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Title: Island Life; Or, The Phenomena and Causes of Insular Faunas
and Floras

Author: Alfred Russel Wallace

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FRONTISPIECE

ISLAND LIFE
OR

THE PHENOMENA AND CAUSES OF

INSULAR FAUNAS AND FLORAS
INCLUDING A REVISION AND ATTEMPTED SOLUTION
OF
THE PROBLEM OF

GEOLOGICAL CLIMATES

BY

ALFRED RUSSEL WALLACE
AUTHOR OF "THE MALAY ARCHIPELAGO," "THE GEOGRAPHICAL DISTRIBUTION OF
ANIMALS,"
"DARWINISM," ETC.

SECOND AND REVISED EDITION

London

MACMILLAN AND CO.
AND NEW YORK

1895

The Right of Translation and Reproduction is Reserved

Richard Clay and Sons, Limited,
LONDON AND BUNGAY.

First Edition printed 1880 (Med. 8vo).
Second Edition 1892 (Extra cr. 8vo). Reprinted 1895.

TO

SIR JOSEPH DALTON HOOKER,
K.C.S.I., C.B., F.R.S., ETC., ETC.

WHO, MORE THAN ANY OTHER WRITER,

HAS ADVANCED OUR KNOWLEDGE OF THE
GEOGRAPHICAL

DISTRIBUTION OF PLANTS, AND ESPECIALLY

OF INSULAR FLORAS,

I Dedicate this Volume;
ON A KINDRED SUBJECT,

AS A TOKEN OF ADMIRATION AND REGARD.

CORRECTIONS IN PRESENT ISSUE.

The first issue of this Edition being exhausted, the opportunity is taken of
making a few corrections, the most important of which are here stated:—
Page 163. Statement modified as to supposed glaciation of South Africa.
Pages 174 and 338. Many geologists now hold that there was no great
submergence during the glacial epoch. The passages referring to it have
therefore been re-written.
Page 182. Colonel Fielden's explanation of the occurrence of large trees on
shores and in recent drift in high latitudes, is now added.
" 272. A species of Carex peculiar to Bermuda is now given.
" 356. Geomalacus maculosus, as a peculiar British species, is now
omitted.
Verbal alterations have also been made at pages 41, 105, 356, and 360.

PREFACE TO THE SECOND EDITION

This edition has been carefully revised throughout, and owing to the great
increase to our knowledge of the Natural History of some of the islands

during the last twelve years considerable additions or alterations have been
required. The more important of these changes are the following:—
Chapter VII. The account of the migrations of animals and plants during
and since the Glacial Epoch, has been modified to accord with newer
information.
Chapters VIII and IX. The discussion of the causes of Glacial Epochs and
Mild Arctic Climates has been somewhat modified in view of the late Dr.
Croll's remarks, and the argument rendered clearer.
Chapter XIII. Several additions to the Fauna of the Galapagos have been
noted.
Chapter XV. Considerable additions have been made to this chapter
embodying the recent discoveries of birds and insects new to the Sandwich
Islands, while a much fuller account has been given of its highly peculiar
and very interesting flora.
Chapter XVI. Important additions and corrections have been made in the
lists of peculiar British animals and plants embodying the most recent
information.
Chapter XVII. Very large additions have been made to the mammalia and
birds of Borneo, and full lists of the peculiar species are given.
Chapter XVIII. A more accurate account is given of the birds of Japan.
Chapter XIX. The recent additions to the mammals and birds of
Madagascar are embodied in this chapter, and a fuller sketch is given of the
rich and peculiar flora of the island.
Chapter XXI. and XXII. Some important additions have been made to these
chapters owing to more accurate information as to the depth of the sea
around New Zealand, and to the discovery of abundant remains of fossil
plants of the tertiary and cretaceous periods both in New Zealand and
Australia.
In the body of the work I have in each case acknowledged the valuable
information given me by naturalists of eminence in their various
departments, and I return my best thanks to all who have so kindly assisted

me. I am however indebted in a special manner to one gentleman—Mr.
Theo. D. A. Cockerell, now Curator of the Museum of the Jamaica Institute
—who supplied me with a large amount of information by searching the
most recent works in the scientific libraries, by personal inquiries among
naturalists, and also by giving me the benefit of his own copious notes and
observations. Without his assistance it would have been difficult for me to
have made the present edition so full and complete as I hope it now is. In a
work of such wide range, and dealing with so large a body of facts some
errors will doubtless be detected, though, I trust few of importance.

Parkstone, Dorset, December, 1891.

PREFACE TO THE FIRST EDITION

The present volume is the result of four years' additional thought and
research on the lines laid down in my Geographical Distribution of
Animals, and may be considered as a popular supplement to and completion
of that work.
It is, however, at the same time a complete work in itself: and, from the
mode of treatment adopted, it will, I hope, be well calculated to bring
before the intelligent reader the wide scope and varied interest of this
branch of natural history. Although some of the earlier chapters deal with
the same questions as my former volumes, they are here treated from a
different point of view; and as the discussion of them is more elementary
and at the same time tolerably full, it is hoped that they will prove both
instructive and interesting. The plan of my larger work required that genera
only should be taken account of; in the present volume I often discuss the
distribution of species, and this will help to render the work more
intelligible to the unscientific reader.
The full statement of the scope and object of the present essay given in the
"Introductory" chapter, together with the "Summary" of the whole work and
the general view of the more important arguments given in the
"Conclusion," render it unnecessary for me to offer any further remarks on

these points. I may, however, state generally that, so far as I am able to
judge, a real advance has here been made in the mode of treating problems
in Geographical Distribution, owing to the firm establishment of a number
of preliminary doctrines or "principles," which in many cases lead to a far
simpler and yet more complete solution of such problems than have been
hitherto possible. The most important of these doctrines are those which
establish and define—(1) The former wide extension of all groups now
discontinuous, as being a necessary result of "evolution"; (2) The
permanence of the great features of the distribution of land and water on the
earth's surface; and, (3) The nature and frequency of climatal changes
throughout geological time.

I have now only to thank the many friends and correspondents who have
given me information or advice. Besides those whose assistance is
acknowledged in the body of the work, I am especially indebted to four
gentlemen who have been kind enough to read over the proofs of chapters
dealing with questions on which they have special knowledge, giving me
the benefit of valuable emendations and suggestions. Mr. Edward R. Alston
has looked over those parts of the earlier chapters which relate to the
mammals of Europe and the North Temperate zone; Mr. S. B. J. Skertchley,
of the Geological Survey, has read the chapters which discuss the glacial
epoch and other geological questions; Professor A. Newton has looked over
the passages referring to the birds of the Madagascar group; while Sir
Joseph D. Hooker has given me the invaluable benefit of his remarks on my
two chapters dealing with the New Zealand flora.

Croydon, August, 1880.

CONTENTS
PART I

THE DISPERSAL OF ORGANISMS; ITS PHENOMENA, LAWS, AND
CAUSES

CHAPTER I

INTRODUCTORY
Remarkable Contrasts in the 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.
Pages 3-11

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
Pages 12-30

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 Palæarctic
Region—Characteristic Features of the Palæarctic 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
Pages 31-54

CHAPTER IV

EVOLUTION AS THE KEY TO DISTRIBUTION
Importance of the Doctrine of Evolution—The Origin of New Species—
Variation in Animals—The amount of Variation in North American Birds—
How New Species Arise from a Variable Species—Definition and Origin of
Genera—Cause of the Extinction of Species—The Rise and Decay of
Species and Genera—Discontinuous Specific Areas, why Rare—
Discontinuity of the Area of Parus Palustris—Discontinuity of Emberiza
Schœniclus—The European and Japanese Jays—Supposed examples of
Discontinuity among North American Birds—Distribution and Antiquity of
Families—Discontinuity a Proof of Antiquity—Concluding remarks
Pages 55-71

CHAPTER V

THE POWERS OF DISPERSAL OF ANIMALS AND PLANTS
Statement of the General Question of Dispersal—The Ocean as a Barrier to
the Dispersal of Mammals—The Dispersal of Birds—The Dispersal of
Reptiles—The Dispersal of Insects—The Dispersal of Land Mollusca—
Great Antiquity of Land-shells—Causes Favouring the Abundance of Land-
shells—The Dispersal of Plants—Special Adaptability of Seeds for

Dispersal—Birds as Agents in the Dispersal of Seeds—Ocean Currents as
Agents in Plant Dispersal—Dispersal along Mountain Chains—Antiquity of
Plants as Effecting their Distribution
Pages 72-82

CHAPTER VI

GEOGRAPHICAL AND GEOLOGICAL CHANGES: THE
PERMANENCE OF CONTINENTS
Changes of Land and Sea, their Nature and Extent—Shore-Deposits and
Stratified Rocks—The Movements of Continents—Supposed Oceanic
Formations; the Origin of Chalk—Fresh-water and Shore-deposits as
Proving the Permanence of Continents—Oceanic Islands as Indications of
the Permanence of Continents and Oceans—General Stability of Continents
with Constant Change of Form—Effect of Continental Changes on the
Distribution of Animals—Changed Distribution Proved by the Extinct
Animals of Different Epochs—Summary of Evidence for the General
Permanence of Continents and Oceans.
Pages 83-105

CHAPTER VII

CHANGES OF CLIMATE WHICH HAVE INFLUENCED THE
DISPERSAL OF ORGANISMS: THE GLACIAL EPOCH
Proofs of the Recent Occurrence of a Glacial Epoch—Moraines—Travelled
Blocks—Glacial Deposits of Scotland: the "Till"—Inferences from the
Glacial Phenomena of Scotland—Glacial Phenomena of North America—
Effects of the Glacial Epoch on Animal Life—Warm and Cold Periods—
Palæontological Evidence of Alternate Cold and Warm Periods—Evidence
of Interglacial Warm Periods on the Continent and in North America—
Migrations and Extinctions of Organisms Caused by the Glacial Epoch
Pages 106-124

