to any former union, or even close approximation
of the Australian and South American continents.
_Summary and Conclusion._
Let us now briefly review the conclusions at which we have arrived.
If we look back to remote Tertiary times, we shall probably find that
all our great continents and oceans were then in existence, and even
bore a general resemblance to the forms and outlines now so familiar
to us. But in many details, and especially in their amount of
communication with each other, we should observe important changes.
The first thing we should notice would be a more complete separation
of the northern and the southern continents. Now, there is only one
completely detached southern land--Australia; but at that period
Africa and South America were also vast islands or archipelagos,
completely separated from their sister continents. Examining them
more closely, we should observe that the great Euro-Asiatic continent
had a considerable extension to the south-east, over what are now
the shallow seas of Japan, China, and Java. In the south-west it
would include Northern Africa, the Mediterranean then forming two
inland seas; while to the west and north-west it would include the
British Isles, and perhaps extend even to Iceland and Greenland.
As a balance to these extensions, much of Northern Siberia and
North-Western Asia may have been under water; the peninsula of India
would be an island with a considerable south-west extension over
what are now the Laccadive and Maldive coral-reefs. The Himalayas
would be a moderate range of hills; the great desert plateau of
Central Asia a fertile plain; the greater part of the continent would
enjoy a tropical or subtropical climate, while even the extreme
north would support a luxuriant vegetation. This great continent
would abound in animal life, and would be especially remarkable
for its mammalia, which would comprise ancestral forms of all our
existing higher types, along with a number of those lower grades
of organisation (such as lemurs and opossums) now found chiefly in
the southern hemisphere.
Connected with this continent by what is now Behring Straits and the
Sea of Kamschatka, we should find North America, perhaps somewhat
diminished in the east, but more extensive in the south and north,
and abounding as now with great inland lakes which were situated
to the west of the present lake district. This continent seems to
have had a less tropical climate and vegetation than prevailed
in the eastern hemisphere, but it supported an almost equally
varied though very distinct fauna. Ancestral horses no larger than
dogs; huge tapir-like and pig-like animals; strange forms allied
to rhinoceroses; the Dinocerata--huge horned animals allied to
elephants and to generalised Ungulata; and the Tillodontia, still
more unlike anything now living, since they combined characters now
found separated in the carnivora, the Ungulata, and the rodents.
Ancestral Primates, allied to both the lemurs and the South American
monkeys, also inhabited this continent.
The great land masses of the northern hemisphere thus appear to have
possessed between them all the higher types of animal life; and
these seem to have been developed for a time in one continent and
then to have been in part transferred by migration to the other,
where alone they have sometimes maintained themselves. Thus, the
elephants and the camels appear to have descended from what were once
exclusively American types, while the opossums were as certainly
European. Many groups, however, never passed out of the continent
in which they originated--the civets, hyænas, and the giraffes
being wholly eastern, while the Oreodontidæ and Brontotheridæ were
no less exclusively western.
South America seems to have been united to the northern continent
once at least in Secondary or early Tertiary times, since it was
inhabited in the Eocene period by many forms of mammalia, such as
rodents, felines, and some ancient forms of Ungulata. It must also
have possessed the ancestors of the Edentata (though they have
not yet been discovered), or we should not find such a variety of
strange and gigantic forms of this order in later Tertiary deposits
in this part of the world only. During the greater part of the
Tertiary period, therefore, South America must have been separated
from the North and protected from incursions of the higher forms
of mammalia which were there so abundant. Thus only does it seem
possible to understand the unchecked development of so many large
but comparatively helpless animals as the Edentata of the Pampas
and the Brazilian caves--a development only comparable with that
of the Australian marsupials, still more completely shut off from
all competition with higher forms of life.
In Africa the evidence of a long period of insulation is somewhat
more complex and less easily apparent, but, it seems to me, equally
conclusive. We have first, the remarkable fauna of Madagascar, in
which lemurs and insectivora predominate, with a few low forms of
carnivora; but none of the higher animals, such as apes, antelopes,
buffaloes, rhinoceroses, elephants, lions, leopards, and hyænas,
which swarm on the continent. The separation of Madagascar from
Africa must therefore have occurred before these important groups
existed there. Now, we know that all these large animals lived
in Europe and Asia during late Miocene times, while lemurs are
only known there during the Eocene period, and were probably more
abundant in late Mesozoic times. It is almost certain, therefore,
that Southern Africa must have been cut off from Europe and Asia
during the whole intervening period, or the same development of high
forms and extinction of low would have gone on in the one country
as in the other. The persistence of a number of low and isolated
types in South and West Africa, which are probably a remnant of the
ancient fauna of the country, is also favourable to this view. At
the time we are considering, therefore, we look upon tropical and
South Africa, with Madagascar, as forming a completely isolated
land or archipelago; while the Seychelles and Chagos banks, with
Bourbon and Mauritius, perhaps, formed another island or group
permanently separated from the larger masses. The extra-tropical
portion of South Africa was also probably more extensive, affording
an area in which its remarkable flora was being developed.