CHAPTER VIII

THE CAUSES OF GLACIAL EPOCHS

Various Suggested Causes—Astronomical Causes of Changes of Climate—
Difference of Temperature Caused by Varying Distances of the Sun—
Properties of Air and Water, Snow and Ice, in Relation to Climate—Effects
of Snow on Climate—High Land and Great Moisture Essential to the
Initiation of a Glacial Epoch—Perpetual Snow nowhere Exists on Lowlands
—Conditions Determining the Presence or Absence of Perpetual Snow—
Efficiency of Astronomical causes in Producing Glaciation—Action of
Meteorological Causes in Intensifying Glaciation—Summary of Causes of
Glaciation—Effect of Clouds and Fog in Cutting off the Sun's Heat—South
Temperate America as Illustrating the Influence of Astronomical Causes on
Climate—Geographical Changes how far a Cause of Glaciation—Land
Acting as a Barrier to Ocean-currents—The Theory of Interglacial Periods
and their Probable Character—Probable Effect of Winter in aphelion on the
Climate of Britain—The Essential Principle of Climatal Change Restated—
Probable Date of the Last Glacial Epoch—Changes of the Sea-level
Dependent on Glaciation—The Planet Mars as Bearing on the Theory of
Excentricity as a Cause of Glacial Epochs
Pages 125-168

CHAPTER IX

ANCIENT GLACIAL EPOCHS, AND MILD CLIMATES IN THE
ARCTIC REGIONS
Mr. 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 Palæozoic Epochs—Warm Arctic Climates in Early
Secondary and Palæozoic 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.
Pages 169-209

CHAPTER X

THE EARTH'S AGE, AND THE RATE OF DEVELOPMENT OF
ANIMALS AND PLANTS
Various Estimates of Geological Time—Denudation and Deposition of
Strata as a Measure of Time—How to Estimate the Thickness of the
Sedimentary Rocks—How to Estimate the Average Rate of Deposition of
the Sedimentary Rocks—The Rate of Geological Change Probably Greater
in very Remote Times—Value of the Preceding Estimate of Geological
Time—Organic Modification Dependent on Change of Conditions—
Geographical Mutations as a Motive Power in Bringing about Organic
Changes—Climatal Revolutions as an Agent in Producing Organic Changes
—Present Condition of the Earth One of Exceptional Stability as Regards
Climate—Date of Last Glacial Epoch and its Bearing on the Measurement
of Geological Time—Concluding Remarks
Pages 210-237

PART II

INSULAR FAUNAS AND FLORAS

CHAPTER XI

THE CLASSIFICATION OF ISLANDS
Importance of Islands in the Study of the Distribution of Organisms—
Classification of Islands with Reference to Distribution—Continental
Islands—Oceanic Islands
Pages 241-245

CHAPTER XII

OCEANIC ISLANDS:—THE AZORES AND BERMUDA

The Azores, or Western Islands
Position and Physical Features—Chief Zoological Features of the Azores—
Birds—Origin of the Azorean Bird-fauna—Insects of the Azores—Land-
shells of the Azores—The Flora of the Azores—The Dispersal of Seeds—
Birds as seed-carriers—Facilities for Dispersal of Azorean Plants—
Important Deduction from the Peculiarities of the Azorean Fauna and Flora
Pages 246-262

Bermuda
Position and Physical Features—The Red Clay of Bermuda—Zoology of
Bermuda—Birds of Bermuda—Comparison of the Bird-faunas of Bermuda
and the Azores—Insects of Bermuda—Land Mollusca—Flora of Bermuda
—Concluding Remarks on the Azores and Bermuda
Pages 263-274

CHAPTER XIII

THE GALAPAGOS ISLANDS
Position and Physical Features—Absence of Indigenous Mammalia and
Amphibia—Reptiles—Birds—Insects and Land-shells—The Keeling
Islands as Illustrating the Manner in which Oceanic Islands are Peopled—
Flora of the Galapagos—Origin of the Flora of the Galapagos—Concluding
remarks
Pages 273-291

CHAPTER XIV

ST. HELENA
Position and Physical Features of St. Helena—Change Effected by
European Occupation—The Insects of St. Helena—Coleoptera—
Peculiarities and Origin of the Coleoptera of St. Helena—Land-shells of St.
Helena—Absence of Fresh-water Organisms—Native Vegetation of St.

Helena—The Relations of the St. Helena Compositæ—Concluding
Remarks on St. Helena
Pages 292-309

CHAPTER XV

THE SANDWICH ISLANDS
Position and Physical Features—Zoology of the Sandwich Islands—Birds
—Reptiles—Land-shells—Insects—Vegetation of the Sandwich Islands—
Peculiar Features of the Hawaiian Flora—Antiquity of the Hawaiian Fauna
and Flora—Concluding Observations on the Fauna and Flora of the
Sandwich Islands—General Remarks on Oceanic Islands
Pages 310-330

CHAPTER XVI

CONTINENTAL ISLANDS OF RECENT ORIGIN: GREAT BRITAIN
Characteristic Features of Recent Continental Islands—Recent Physical
Changes of the British Isles—Proofs of Former Elevation—Submerged
Forests—Buried River Channels—Time of Last Union with the Continent
—Why Britain is Poor in Species—Peculiar British Birds—-Fresh-water
Fishes—Cause of Great Speciality in Fishes—Peculiar British Insects—
Lepidoptera Confined to the British Isles—Peculiarities of the Isle of Man
Lepidoptera—Coleoptera Confined to the British Isles—Trichoptera
Peculiar to the British Isles—Land and Fresh-water Shells—Peculiarities of
the British Flora—Peculiarities of the Irish Flora—Peculiar British Mosses
and Hepaticæ—Concluding Remarks on the Peculiarities of the British
Fauna and Flora
Pages 331-372

CHAPTER XVII

BORNEO AND JAVA
Position and Physical Features of Borneo—Zoological Features of Borneo:
Mammalia—Birds—The Affinities of the Borneo Fauna—Java, its Position

and Physical Features—General Character of the Fauna of Java—
Differences Between the Fauna of Java and that of the other Malay Islands
—Special Relations of the Javan Fauna to that of the Asiatic Continent—
Past Geographical Changes of Java and Borneo—The Philippine Islands—
Concluding Remarks on the Malay Islands
Pages 373-390

CHAPTER XVIII

JAPAN AND FORMOSA
Japan, its Position and Physical Features—Zoological Features of Japan—
Mammalia—Birds—Birds Common to Great Britain and Japan—Birds
Peculiar to Japan—Japan Birds Recurring in Distant Areas—Formosa—
Physical Features of Formosa—Animal Life of Formosa—Mammalia—
Land Birds Peculiar to Formosa—Formosan Birds Recurring in India or
Malaya—Comparison of Faunas of Hainan, Formosa, and Japan—General
Remarks on Recent Continental Islands
Pages 391-410

CHAPTER XIX

ANCIENT CONTINENTAL ISLANDS: THE MADAGASCAR GROUP
Remarks on Ancient Continental Islands—Physical Features of Madagascar
—Biological Features of Madagascar—Mammalia—Reptiles—Relation of
Madagascar to Africa—Early History of Africa and Madagascar—
Anomalies of Distribution and how to Explain Them—The Birds of
Madagascar as Indicating a Supposed Lemurian Continent—Submerged
Islands Between Madagascar and India—Concluding Remarks on
"Lemuria"—The Mascarene Islands—The Comoro Islands—The
Seychelles Archipelago—Birds of the Seychelles—Reptiles and Amphibia
—Fresh-water Fishes—Land Shells—Mauritius, Bourbon, and Rodriguez
—Birds—Extinct Birds and their Probable Origin—Reptiles—Flora of
Madagascar and the Mascarene Islands—Curious Relations of Mascarene
Plants—Endemic Genera of Mauritius and Seychelles—Fragmentary
Character of the Mascarene Flora—Flora of Madagascar Allied to that of

South Africa—Preponderance of Ferns in the Mascarene Flora—
Concluding Remarks on the Madagascar Group
Pages 411-449

CHAPTER XX

ANOMALOUS ISLANDS: CELEBES
Anomalous Relations of Celebes—Physical Features of the Island—
Zoological Character of the Islands Around Celebes—The Malayan and
Australian Banks—Zoology of Celebes: Mammalia—Probable Derivation
of the Mammals of Celebes—Birds of Celebes—Bird-types Peculiar to
Celebes—Celebes not Strictly a Continental Island—Peculiarities of the
Insects of Celebes—Himalayan Types of Birds and Butterflies in Celebes—
Peculiarities of Shape and Colour of Celebesian Butterflies—Concluding
Remarks—Appendix on the Birds of Celebes
Pages 450-470

CHAPTER XXI

ANOMALOUS ISLANDS: NEW ZEALAND
Position and Physical Features of New Zealand—Zoological Character of
New Zealand—Mammalia—Wingless Birds Living and Extinct—Recent
Existence of the Moa—Past Changes of New Zealand deduced from its
Wingless Birds—Birds and Reptiles of New Zealand—Conclusions from
the Peculiarities of the New Zealand Fauna
Pages 471-486

CHAPTER XXII

THE FLORA OF NEW ZEALAND: ITS AFFINITIES AND PROBABLE
ORIGIN
Relations of the New Zealand Flora to that of Australia—General Features
of the Australian Flora—The Floras of South-eastern and South-western
Australia—Geological Explanation of the Differences of these Two Floras
—The Origin of the Australian Element in the New Zealand Flora—

Tropical Character of the New Zealand Flora Explained—Species Common
to New Zealand and Australia mostly Temperate Forms—Why Easily
Dispersed Plants have often Restricted Ranges—Summary and Conclusion
on the New Zealand Flora
Pages 487-508

CHAPTER XXIII

ON THE ARCTIC ELEMENT IN SOUTH TEMPERATE FLORAS
European Species and Genera of Plants in the Southern Hemisphere—
Aggressive Power of the Scandinavian Flora—Means by which Plants have
Migrated from North to South—Newly Moved Soil as Affording Temporary
Stations to Migrating Plants—Elevation and Depression of the Snow-line as
Aiding the Migration of Plants—Changes of Climate Favourable to
Migration—The Migration from North to South has been Long going on—
Geological Changes as Aiding Migration—Proofs of Migration by way of
the Andes—Proofs of Migration by way of the Himalayas and Southern
Asia—Proofs of Migration by way of the African Highlands—Supposed
Connection of South Africa and Australia—The Endemic Genera of Plants
in New Zealand—The Absence of Southern Types from the Northern
Hemisphere—Concluding Remarks on the New Zealand and South
Temperate Floras
Pages 509-530

CHAPTER XXIV

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

MAPS AND ILLUSTRATIONS
PAGE
1. Map showing the Distribution of the true Frontispiece.
Jays
2. Map showing the Zoological Regions To face 31
3. Map showing the Distribution of Parus
To face 66
Palustris
4. A Glacier with Moraines (From Sir C.
109
Lyell's Principles of Geology)
5. Map of the Ancient Rhone Glacier (From
110
Sir C. Lyell's Antiquity of Man)
6. Diagram showing the effects of
127
Excentricity and Precession on Climate
7. Diagram of Excentricity and Precession 129
8. Map showing the Extent of the North and
138
South Polar Ice
9. Diagram showing Changes of Excentricity
171
during Three Million Years
10. Outline Map of the Azores 248
11. Map of Bermuda and the American Coast 263
12. Section of Bermuda and adjacent Sea-
264
bottom
13. Map of the Galapagos and adjacent Coasts
276
of South America
14. Map of the Galapagos 277
15. Map of the South Atlantic, showing
293
position of St. Helena
16. Map of the Sandwich Islands 311
17. Map of the North Pacific, with its
312
submerged Banks
18. Map showing the Bank connecting Britain
333
with the Continent

19. Map of Borneo and Java, showing the Great
373
Submarine Bank of South-Eastern Asia
20. Map of Japan and Formosa 392
21. Physical Sketch Map of Madagascar (From
413
Nature)
22. Map of Madagascar Group, showing Depths
415
of Sea
23. Map of the Indian Ocean 424
24. Map of Celebes and the surrounding
451
Islands
25. Map showing Depths of Sea around
471
Australia and New Zealand
26. Map showing the probable condition of
496
Australia during the Cretaceous Epoch

ISLAND LIFE
PART I

THE DISPERSAL OF ORGANISMS
ITS PHENOMENA, LAWS, AND CAUSES

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, 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 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 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 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 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 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;
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 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.

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 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. 55°, 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 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 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.
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 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
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 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
Dendrœca, 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. cœrulea 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 64° 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 61° 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 western portion of its area. Our little
favourite, the blue tit (Parus cœruleus) 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. cœruleus in Western
Europe as far as St. Petersburg and Austria, rarely straggling to Denmark,

while it stretches all across Central Asia between the latitudes 35° and 56°
N. as far as the Amoor valley. Besides these wide-ranging species there are
several others which are more restricted. Parus teneriffæ, 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 64° 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 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 Palæarctic[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 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 Abbé 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 Abbé 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 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 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 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.
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 9° 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. 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 Centetidæ, 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, Octodontidæ and
Echimyidæ, 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 Tragulidæ 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 Simiidæ 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 Echimyidæ 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 Megalæmidæ or barbets are gaily
coloured 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 Dendrophidæ or tree-
snakes, and the Dryiophidæ 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 Amphisbænidæ 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 Cæciliadæ are tropicopolitan, as are also the
toads of the family Engystomatidæ. Insects also furnish some analogous
cases, three genera of Cicindelidæ, (Pogonostoma, Ctenostoma, and
Peridexia) showing a decided connection between this family in South
America and Madagascar; while the beautiful family of diurnal moths,
Uraniidæ, 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 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
(Sylviadæ) 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 Cebidæ 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 Bovidæ, 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
(Chrysochloridæ) consisting of two genera and three species, confined to
extratropical South Africa; the hill-tits (Liotrichidæ), 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
Pteroptochidæ, large wren-like birds, consisting of eight genera and
nineteen species, almost entirely confined to temperate South America and
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 (Uropeltidæ),
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.

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 Palæarctic Region—Characteristic features of the Palæarctic
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
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 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 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 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. ,, œnanthe Wheatear (also N. America).
10. Acrocephalus
Great Reed-Warbler.
arundinaceus
11. Sylvia curruca Lesser Whitethroat.
12. Parus major Great Titmouse.
13. Motacilla
Grey Wagtail (also China and Malaya).
sulphurea
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.
20. Emberiza
Reed Bunting.
schœniclus
21. ,, citrinella Yellow-hammer.
22. Fringilla
Brambling.
montifringilla
23. Passer montanus Tree Sparrow (also S. Asia).
24. ,, domesticus House Sparrow.
25. Coccothraustes
Hawfinch.
vulgaris
26. Carduelis spinus Siskin (also China).
27. Loxia curvirostra Crossbill.
28. Sturnus vulgaris Starling.
29. Pyrrhocorax
Chough.
graculus
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
Grasshopper Warbler.
nævius
5. Phylloscopus
Yellow-browed Warbler.
superciliosus
6. Certhia familiaris Tree-creeper.
7. Parus cœruleus 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
Snow-Bunting (also N. America).
nivalis
15. ,,
Lapland Bunting.
lapponicus
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
Nutcracker.
caryocatactes

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. ,,
Pied Flycatcher (also Central Africa).
atricapilla
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.
11. Ruticilla
Redstart.
phænicurus
12. ,, tithys Black Redstart.
13. Saxicola rubetra Whinchat.
14. Aëdon galactodes Rufous Warbler.
15. Acrocephalus
Reed Warbler.
streperus
16. ,,
Sedge Warbler.
schænobenus
17. Melizophilus
Dartford Warbler.
undatus
18. Sylvia rufa Greater Whitethroat.
19. ,, salicaria Garden Warbler.
20. ,, atricapilla Blackcap.
21. ,, orphea Orphean Warbler.
22. Phylloscopus
Wood Wren.
sibilatrix
23. ,,
Willow Wren.
trochilus
24. ,,
Chiffchaff.
collybita
25. Regulus cristatus Golden-crested Wren.
26. ,, ignicapillus Fire-crested Wren.

27. Troglodytes
Wren.
parvulus
28. Sitta cæsia 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
Short-toed Lark.
brachydactyla
34. Emberiza miliaria Common Bunting.
35. ,, cirlus Cirl Bunting.
36. ,, hortulana Ortolan.
37. Fringilla cœlebs Chaffinch.
38. Coccothraustes
Greenfinch.
chloris
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
Aquatic Warbler.
aquaticus
3. ,,
Savi's Warbler.
luscinioides
4. Motacilla lugubris Pied Wagtail.
5. Pyrrhula europæa 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
White-winged Lark.
sibirica
5. Euspiza
Black-headed Bunting.
melanocephala
6. Linota flavirostris Twite.
7. Corvus frugilegus Rook.

VI. British Birds confined to Europe.
Dipper (closely allied races inhabit other
1. Cinclus aquaticus
parts of the Palæarctic Region).
2. Parus cristatus Crested Titmouse.
3. Anthus obscurus Rock Pipit.
Lesser Redpoll (closely allied races in N.
4. Linota rufescens
Asia and N. America).
Parrot Crossbill (a closely allied form in
5. Loxia pityopsittacus
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 Picariæ (the perching and climbing birds), and of these no less
than 77, or more than half, are absolutely identical 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
"Palæarctic" by Mr. Sclater, meaning the "northern old-world" region—a
name now well known to naturalists.
The Limits of the Palæarctic 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 Palæarctic 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 Palæarctic region, and Baluchistan should probably go with
these. The boundary in the north-western part of India is again difficult to
determine, but it 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 Palæarctic 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 China and East Thibet in about 32° N. latitude; where, at
Moupin, a French missionary, Père David, made extensive collections
showing this district to be at the junction of the tropical and temperate
faunas. Japan, as a whole, is decidedly Palæarctic, 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 Palæarctic Region.—Having thus
demonstrated the unity of the Palæarctic 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 Talpidæ or moles, the family being confined to the
Palæarctic 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 Bovidæ, 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 most
important. The true pheasants (Phasianus) are wholly Palæarctic, 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
Palæarctic 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 Carabidæ,
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.
Having given so full an exposition of the facts which determine the extent
and boundaries of the Palæarctic 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 Palæarctic region.
Taking first the mammalia, we find the following remarkable animals at
once separating it from the Palæarctic and every other region. The gorilla
and chimpanzee, the baboons, numerous lemurs, the spotted hyæna, the
aard-wolf and hyæna-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 Palæarctic 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, Potamogalidæ; the
goldenmoles, also forming a peculiar family, Chrysochloridæ; as do the
elephant-shrews, Macroscelididæ; the singular aard-varks, or earth-pigs,
forming a peculiar family of Edentata called Orycteropodidæ; 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 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 (Balæniceps), and the true ostriches. There are also three quite
peculiar African families, the Musophagidæ or plantain-eaters, including
the elegant crested touracos; the curious little finch-like colies (Coliidæ),
and the Irrisoridæ, 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 Cetoniidæ; and some of the Achatinæ, 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 Palæarctic 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
(Timaliidæ), its beautiful little hill-tits (Liotrichidæ), its green bulbuls
(Phyllornithidæ), its many varieties of the crow-family, its beautiful gapers
and pittas adorned with the most delicate colours, its great variety of
hornbills, and its magnificent Phasianidæ, 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 æneus 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 magnificent golden and green Papilionidæ 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 Carabidæ, while the
catoxantha is equally a giant among the Buprestidæ. 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 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 (Meliphagidæ), 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 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 Palæarctic 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 Palæarctic.
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
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
(Geomyidæ), 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 Palæarctic 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 latter are really quite distinct, and
that there is not a single true flycatcher of the family Muscicapidæ, or a
single starling of the family Sturnidæ in the whole continent, while there are
very few true warblers (Sylviidæ), their place being taken by the quite
distinct families Mniotiltidæ or wood-warblers, and Vireonidæ or greenlets.
In like manner the flycatchers of America belong to the totally distinct
family of tyrant-birds, Tyrannidæ, and those that look like starlings to the
hang-nests, Icteridæ; 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
Iguanidæ. Nowhere in the world are the tailed batrachians so largely

developed as in this region, the Sirens and the Amphiumidæ 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 Palæarctic. 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
Palæarctic, Ethiopian, and Oriental regions. If, however, we compare the
number of species, which are common to the Nearctic and Palæarctic
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 Cœrebidæ; 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 Dendrocolaptidæ; 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 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 Viverridæ 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 Picariæ (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, 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 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 Palæarctic 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.
Palæarctic 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.
South America, with tropical N. America and W.
Neotropical
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 æ a r c t i c
| | |
| | Oriental
| Ethiopian |
Neo- |
Tropical Australian

CHAPTER IV

EVOLUTION THE KEY TO DISTRIBUTION

Importance of the Doctrine of Evolution—The Origin of New Species
—Variation in Animals—The Amount of Variation in North American
Birds—How New Species arise from a Variable Species—Definition
and Origin of Genera—Cause of the Extinction of Species—The Rise
and Decay of Species and Genera—Discontinuous Specific Areas, why
Rare—Discontinuity of the Area of Parus Palustris—Discontinuity of
Emberiza Schœniclus—The European and Japanese Jays—Supposed
Examples of Discontinuity among North American Birds—
Distribution and Antiquity of Families—Discontinuity a proof of
Antiquity—Concluding Remarks.