Turning to Australia, we should probably find it, at this remote
period, more extensive than it is now, including in its area New
Guinea and some of the adjacent islands, as well as Tasmania; while
another extensive land probably occupied the site of the New Zealand
group. It may be considered certain that, whatever elevations and
subsidences these countries may have undergone, they have not been
connected either with Asia, Africa, or South America during the
whole Tertiary period.
In conclusion, I would especially remark that the various changes in
the outlines and mutual relations of our continents, which I have
now endeavoured to establish, must not be supposed to have been
all strictly contemporaneous. Some may have been a little earlier
or a little later than others; some changes may have been slower,
others more rapid; some may have had but a short duration, while
others may have persisted through considerable geological periods.
But, notwithstanding this uncertainty as to details, the great
features of the geographical revolutions which I have indicated,
appear to be established by a mass of concurring evidence; and the
lesson they teach us is, that although almost the whole of what is
now dry land has undoubtedly once lain deep beneath the waters of
the ocean, yet such changes on a great scale are excessively slow
and gradual; so that, when compared with the highest estimates of
the antiquity of the human race, or even with that of most of the
higher animals, our existing continents and oceans may be looked
upon as permanent features of the earth’s surface.
ERRATUM.
At page 59 I have said that there are only three or four species
of Mimosa which are sensitive. This is a mistake, as the greater
portion of the species in the extensive genus Mimosa, as well as
some species of several other genera of Leguminosæ, and also of
Oxalidaceæ, possess this curious property. I cannot find, however,
that any one has suggested in what way the sensitiveness may have
been useful to the species which first acquired it. My guess at an
explanation may therefore induce botanists who are acquainted with
the various species in a state of nature, to suggest some better
solution of the problem.
INDEX.
A.
_Abrus precatoria_, perhaps a case of mimicry, 226
Absorption-colours or pigments, 183
Acræidæ, warning colours of, 174
Adaptive characters, 150, 155
Affinities, how to determine doubtful, 148
African large mammalia, recent immigrants, 323
Allen, Mr. Grant, on protective colours of fruits, 225
Alpine flowers, why so beautiful, 232
Amboyna, large-sized butterflies of, 258
American monkeys, 118
American Continents, past history of, 332
Ancient races of North and South America, 298
Andaman Islands, pale butterflies of, 260
white-marked birds of, 263
Anderson, Mr. W. Marshall, on cranium from N. American mound, 296
Andes, very rich in humming-birds, 139
Animal colours, how produced, 184
life in tropical forests, 70
Anthribidæ, 95
Ants, wasps, and bees, 80
numbers of, in India and Malaya, 81-88
destructive to insect-specimens, 85
and vegetation, special relation between, 89
Apatura and Heterochroa, resemblance of species of, 257
Apes, 116
Aqueous vapour of atmosphere, its influence on temperature, 9
quantity at Batavia and Clifton, 10
Arctic plants, large leaves of, 236
flowers and fruits brightly coloured, 237
Areca palm, 45
_Arenga saccharifera_, 43
Argus-pheasant, wonderful plumage of, 205
Arums, 48
Assai of the Amazon, 43
Auckland Isles, handsome flowers of, 238
Audubon, on the ruby humming-birds, 130, 137
Australian Region, mammalia of, 340
birds of, 340
extinct fauna of, 341
its supposed union with S. America, 341
Azara, on food of humming-birds, 135
B.