In the preceding chapters we have explained the general nature of the
phenomena presented by the distribution of animals, and have illustrated
and defined the new geographical division of the earth which is found best
to agree with them. Before we go further into the details of our subject, and
especially before we attempt to trace the causes which have brought about

the existing biological relations of the islands of the globe, it is absolutely
necessary to have a clear comprehension of the collateral facts and general
principles to which we shall most frequently have occasion to refer. These
may be briefly defined as, the powers of dispersal of animals and plants
under different conditions, such as geological and climatal changes, and the
origin and development of species and groups by natural selection. This last
is of the most fundamental importance, and its bearing on the dispersal of
animals has been much neglected. We therefore devote the present chapter
to its consideration.
As we have already shown in our first chapter that the distribution of
species, of genera, and of families, present almost exactly the same general
phenomena in varying degrees of complexity, and that almost all the
interesting problems we have to deal with depend upon the mode of
dispersal of one or other of these; and as, further, our knowledge of most of
these groups, in the higher animals at least, is confined to the tertiary period
of geology, it is therefore unnecessary for us to enter into any questions
involving the origin of more comprehensive groups, such as classes or
orders. This enables us to avoid most of the disputed questions as to the
development of animals, and to confine ourselves to those general
principles regulating the origin and development of species and genera
which were first laid down by Mr. Darwin thirty years ago, and have now
come to be adopted by naturalists as established propositions in the theory
of evolution.
The Origin of New Species.—How, then, do new species arise, supposing
the world to have been, physically, much as we now see it; and what
becomes of them after they have arisen? In the first place we must
remember that new species can only be formed when and where there is
room for them. If a continent is fully stocked with animals, each species
being so well adapted for its mode of life that it can overcome all the
dangers to which it is exposed, and maintain on the average a tolerably
uniform population, then, so long as no change takes place, no new species
will arise. For every place or station is supposed to be filled by creatures in
all respects adapted to surrounding conditions, able to defend themselves
from all enemies, and to obtain food notwithstanding the rivalry of many
competitors. But such a perfect balance of organisms nowhere exists upon
the earth, and probably never has existed. The well-known fact that some

species are very common, while others are very rare, is an almost certain
proof that the one is better adapted to its position than the other; and this
belief is strengthened when we find the individuals of one species ranging
into different climates, subsisting on different food, and competing with
different sets of animals, while the individuals of another species will be
limited to a small area beyond which they seem unable to extend. When a
change occurs, either of climate or geography, some of the small and ill-
adapted species will probably die out altogether, and thus leave room for
others to increase, or for new forms to occupy their places.
But the change will most likely affect even flourishing species in different
ways, some beneficially, others injuriously. Or, again, it may affect a great
many injuriously, to such an extent as to require some change in their
structure or habits to enable them to get on as well as before. Now
"variation" and the "struggle for existence" come into play. All the weaker
and less perfectly organised individuals die out, while those which vary in
such a way as to bring them into more harmony with the new conditions
constantly survive. If the change of conditions has been considerable, then,
after a few centuries, or perhaps even a few generations, one or more new
species will be almost sure to be formed.
Variation in Animals.—To make this more intelligible to those who have
not considered the subject, and to obviate the difficulty many feel about
"favourable variations occurring at the right time," it will be well to discuss
this matter a little more fully. Few persons consider how largely and
universally all animals are varying. We know, however, that in every
generation, if we could examine all the individuals of any common species,
we should find considerable differences, not only in size and colour, but in
the form and proportions of all the parts and organs of the body. In our
domesticated animals we know this to be the case, and it is by means of the
continual selection of such slight varieties to breed from that all our
extremely different domestic breeds have been produced. Think of the
difference in every limb, and every bone and muscle, and probably in every
part, internal and external of the whole body, between a greyhound and a
bull-dog! Yet, if we had the complete series of ancestors of these two breeds
before us, we should probably find that in no one generation was there a
greater difference than now occurs in the same breed, or sometimes even
the same litter. It is often thought, however, that wild species do not vary

sufficiently to bring about any such change as this in the same time; and
though naturalists are well aware that this is a mistake, it is only recently
that they have been able to adduce positive proof of their opinion.
The Amount of Variation in North American Birds.—An American
naturalist, Mr. J. A. Allen, has made elaborate observations and
measurements of the birds of the United States, and he finds a wonderful
and altogether unsuspected amount of variation between individuals of the
same species. They differ in the general tint, and in the markings and
distribution of the colours; in size and proportions; in the length of the
wings, tail, bill, and feet; in the length of particular feathers, altering the
shape of the wing or tail; in the length of the tarsi and of the separate toes,
and in the length, width, thickness, and curvature of the bill. These
variations are very considerable, often reaching to one-sixth or one-seventh
of the average dimensions, and sometimes more. Thus Turdus fuscescens
(Wilson's thrush) varied in length of wing from 3.58 to 4.16 inches, and in
the tail from 3.55 to 4.00 inches; and in twelve specimens, all taken in the
same locality, the wing varied in length from 14.5 to 21 per cent., and the
tail from 14 to 22.5 per cent. In Sialia sialis (the blue bird) the middle toe
varied from .77 to .91 inch, and the hind toe from .58 to .72 inch, or more
than 21.5 per cent. on the mean, while the bill varied from .45 to .56 inch in
length, and from .30 to .38 inch in width, or about 20 per cent. in both
cases. In Dendrœca coronata (the yellow-crowned warbler) the quills vary
in proportionate length, so that the 1st, the 2nd, the 3rd, or the 4th, is
sometimes longest; and a similar variation of the wing involving a change
of proportion between two or more of the feathers is recorded in eleven
species of birds. Colour and marking vary to an equal extent; the dark
streaks on the under surface of Melospiza melodia (the American song-
sparrow) being sometimes reduced to narrow lines. while in other
specimens they are so enlarged as to cover the greater part of the breast and
sides of the body, sometimes uniting on the middle of the breast into a
nearly continuous patch. In one of the small spotted wood-thrushes, Turdus
fuscescens, the colours are sometimes very pale, and the markings on the
breast reduced to indistinct narrow lines, while in other specimens the
general colour is much darker, and the breast markings dark, broad, and
triangular. All the variations here mentioned occur between adult males, so

that there is no question of differences of age or sex, and the pair last
referred to were taken at the same place and on the same day.[9]
These interesting facts entirely support the belief in the variability of all
animals in all their parts and organs, to an extent amply sufficient for
natural selection to work with. We may, indeed, admit that these are
extreme cases, and that the majority of species do not vary half or a quarter
so much as shown in the examples quoted, and we shall still have ample
variation for all purposes of specific modification. Instead of an extreme
variation in the dimensions and proportions of the various organs of from
10 to 25 per cent. as is here proved to occur, we may assume from 3 to 6 per
cent. as generally occurring in the majority of species; and if we further
remember that the above excessive variations were found by comparing a
number of specimens of each species, varying from 50 to 150 only, we may
be sure that the smaller variations we require must occur in considerable
numbers among the thousands or millions of individuals of which all but
the very rare species consist. If, therefore, we were to divide the population
of any species into three groups of equal extent, with regard to any
particular character—as length of wing, or of toes, or thickness or curvature
of bill, or strength of markings—we should have one group in which the
mean or average character prevailed with little variation, one in which the
character was greatly, and one in which it was little, developed. If we
formed our groups, not by equal numbers, but by equal amount of variation,
we should probably find, in accordance with the law of averages, that the
central group in which the mean characteristics prevailed was much more
numerous than the extremes, perhaps twice, or even three times, as great as
either of them, and forming such a series as the following—10 maximum,
30 mean, 10 minimum development. In ordinary cases we have no reason to
believe that the mean characters or the amount of variation of a species
changes materially from year to year or from century to century, and we
may therefore look upon the central group as the type of the species which
is best adapted to the conditions in which it has actually to exist. This type
will therefore always form the majority, because the struggle for existence
will lead to the continual suppression of the less perfectly adapted extremes.
But sometimes a species has a wide range into countries which differ in
physical conditions, and then it often happens that one or other of the
extremes will predominate in a portion of its range. These form local

varieties, but as they occur mixed with the other forms, they are not
considered to be distinct species, although they may differ from the other
extreme form quite as much as species often do from each other.[10]
How New Species arise from a Variable Species.—It is now very easy to
understand how, from such a variable species, one or more new species may
arise. The peculiar physical or organic conditions that render one part of the
area better adapted to an extreme form may become intensified, and the
most extreme variations thus having the advantage, they will multiply at the
expense of the rest. If the change of conditions spreads over the whole area
occupied by the species, this one extreme form will replace the others;
while if the area should be cut in two by subsidence or elevation, the
conditions of the two parts may be modified in opposite directions, so as to
be each adapted to one extreme form; in which case the original type will
become extinct, being replaced by two species, each formed by a
combination of certain extreme characters which had before existed in some
of its varieties.
The changes of conditions which lead to such selection of varieties are very
diverse in nature, and new species may thus be formed, diverging in many
ways from the original stock. The climate may change from moist to dry, or
the reverse, or the temperature may increase or diminish for long periods, in
either case requiring a corresponding change of constitution, of covering, of
vegetable or of insect food, to be met by the selection of variations of
colour or of swiftness, of length of bill or of strength of claws. Again,
competitors or enemies may arrive from other regions, giving the advantage
to such varieties as can change their food, or by swifter flight or greater
wariness can escape their new foes. We may thus easily understand how a
series of changes may occur at distant intervals, each leading to the
selection and preservation of a special set of variations, and thus what was a
single species may become transformed into a group of allied species
differing from each other in a variety of ways, just as we find them in
nature.
Among these species, however, there will be some which will have become
adapted to very local or special conditions, and will therefore be
comparatively few in number and confined to a limited area; while others,
retaining the more general characters of the parent form, but with some

important change of structure, will be better adapted to succeed in the
struggle for existence with other animals, will spread over a wider area, and
increase so as to become common species. Sometimes these will acquire
such a perfection of organisation by successive favourable modifications
that they will be able to spread greatly beyond the range of the parent form.
They then become what are termed dominant species, maintaining
themselves in vigour and abundance over very wide areas, displacing other
species with which they come into competition, and, under still further
changes of conditions, becoming the parents of a new set of diverging
species.
Definition and Origin of Genera.—As some of the most important and
interesting phenomena of distribution relate to genera rather than to single
species, it will be well here to explain what is meant by a genus, and how
genera are supposed to arise.
A genus is a group of allied species which differs from all other groups in
some well marked characters, usually of a structural rather than a
superficial nature. Species of one genus usually differ from each other in
size, in colour or marking, in the proportions of the limbs or other organs,
and in the form and size of such superficial appendages as horns, crests,
manes, &c.; but they generally agree in the form and structure of important
organs, as the teeth, the bill, the feet, and the wings. When two groups of
species differ from each other constantly in one or more of these latter
particulars they are said to belong to different genera. We have already seen
that species vary in these more important as well as in the more superficial
characters. If, then, in any part of the area occupied by a species some
change of habits becomes useful to it, all such structural variations as
facilitate the change will be accumulated by natural selection, and when
they have become fixed in the proportions most beneficial to the animal, we
shall have the first species of a new genus.
A creature which has been thus modified in important characters will be a
new type, specially adapted to fill a new place in the economy of nature. It
will almost certainly have arisen from an extensive or dominant species,
because only such are sufficiently rich in individuals to afford an ample
supply of the necessary variations, and it will inherit the vigour of
constitution and adaptability to a wide range of conditions which gave

success to its ancestors. It will therefore have every chance in its favour in
the struggle for existence; it may spread widely and displace many of its
nearest allies, and in doing so will itself become modified superficially and
become the parent of a number of subordinate species. It will now have
become a dominant genus, occupying an entire continent, or perhaps even
two or more continents, spreading in every direction till it comes in contact
with competing forms better adapted to the different environments. Such a
genus may continue to exist during long geological epochs; but the time
will generally come when either physical changes, or competing forms, or
new enemies are too much for it, and it begins to lose its supremacy. First
one then another of its component species will dwindle away and become
extinct, till at last only a few species remain. Sometimes these soon follow
the others and the whole genus dies out, as thousands of genera have died
out during the long course of the earth's life-history; but it will also
sometimes happen that a few species will continue to maintain themselves
in areas where they are removed from the influences that exterminated their
fellows.
Cause of the Extinction of Species.—There is good reason to believe that
the most effective agent in the extinction of species is the pressure of other
species, whether as enemies or merely as competitors. If therefore any
portion of the earth is cut off from the influx of new or more highly
organised animals, we may there expect to find the remains of groups which
have elsewhere become extinct. In islands which have been long separated
from their parent continents these conditions are exactly fulfilled, and it is
in such places that we find the most striking examples of the preservation of
fragments of primeval groups of animals, often widely separated from each
other, owing to their having been preserved at remote portions of the area of
the once widespread parental group. There are many other ways in which
portions of dying out groups may be saved. Nocturnal or subterranean
modes of life may save a species from enemies or competitors, and many of
the ancient types still existing have such habits. The dense gloom of
equatorial forests also affords means of concealment and protection, and we
sometimes find in such localities a few remnants of low types in the midst
of a general assemblage of higher forms. Some of the most ancient types
now living inhabit caves like the Proteus, or bury themselves in mud like
the Lepidosiren, or in sand like the Amphioxus, the last being the most

primitive of all vertebrates; while the Galeopithecus and Tarsius of the
Malay islands and the potto of West Africa, survive amid the higher
mammalia of the Asiatic and African continents owing to their nocturnal
habits and concealment in the densest forests.
The Rise and Decay of Species and Genera.—The preceding sketch of the
mode in which species and genera have arisen, have come to maturity, and
then decay, leads us to some very important conclusions as to the mode of
distribution of animals. When a species or a genus is increasing and
spreading, it necessarily occupies a continuous area which gets larger and
larger till it reaches a maximum; and we accordingly find that almost all
extensive groups are thus continuous. When decay commences, and the
group, ceasing to be in harmony with its environment, is encroached upon
by other forms, the continuity may frequently be broken. Sometimes the
outlying species may be the first to become extinct, and the group may
simply diminish in area while keeping a compact central mass; but more
often the process of extinction will be very irregular, and may even divide
the group into two or more disconnected portions. This is the more likely to
be the case because the most recently formed species, probably adapted to
local conditions and therefore most removed from the general type of the
group, will have the best chance of surviving, and these may exist at several
isolated points of the area once occupied by the whole group. We may thus
understand how the phenomenon of discontinuous areas has come about,
and we may be sure that when allied species or varieties of the same species
are found widely separated from each other, they were once connected by
intervening forms or by each extending till it overlapped the other's area.
Discontinuous Specific Areas, why Rare.—But although discontinuous
generic areas, or the separation from each other of species whose ancestors
must once have occupied conterminous or overlapping areas, is of frequent
occurrence, yet undoubted cases of discontinuous specific areas are very
rare, except, as already stated, when one portion of a species inhabits an
island. A few examples among mammalia have been referred to in our first
chapter, but it may be said that these are examples of the very common
phenomenon of a species being only found in the station for which its
organisation adapts it; so that forest or marsh or mountain animals are of
course only found where there are forests, marshes, or mountains. This may
be true, and when the separate forests or mountains inhabited by the same

species are not far apart there is little that needs explanation; but in one of
the cases referred to there was a gap of a thousand miles between two of the
areas occupied by the species, and this being too far for the animal to
traverse through an uncongenial territory, we are forced to the conclusion
that it must at some former period and under different conditions have
occupied a considerable portion of the intervening area.
Among birds such cases of specific discontinuity are very rare and hardly
ever quite satisfactory. This may be owing to birds being more rapidly
influenced by changed conditions, so that when a species is divided the two
portions almost always become modified into varieties or distinct species;
while another reason may be that their powers of flight cause them to
occupy on the average wider and less precisely defined areas than do the
species of mammalia. It will be interesting therefore to examine the few
cases on record, as we shall thereby obtain additional knowledge of the
steps and processes by which the distribution of varieties and species has
been brought about.

Discontinuity of the Area of Parus palustris.—Mr. Seebohm, who has
travelled and collected in Europe, Siberia, and India, and possesses
extensive and accurate knowledge of Palæarctic birds, has recently called
attention to the varieties and sub-species of the marsh tit (Parus palustris),
of which he has examined numerous specimens ranging from England to
Japan.[11] The curious point is that those of Southern Europe and of China
are exactly alike, while all over Siberia a very distinct form occurs, forming
the sub-species P. borealis.[12] In Japan and Kamschatka other varieties are
found, which have been named respectively P. japonicus and P.
camschatkensis and another P. songarus in Turkestan and Mongolia. Now it
all depends upon these forms being classed as sub-species or as true species
whether this is or is not a case of discontinuous specific distribution. If
Parus borealis is a distinct species from Parus palustris, as it is reckoned in
Gray's Hand List of Birds, and also in Sharpe and Dresser's Birds of Europe,
then Parus palustris has a most remarkable discontinuous distribution, as
shown in the accompanying map, one portion of its area comprising Central
and South Europe and Asia Minor, the other an undefined tract in Northern
China, the two portions being thus situated in about the same latitude and
having a very similar climate, but with a distance of about 4,000 miles
between them. If, however, these two forms are reckoned as sub-species
only, then the area of the species becomes continuous, while only one of its

varieties or sub-species has a discontinuous area. It is a curious fact that P.
palustris and P. borealis are found together in Southern Scandinavia and in
some parts of Central Europe, and are said to differ somewhat in their note
and their habits, as well as in colouration.
Discontinuity of Emberiza schœniclus.—The other case is that of our reed
bunting (Emberiza schœniclus), which ranges over almost all Europe and
Western Asia as far as the Yenesai valley and North-west India. It is then
replaced by another smaller species, E. passerina, which ranges eastwards
to the Lena river, and in winter as far south as Amoy in China; but in Japan
the original species appears again, receiving a new name (E. pyrrhulina),
but Mr. Seebohm assures us that it is quite indistinguishable from the
European bird. Although the distance between these two portions of the
species is not so great as in the last example, being about 2,000 miles, in
other respects the case is an interesting one, because the forms which
occupy the intervening space are recognised by Mr. Seebohm himself as
undoubted species.[13]
The European and Japanese Jays.—Another case somewhat resembling
that of the marsh tit is afforded by the European and Japanese jays
(Garrulus glandarius and G. japonicus). Our common jay inhabits the
whole of Europe except the extreme north, but is not known to extend
anywhere into Asia, where it is represented by several quite distinct species.
(See Map, Frontispiece.) But the great central island of Japan is inhabited
by a jay (G. japonicus) which is very like ours, and was formerly classed as
a sub-species only, in which case our jay would be considered to have a
discontinuous distribution. But the specific distinctness of the Japanese bird
is now universally admitted, and it is certainly a very remarkable fact that
among the twelve species of jays which together range over all temperate
Europe and Asia, one which is so closely allied to our English bird should
be found at the remotest possible point from it. Looking at the map
exhibiting the distribution of the several species, we can hardly avoid the
conclusion that a bird very like our jay once occupied the whole area of the
genus, that in various parts of Asia it became gradually modified into a
variety of distinct species in the manner already explained, a remnant of the
original type being preserved almost unchanged in Japan, owing probably
to favourable conditions of climate and protection from competing forms.

Supposed Examples of Discontinuity among North American Birds.—In
North America, the eastern and western provinces are so different in climate
and vegetation, and are besides separated by such remarkable physical
barriers—the arid central plains and the vast ranges of the Rocky Mountains
and Sierra Nevada, that we can hardly expect to find species whose areas
may be divided maintaining their identity. Towards the north however the
above-named barriers disappear, the forests being almost continuous from
east to west, while the mountain range is broken up by passes and valleys. It
thus happens that most species of birds which inhabit both the eastern and
western coasts of the North American continent have maintained their
continuity towards the north, while even when differentiated into two or
more allied species their areas are often conterminous or overlapping.
Almost the only bird that seems to have a really discontinuous range is the
species of wren, Thryothorus bewickii, of which the type form ranges from
the east coast to Kansas and Minnesota, while a longer-billed variety, T.
bewickii spilurus, is found in the wooded parts of California and as far
north as Puget Sound. If this really represents the range of the species there
remains a gap of about 1,000 miles between its two disconnected areas.
Other cases are those of Vireo bellii of the middle United States and the
sub-species pusillus of California; and of the purple red-finch, Carpodacus
purpureus, with its variety C. californicus; but unfortunately the exact
limits of these varieties are in neither case known, and though each one is
characteristic of its own province, it is possible that they may somewhere
become conterminous, though in the case of the red-finches this does not
seem likely to be the fact.
In a later chapter we shall have to point out some remarkable cases of this
kind where one portion of the species inhabits an island; but the facts now
given are sufficient to prove that the discontinuity of the area occupied by a
single homogeneous species, by two varieties of a species, by two well-
marked sub-species, and by two closely allied but distinct species, are all
different phases of one phenomenon—the decay of ill-adapted, and their
replacement by better-adapted forms, under the pressure of a change of
conditions either physical or organic. We may now proceed with our sketch
of the mode of distribution of higher groups.