Bamboos, 52
uses of, 53-58
Bananas, wild, 47
Banana, 48
Barber, Mrs. on colour changes of pupa of _Papilio nireus_, 168
Barbets, 105
Bark, varieties of in tropical forests, 33
Barometer, range of, at Batavia, 24
Batavia, Meteorology of, 4
and London, diagram of mean temperatures, 5
greatest rainfall at, 24
range of barometer at, 24
Bates, Mr., on climate at the Equator, 24
on scarcity of forest-flowers on Amazon, 61
on animal life in Amazon valley, 70
on abundance of butterflies at Ega, 75
on importance of study of butterflies, 78
on leaf-cutting ants, 86
on blind ants, 88
on bird-catching spider, 97
on use of toucan’s bill, 106
on large serpents, 115
on the habits of humming-birds, 132
Bats, 118
Beetles, 94
abundance of, in New Forest-clearings, 96
probable use of horns of, 202
Belt, Mr., on virgin forests of Nicaragua, 62
on aspects of tropical vegetation, 67
on leaf-cutting ants, 86
on an Acacia inhabited by ants, 89
on uses of ants to the trees they live on, 90
on a leaf-like locust, 93
on tree-frogs, 116
on the habits of humming-birds, 133, 134
on uneatable bright-coloured frog, 175
on use of light of glow-worm, 205
Betel-nut, 45
Bill of humming-birds, 129
Biology, by-paths of, illustrated, 251
Birds, 99
how many known, 124
cases of local variation of colour among, 262
influence of locality on colours of, 255
which fertilize flowers, 273, 274
and insects blown to oceanic islands, 308
of Palæarctic Region, 316
of Ethiopian Region, 318
of Oriental Region, 320
Bonelli, Mr., on the Sappho comet humming-bird, 132
Bullock on food of humming-birds, 153
Buprestidæ, 94
Burchell, Dr., on the “stone mesembryanthemum,” 223
Butterflies, abundance of, in tropical forests, 72
conspicuousness of in tropical forests, 73
colours and form of, 74
peculiar habits of tropical, 76
tropical and temperate compared as to colour, 164
females do not choose their partners, 200
with gaily-coloured females, 204
numbers and variety of, 255
influence of locality on colours of, 255
Buttressed trees, 31
C.
Calamus, 41
Callithea, imitated by species of Catagramma and Agrias, 257
_Callithea markii_, 75
_Campylopterus hemileucurus_, pugnacious and ornamental, 214
Cattleyas, 51
Cecropias, trees inhabited by ants, 89
Celebes, large and peculiarly formed butterflies of, 259
white-marked birds of, 263
Centipedes, 97
Ceylon and Malaya, resemblances of fauna of, 327
Chameleons, 113
Chameleon, cause of changes of its colour, 170
Chemical action changes colours, 183
Chili, humming-birds of, 141
Chiroptera, 119
_Chrysobactron Rossii_, 238
Clark, Rev. Hamlet on leaf-cutting ants, 86
Climate of Equator, general features of, 17
Climates of Timor, Angola, and Scotland compared, 14
Climbing plants of tropical forests, 37
uses of, 39
Cockatoos, 100
Cœlogynes, 51
Coloration of tropical birds, 110
Colour, cause of change of, in humming-birds, 144
Colour in nature, problems of, 159
how far constant, 161
as affected by heat and light, 161
of tropical birds, 163
of tropical butterflies, 164
of temperate and tropical flowers, 165
changes of, in animals produced by coloured light, 167
voluntary change of, in animals, 170
not usually influenced by coloured light, 171
Colour, the nature of, 180
how produced, 183
changed by heat, 183
a normal product of organization, 185
as a means of recognition, 196
proportionate to integumentary development, 198
not caused by female selection, 198
Colour absent in wind-fertilized flowers, 233
same theory of, in animals and plants, 234
of flowers and their distribution, 235
Colour, nomenclature of, formerly imperfect, 247
Colour-development as illustrated by humming-birds, 212
local causes of, 216
in animals, summary, 216
Colour-perception, supposed recent growth of, 244
Colour-sense, origin of the, 241
need for, 243
not of recent origin, 246
not wholly explicable, 248
Colours, classification of organic, 172
protective, 172
warning, 174
sexual, 177
typical, 179
of animals, how produced, 184
theory of protective, 187
theory of warning, 189
theory of sexual, 192
theory of typical, 215
Colours and ornaments of humming-birds, 127
Colours of fruits, attractive, 224
protective, 225
Colours, which first perceived, 243
_Cometes sparganurus_, very pugnacious, 214
Compositæ, arborescent in oceanic islands, 276
Continent, past changes of the great Eastern, 321
Continents of Tertiary period, probable aspect of, 343
Copridæ, 95
probable use of horns of, 202
Crematogaster, genus of ants, 83
Cross-fertilization of flowers, use of, 228
complex arrangements for, 229
Cuckoos, 104
D.
Danaidæ, warning colours of, 174
Danainæ, Acræinæ and Heliconiinæ, local resemblances of, 256
_Daphne pontica_, 230