Distribution and Antiquity of Families.—Just as genera are groups of allied
species distinguished from all other groups by some well-marked structural
characters, so families are groups of allied genera distinguished by more
marked and more important characters, which are generally accompanied
by a peculiar outward form and style of colouration, and by distinctive
habits and mode of life. As a genus is usually more ancient than any of the
species of which it is composed, because during its growth and
development the original rudimentary species becomes supplanted by more
and more perfectly adapted forms, so a family is usually older than its
component genera, and during the long period of its life-history may have
survived many and great terrestrial and organic changes. Many families of
the higher animals have now an almost worldwide extension, or at least
range over several continents; and it seems probable that all families which
have survived long enough to develop a considerable variety of generic and
specific forms have also at one time or other occupied an extensive area.
Discontinuity a Proof of Antiquity.—Discontinuity will therefore be an
indication of antiquity, and the more widely the fragments are scattered the
more ancient we may usually presume the parent group to be. A striking
example is furnished by the strange reptilian fishes forming the order or
sub-order Dipnoi, which includes the Lepidosiren and its allies. Only three
or four living species are known, and these inhabit tropical rivers situated in
the remotest continents. The Lepidosiren paradoxa is only known from the
Amazon and some other South American rivers. An allied species,
Lepidosiren annectens, sometimes placed in a distinct genus, inhabits the
Gambia in West Africa, while the recent discovery in Eastern Australia of
the Ceratodus or mud-fish of Queensland, adds another form to the same
isolated group. Numerous fossil teeth, long known from the Triassic beds of
this country, and also found in Germany and India in beds of the same age,
agree so closely with those of the living Ceratodus that both are referred to
the same genus. No more recent traces of any such animal have been
discovered, but the Carboniferous Ctenodus and the Devonian Dipterus
evidently belong to the same group, while in North America the Devonian
rocks have yielded a gigantic allied form which has been named Heliodus
by Professor Newberry. Thus an enormous range in time is accompanied by
a very wide and scattered distribution of the existing species.

Whenever, therefore, we find two or more living genera belonging to the
same family or order but not very closely allied to each other, we may be
sure that they are the remnants of a once extensive group of genera; and if
we find them now isolated in remote parts of the globe, the natural
inference is that the family of which they are fragments once had an area
embracing the countries in which they are found. Yet this simple and very
obvious explanation has rarely been adopted by naturalists, who have
instead imagined changes of land and sea to afford a direct passage from the
one fragment to the other. If there were no cosmopolitan or very wide-
spread families still existing, or even if such cases were rare, there would be
some justification for such a proceeding; but as about one-fourth of the
existing families of land mammalia have a range extending to at least three
or four continents, while many which are now represented by disconnected
genera are known to have occupied intervening lands or to have had an
almost continuous distribution in tertiary times, all the presumptions are in
favour of the former continuity of the group. We have also in many cases
direct evidence that this former continuity was effected by means of
existing continents, while in no single case has it been shown that such a
continuity was impossible, and that it either was or must have been effected
by means of continents now sunk beneath the ocean.
Concluding Remarks.—When writing on the subject of distribution it
usually seems to have been forgotten that the theory of evolution absolutely
necessitates the former existence of a whole series of extinct genera filling
up the gap between the isolated genera which in many cases now alone
exist; while it is almost an axiom of "natural selection" that such numerous
forms of one type could only have been developed in a wide area and under
varied conditions, implying a great lapse of time. In our succeeding
chapters we shall show that the known and probable changes of sea and
land, the known changes of climate, and the actual powers of dispersal of
the different groups of animals, were such as would have enabled all the
now disconnected groups to have once formed parts of a continuous series.
Proofs of such former continuity are continually being obtained by the
discovery of allied extinct forms in intervening lands, but the extreme
imperfection of the geological record as regards land animals renders it
unlikely that this proof will be forthcoming in the majority of cases. The
notion that if such animals ever existed their remains would certainly be

found, is a superstition which, notwithstanding the efforts of Lyell and
Darwin, still largely prevails among naturalists; but until it is got rid of no
true notions of the former distribution of life upon the earth can be attained.

CHAPTER V

THE POWERS OF DISPERSAL OF ANIMALS AND PLANTS

Statement of the general question of Dispersal—The Ocean as a
Barrier to the Dispersal of Mammals—The Dispersal of Birds—The
Dispersal of Reptiles—The Dispersal of Insects—The Dispersal of
Land Mollusca—Great Antiquity of Land-shells—Causes favouring
the Abundance of Land-shells—The Dispersal of Plants—Special
adaptability of Seeds for Dispersal—Birds as agents in the Dispersal of
Seeds—Ocean Currents as agents in Plant Dispersal—Dispersal along
Mountain-chains—Antiquity of Plants as affecting their Distribution.

In order to understand the many curious anomalies we meet with in
studying the distribution of animals and plants, and to be able to explain
how it is that some species and genera have been able to spread widely over
the globe, while others are confined to one hemisphere, to one continent, or
even to a single mountain or a single island, we must make some inquiry
into the different powers of dispersal of animals and plants, into the nature
of the barriers that limit their migrations, and into the character of the
geological or climatal changes which have favoured or checked such
migrations.
The first portion of the subject—that which relates to the various modes by
which organisms can pass over wide areas of sea and land—has been fully
treated by Sir Charles Lyell, by Mr. Darwin, and many other writers, and it
will only be necessary here to give a very brief notice of the best known
facts on the subject, which will be further referred to when we come to
discuss the particular cases that arise in regard to the faunas and floras of
remote islands. But the other side of the question of dispersal—that which
depends on geological and climatal changes—is in a far less satisfactory

condition, for, though much has been written upon it, the most contradictory
opinions still prevail, and at almost every step we find ourselves on the
battle-field of opposing schools in geological or physical science. As,
however, these questions lie at the very root of any general solution of the
problems of distribution, I have given much time to a careful examination
of the various theories that have been advanced, and the discussions to
which they have given rise; and have arrived at some definite conclusions
which I venture to hope may serve as the foundation for a better
comprehension of these intricate problems. The four chapters which follow
this are devoted to a full examination of these profoundly interesting and
important questions, after which we shall enter upon our special inquiry—
the nature and origin of insular faunas and floras.
The Ocean as a Barrier to the Dispersal of Mammals.—A wide extent of
ocean forms an almost absolute barrier to the dispersal of all land animals,
and of most of those which are aerial, since even birds cannot fly for
thousands of miles without rest and without food, unless they are aquatic
birds which can find both rest and food on the surface of the ocean. We may
be sure, therefore, that without artificial help neither mammalia nor land
birds can pass over very wide oceans. The exact width they can pass over is
not determined, but we have a few facts to guide us. Contrary to the
common notion, pigs can swim very well, and have been known to swim
over five or six miles of sea, and the wide distribution of pigs in the islands
of the Eastern Hemisphere may be due to this power. It is almost certain,
however, that they would never voluntarily swim away from their native
land, and if carried out to sea by a flood they would certainly endeavour to
return to the shore. We cannot therefore believe that they would ever swim
over fifty or a hundred miles of sea, and the same may be said of all the
larger mammalia. Deer also swim well, but there is no reason to believe that
they would venture out of sight of land. With the smaller, and especially
with the arboreal mammalia, there is a much more effectual way of passing
over the sea, by means of floating trees, or those floating islands which are
often formed at the mouths of great rivers. Sir Charles Lyell describes such
floating islands which were encountered among the Moluccas, on which
trees and shrubs were growing on a stratum of soil which even formed a
white beach round the margin of each raft. Among the Philippine Islands
similar rafts with trees growing on them have been seen after hurricanes;

and it is easy to understand how, if the sea were tolerably calm, such a raft
might be carried along by a current, aided by the wind acting on the trees,
till after a passage of several weeks it might arrive safely on the shores of
some land hundreds of miles away from its starting-point. Such small
animals as squirrels and field-mice might have been carried away on the
trees which formed part of such a raft, and might thus colonise a new
island; though, as it would require a pair of the same species to be thus
conveyed at the same time, such accidents would no doubt be rare. Insects,
however, and land-shells would almost certainly be abundant on such a raft
or island, and in this way we may account for the wide dispersal of many
species of both these groups.
Notwithstanding the occasional action of such causes, we cannot suppose
that they have been effective in the dispersal of mammalia as a whole; and
whenever we find that a considerable number of the mammals of two
countries exhibit distinct marks of relationship, we may be sure that an
actual land connection, or at all events an approach to within a very few
miles of each other, has at one time existed. But a considerable number of
identical mammalian families and even genera are actually found in all the
great continents, and the present distribution of land upon the globe renders
it easy to see how they have been able to disperse themselves so widely. All
the great land masses radiate from the arctic regions as a common centre,
the only break being at Behrings Strait, which is so shallow that a rise of
less than a thousand feet would form a broad isthmus connecting Asia and
America as far south as the parallel of 60° N. Continuity of land therefore
may be said to exist already for all parts of the world (except Australia and
a number of large islands, which will be considered separately), and we
have thus no difficulty in the way of that former wide diffusion of many
groups, which we maintain to be the only explanation of most anomalies of
distribution other than such as may be connected with unsuitability of
climate.
The Dispersal of Birds.—Wherever mammals can migrate other vertebrates
can generally follow with even greater facility. Birds, having the power of
flight, can pass over wide arms of the sea, or even over extensive oceans,
when these are, as in the Pacific, studded with islands to serve as resting
places. Even the smaller land-birds are often carried by violent gales of
wind from Europe to the Azores, a distance of nearly a thousand miles, so

that it becomes comparatively easy to explain the exceptional distribution
of certain species of birds. Yet on the whole it is remarkable how closely the
majority of birds follow the same laws of distribution as mammals, showing
that they generally require either continuous land or an island-strewn sea as
a means of dispersal to new homes.
The Dispersal of Reptiles.—Reptiles appear at first sight to be as much
dependent on land for their dispersal as mammalia, but they possess two
peculiarities which favour their occasional transmission across the sea—the
one being their greater tenacity of life, the other their oviparous mode of
reproduction. A large boa-constrictor was once floated to the island of St.
Vincent, twisted round the trunk of a cedar tree, and was so little injured by
its voyage that it captured some sheep before it was killed. The island is
nearly two hundred miles from Trinidad and the coast of South America,
whence the reptile almost certainly came.[14] Snakes are, however,
comparatively scarce on islands far from continents, but lizards are often
abundant, and though these might also travel on floating trees, it seems
more probable that there is some as yet unknown mode by which their eggs
are safely, though perhaps very rarely, conveyed from island to island.
Examples of their peculiar distribution will be given when we treat of the
fauna of some islands in which they abound.
The Dispersal of Amphibia and Fresh-water Fishes.—The two lower
groups of vertebrates, Amphibia and fresh-water fishes, possess special
facilities for dispersal, in the fact of their eggs being deposited in water, and
in their aquatic or semi-aquatic habits. They have another advantage over
reptiles in being capable of flourishing in arctic regions, and in the power
possessed by their eggs of being frozen without injury. They have thus, no
doubt, been assisted in their dispersal by floating ice, and by that
approximation of all the continents in high northern latitudes which has
been the chief agent in producing the general uniformity in the animal
productions of the globe. Some genera of Batrachia have almost a world-
wide distribution; while the tailed Batrachia, such as the newts and
salamanders, are almost entirely confined to the northern hemisphere, some
of the genera spreading over the whole of the north temperate zone. Fresh-
water fishes have often a very wide range, the same species being
sometimes found in all the rivers of a continent. This is no doubt chiefly
due to the want of permanence in river basins, especially in their lower

portions, where streams belonging to distinct systems often approach each
other and may be made to change their course from one to the other basin
by very slight elevations or depressions of the land. Hurricanes and water-
spouts also often carry considerable quantities of water from ponds and
rivers, and thus disperse eggs and even small fishes. As a rule, however, the
same species are not often found in countries separated by a considerable
extent of sea, and in the tropics rarely the same genera. The exceptions are
in the colder regions of the earth, where the transporting power of ice may
have come into play. High ranges of mountains, if continuous for long
distances, rarely have the same species of fish in the rivers on their two
sides. Where exceptions occur, it is often due to the great antiquity of the
group, which has survived so many changes in physical geography that it
has been able, step by step, to reach countries which are separated by
barriers impassable to more recent types. Yet another and more efficient
explanation of the distribution of this group of animals is the fact that many
families and genera inhabit both fresh and salt water; and there is reason to
believe that many of the fishes now inhabiting the tropical rivers of both
hemispheres have arisen from allied marine forms becoming gradually
modified for a life in fresh water. By some of these various causes, or a
combination of them, most of the facts in the distribution of fishes can be
explained without much difficulty.
The Dispersal of Insects.—In the enormous group of insects the means of
dispersal among land animals reach their maximum. Many of them have
great powers of flight, and from their extreme lightness they can be carried
immense distances by gales of wind. Others can survive exposure to salt
water for many days, and may thus be floated long distances by marine
currents. The eggs and larvæ often inhabit solid timber, or lurk under bark
or in crevices of logs, and may thus reach any countries to which such logs
are floated. Another important factor in the problem is the immense
antiquity of insects, and the long persistence of many of the best marked
types. The rich insect fauna of the Miocene period in Switzerland consisted
largely of genera still inhabiting Europe, and even of a considerable number
identical, or almost so, with living species. Out of 156 genera of Swiss
fossil beetles no less than 114 are still living; and the general character of
the species is exactly like that of the existing fauna of the northern
hemisphere in a somewhat more southern latitude. There is, therefore,

evidently no difficulty in accounting for any amount of dispersal among
insects; and it is all the more surprising that with such powers of migration
they should yet be often as restricted in their range as the reptiles or even
the mammalia. The cause of this wonderful restriction to limited areas is,
undoubtedly, the extreme specialisation of most insects. They have become
so exactly adapted to one set of conditions, that when carried into a new
country they cannot live. Many can only feed in the larva state on one
species of plant; others are bound up with certain groups of animals on
whom they are more or less parasitic. Climatal influences have a great
effect on their delicate bodies; while, however well a species may be
adapted to cope with its enemies in one locality, it may be quite unable to
guard itself against those which elsewhere attack it. From this peculiar
combination of characters it happens, that among insects are to be found
examples of the widest and most erratic dispersal and also of the extremest
restriction to limited areas; and it is only by bearing these considerations in
mind that we can find a satisfactory explanation of the many anomalies we
meet with in studying their distribution.
The Dispersal of Land Mollusca.—The only other group of animals we
need now refer to is that of the air-breathing mollusca, commonly called
land-shells. These are almost as ubiquitous as insects, though far less
numerous; and their wide distribution is by no means so easy to explain.
The genera have usually a very wide, and often a cosmopolitan range, while
the species are rather restricted, and sometimes wonderfully so. Not only do
single islands, however small, often possess peculiar species of land-shells,
but sometimes single mountains or valleys, or even a particular mountain
side, possess species or varieties found nowhere else upon the globe. It is
pretty certain that they have no means of passing over the sea but such as
are very rare and exceptional. Some which possess an operculum, or which
close the mouth of the shell with a diaphragm of secreted mucus, may float
across narrow arms of the sea, especially when protected in the crevices of
logs of timber; while in the young state when attached to leaves or twigs
they may be carried long distances by hurricanes.[15] Owing to their
exceedingly slow motion, their powers of voluntary dispersal, even on land,
are very limited, and this will explain the extreme restriction of their range
in many cases.

Great Antiquity of Land-Shells.—The clue to the almost universal
distribution of the several families and of many genera, is to be found,
however, in their immense antiquity. In the Pliocene and Miocene
formations most of the land-shells are either identical with living species or
closely allied to them, while even in the Eocene almost all are of living
genera, and one British Eocene fossil still lives in Texas. Strange to say, no
true land-shells have been discovered in the Secondary formations, but they
must certainly have abounded, for in the far more ancient Palæozoic coal
measures of Nova Scotia two species belonging to the living genera Pupa
and Zonites have been found in considerable abundance.
Land-shells have therefore survived all the revolutions the earth has
undergone since Palæozoic times. They have been able to spread slowly but
surely into every land that has ever been connected with a continent, while
the rare chances of transfer across the ocean, to which we have referred as
possible, have again and again occurred during the almost unimaginable
ages of their existence. The remotest and most solitary of the islands of the
mid-ocean have thus become stocked with them, though the variety of
species and genera bears a direct relation to the facilities of transfer, and the
shell fauna is never very rich and varied, except in countries which have at
one time or other been united to some continental land.
Causes Favouring the Abundance of Land-Shells.—The abundance and
variety of land-shells is also, more than that of any other class of animals,
dependent on the nature of the surface and the absence of enemies, and
where these conditions are favourable their forms are wonderfully
luxuriant. The first condition is the presence of lime in the soil, and a
broken surface of country with much rugged rock offering crevices for
concealment and hibernation. The second is a limited bird and mammalian
fauna, in which such species as are especially shell-eaters shall be rare or
absent. Both these conditions are found in certain large islands, and pre-
eminently in the Antilles, which possess more species of land-shells than
any single continent. If we take the whole globe, more species of land-
shells are found on the islands than on the continents—a state of things to
which no approach is made in any other group of animals whatever, but
which is perhaps explained by the considerations now suggested.

The Dispersal of Plants.—The ways in which plants are dispersed over the
earth, and the special facilities they often possess for migration have been
pointed out by eminent botanists, and a considerable space might be
occupied in giving a summary of what has been written on the subject. In
the present work, however, it is only in two or three chapters that I discuss
the origin of insular floras in any detail; and it will therefore be advisable to
adduce any special facts when they are required to support the argument in
particular cases. A few general remarks only will therefore be made here.
Special Adaptability of Seeds for Dispersal.—Plants possess many great
advantages over animals as regards the power of dispersal, since they are all
propagated by seeds or spores, which are hardier than the eggs of even
insects, and retain their vitality for a much longer time. Seeds may lie
dormant for many years and then vegetate, while they endure extremes of
heat, of cold, of drought, or of moisture which would almost always be fatal
to animal germs. Among the causes of the dispersal of seeds De Candolle
enumerates the wind, rivers, ocean currents, icebergs, birds and other
animals, and human agency. Great numbers of seeds are specially adapted
for transport by one or other of these agencies. Many are very light, and
have winged appendages, pappus, or down, which enable them to be carried
enormous distances. It is true, as De Candolle remarks, that we have no
actual proofs of their being so carried; but this is not surprising when we
consider how small and inconspicuous most seeds are. Supposing every
year a million seeds were brought by the wind to the British Isles from the
Continent, this would be only ten to a square mile, and the observation of a
life-time might never detect one; yet a hundredth part of this number would
serve in a few centuries to stock an island like Britain with a great variety of
continental plants.
When, however, we consider the enormous quantity of seeds produced by
plants, that great numbers of these are more or less adapted to be carried by
the wind, and that winds of great violence and long duration occur in most
parts of the world, we are as sure that seeds must be carried to great
distances as if we had seen them so carried. Such storms carry leaves, hay,
dust, and many small objects to a great height in the air, while many insects
have been conveyed by them for hundreds of miles out to sea and far
beyond what their unaided powers of flight could have effected.

Birds as Agents in the Dispersal of Plants.—Birds are undoubtedly
important agents in the dispersal of plants over wide spaces of ocean, either
by swallowing fruits and rejecting the seeds in a state fit for germination, or
by the seeds becoming attached to the plumage of ground-nesting birds, or
to the feet of aquatic birds embedded in small quantities of mud or earth.
Illustrations of these various modes of transport will be found in Chapter
XII. when discussing the origin of the flora of the Azores and Bermuda.
Ocean-currents as Agents in Plant-dispersal.—Ocean-currents are
undoubtedly more important agents in conveying seeds of plants than they
are in the case of any other organisms, and a considerable body of facts and
experiments have been collected proving that seeds may sometimes be
carried in this way many thousand miles and afterwards germinate. Mr.
Darwin made a series of interesting experiments on this subject, some of
which will be given in the chapter above referred to.
Dispersal along Mountain Chains.—These various modes of transport are,
as will be shown when discussing special cases, amply sufficient to account
for the vegetation found on oceanic islands, which almost always bears a
close relation to that of the nearest continent; but there are other phenomena
presented by the dispersal of species and genera of plants over very wide
areas, especially when they occur in widely separated portions of the
northern and southern hemispheres, that are not easily explained by such
causes alone. It is here that transmission along mountain chains has
probably been effective; and the exact mode in which this has occurred is
discussed in Chapter XXIII., where a considerable body of facts is given,
showing that extensive migrations may be effected by a succession of
moderate steps, owing to the frequent exposure of fresh surfaces of soil or
débris on mountain sides and summits, offering stations on which foreign
plants can temporarily establish themselves.
Antiquity of Plants as affecting their Distribution.—We have already
referred to the importance of great antiquity in enabling us to account for
the wide dispersal of some genera and species of insects and land-shells,
and recent discoveries in fossil botany show that this cause has also had
great influence in the case of plants. Rich floras have been discovered in the
Miocene, the Eocene, and the Upper Cretaceous formations, and these
consist almost wholly of living genera, and many of them of species very

closely allied to existing forms. We have therefore every reason to believe
that a large number of our plant-species have survived great geological,
geographical, and climatal changes; and this fact, combined with the varied
and wonderful powers of dispersal many of them possess, renders it far less
difficult to understand the examples of wide distribution of the genera and
species of plants than in the case of similar instances among animals. This
subject will be further alluded to when discussing the origin of the New
Zealand flora, in Chapter XXII.

CHAPTER VI

GEOGRAPHICAL AND GEOLOGICAL CHANGES: THE
PERMANENCE OF CONTINENTS

Changes of Land, and Sea, their Nature and Extent—Shore-deposits
and Stratified Rocks—The Movements of Continents—Supposed
Oceanic Formations; the Origin of Chalk—Fresh-water and Shore-
deposits as proving the Permanence of Continents—Oceanic Islands as
indications of the Permanence of Continents and Oceans—General
Stability of Continents with constant Change of Form—Effect of
Continental Changes on the Distribution of Animals—Changed
Distribution proved by the Extinct Animals of Different Epochs—
Summary of Evidence for the general Permanence of Continents and
Oceans.

The changes of land and sea which have occurred in particular cases will be
described when we discuss the origin and relations of the faunas of the
different classes of islands. We have here only to consider the general
character and extent of such changes, and to correct some erroneous ideas
which are prevalent on the subject.
Changes of Land and Sea, their Nature and Extent.—It is a very common
belief that geological evidence proves a complete change of land and sea to
have taken place over and over again. Every foot of dry land has
undoubtedly, at one time or other, formed part of a sea-bottom, and we can

hardly exclude the surfaces occupied by volcanic and fresh-water deposits,
since, in many cases, if not in all, these rest upon a substratum of marine
formations. At first sight, therefore, it seems a necessary inference that
when the present continents were under water there must have been other
continents situated where we now find the oceans, from which the
sediments came to form the various deposits we now see. This view was
held by so acute and learned a geologist as Sir Charles Lyell, who says:
—"Continents, therefore, although permanent for whole geological epochs,
shift their positions entirely in the course of ages."[16] Mr. T. Mellard Reade,
late President of the Geological Society of Liverpool, so recently as 1878,
says:—"While believing that the ocean-depths are of enormous age, it is
impossible to resist other evidences that they have once been land. The very
continuity of animal and vegetable life on the globe points to it. The
molluscous fauna of the eastern coast of North America is very similar to
that of Europe, and this could not have happened without littoral continuity,
yet there are depths of 1,500 fathoms between these continents."[17] It is
certainly strange that a geologist should not remember the recent and long-
continued warm climates of the Arctic regions, and see that a connection of
Northern Europe by Iceland with Greenland and Labrador over a sea far
less than a thousand fathoms deep would furnish the "littoral continuity"
required. Again, in the same pamphlet Mr. Reade says:—"It can be
mathematically demonstrated that the whole, or nearly the whole, of the
sea-bottom has been at one time or other dry land. If it were not so, and the
oscillations, of the level of the land with respect to the sea were confined
within limits near the present continents, the results would have been a
gradual diminution instead of development of the calcareous rocks. To state
the case in common language, the calcareous portion of the rocks would
have been washed out during the mutations, the destruction and redeposit of
the continental rocks, and eventually deposited in the depths of the
immutable sea far from land. Immense beds of limestone would now exist
at the bottom of the ocean, while the land would be composed of sandstones
and argillaceous shales. The evidence of chemistry thus confirms the
inductions drawn from the distribution of animal life upon the globe."
So far from this being a "mathematical demonstration," it appears to me to
be a complete misinterpretation of the facts. Animals did not create the lime
which they secrete from the sea-water, and therefore we have every reason

to believe that the inorganic sources which originally supplied it still keep
up that supply, though perhaps in diminished quantity. Again, the great
lime-secreters—corals—work in water of moderate depth, that is, near land,
while there is no proof whatever that there is any considerable accumulation
of limestone at the bottom of the deep ocean. On the contrary, the fact
ascertained by the Challenger, that beyond a certain depth the "calcareous"
ooze ceases, and is replaced by red and grey clays, although the calcareous
organisms still abound in the surface waters of the ocean, shows that the
lime is dissolved again by the excess of carbonic acid usually found at great
depths, and its accumulation thus prevented. As to the increase of
limestones in recent as compared with older formations, it may be readily
explained by two considerations: in the first place, the growth and
development of the land in longer and more complex shore lines and the
increase of sedimentary over volcanic formations may have offered more
stations favourable to the growth of coral; while the solubility of limestone
in rain-water renders the destruction of such rocks more rapid than that of
sandstones and shales, and would thus, by supplying more calcareous
matter in solution for secretion by limestone-forming organisms, lead to
their comparative abundance in later as compared with earlier formations.
However weak we may consider the above-quoted arguments against the
permanence of oceans, the fact that these arguments are so confidently and
authoritatively put forward, renders it advisable to show how many and
what weighty considerations can be adduced to justify the opposite belief,
which is now rapidly gaining ground among students of earth-history.
Shore Deposits and Stratified Rocks.—If we go round the shores of any of
our continents we shall almost always find a considerable belt of shallow
water, meaning thereby water from 100 to 150 fathoms deep. The distance
from the coast line at which such depths are reached is seldom less than
twenty miles, and is very frequently more than a hundred, while in some
cases such shallow seas extend several hundred miles from existing
continents. The great depth of a thousand fathoms is often reached at thirty
miles from shore, but more frequently at about sixty or a hundred miles.
Round the entire African coast for example, this depth is reached at
distances varying from forty to a hundred and fifty miles (except in the Red
Sea and the Straits of Mozambique), the average being about eighty miles.

Now the numerous specimens of sea-bottoms collected during the voyage
of the Challenger show that true shore-deposits—that is, materials denuded
from the land and carried down as sediment by rivers—are almost always
confined within a distance of 50 or 100 miles of the coast, the finest mud
only being sometimes carried 150 or rarely 200 miles. As the sediment
varies in coarseness and density it is evident that it will sink to the bottom at
unequal distances, the bulk of it sinking comparatively near shore, while
only the very finest and almost impalpable mud will be carried out to the
furthest limits. Beyond these limits the only deposits (with few exceptions)
are organic, consisting of the shells of minute calcareous or siliceous
organisms with some decomposed pumice and volcanic dust which floats
out to mid-ocean. It follows, therefore, that by far the larger part of all
stratified deposits, especially those which consist of sand or pebbles or any
visible fragments of rock, must have been formed within 50 or 100 miles of
then existing continents, or if at a greater distance, in shallow inland seas
receiving deposits from more sides than one, or in certain exceptional areas
where deep ocean currents carry the débris of land to greater distances.[18]
If we now examine the stratified rocks found in the very centre of all our
great continents, we find them to consist of sandstones, limestones,
conglomerates, or shales, which must, as we have seen, have been deposited
within a comparatively short distance of a sea-shore. Sir Archibald Geikie
says:—"Among the thickest masses of sedimentary rock—those of the
ancient Palæozoic systems—no features recur more continually than the
alternations of different sediments, and the recurrence of surfaces covered
with well-preserved ripple-marks, trails and burrows of annelides,
polygonal and irregular desiccation marks, like the cracks at the bottom of a
sun-dried muddy pool. These phenomena unequivocally point to shallow
and even littoral waters. They occur from bottom to top of formations,
which reach a thickness of several thousand feet. They can be interpreted
only in one way, viz., that the formations in question began to be laid down
in shallow water; that during their formation the area of deposit gradually
subsided for thousands of feet; yet that the rate of accumulation of sediment
kept pace on the whole with this depression; and hence that the original
shallow-water character of the deposits remained, even after the original
sea-bottom had been buried under a vast mass of sedimentary matter." He
goes on to say, that this general statement applies to the more recent as well

as to the more ancient formations, and concludes—"In short, the more
attentively the stratified rocks of the earth are studied, the more striking
becomes the absence of any formations among them, which can
legitimately be considered those of a deep sea. They have all been deposited
in comparatively shallow water."[19]
The arrangement and succession of the stratified rocks also indicate the
mode and place of their formation. We find them stretching across the
country in one general direction, in belts of no great width though often of
immense length, just as we should expect in shore deposits; and they often
thin out and change from coarse to fine in a definite manner, indicating the
position of the adjacent land from the débris of which they were originally
formed. Again quoting Sir Archibald Geikie:—"The materials carried down
to the sea would arrange themselves then as they do still, the coarser
portions nearest the shore, the finer silt and mud furthest from it. From the
earliest geological times the great area of deposit has been, as it still is, the
marginal belt of sea-floor skirting the land. It is there that nature has always
strewn the dust of continents to be."
The Movements of Continents.—As we find these stratified rocks of
different periods spread over almost the whole surface of existing
continents where not occupied by igneous or metamorphic rocks, it follows
that at one period or another each part of the continent has been under the
sea, but at the same time not far from the shore. Geologists now recognise
two kinds of movements by which the deposits so formed have been
elevated into dry land—in the one case the strata remain almost level and
undisturbed, in the other they are contorted and crumpled, often to an
enormous extent. The former often prevails in plains and plateaus, while the
latter is almost always found in the great mountain ranges. We are thus led
to picture the land of the globe as a flexible area in a state of slow but
incessant change; the changes consisting of low undulations which creep
over the surface so as to elevate and depress limited portions in succession
without perceptibly affecting their nearly horizontal position; and also of
intense lateral compression, supposed to be produced by partial subsidence
along certain lines of weakness in the earth's crust, the effect of which is to
crumple the strata and force up certain areas in great contorted masses,
which, when carved out by subaërial denudation into peaks and valleys,
constitute our great mountain systems.[20] In this way every part of a

continent may again and again have sunk beneath the sea, and yet as a
whole may never have ceased to exist as a continent or a vast continental
archipelago. And, as subsidence will always be accompanied by deposition,
of sediments from the adjacent land, piles of marine strata many thousand
feet thick may have been formed in a sea which was never very deep, by
means of a slow depression either continuous or intermittent, or through
alternate subsidences and elevations, each of moderate amount.
Supposed Oceanic Formations;—the Origin of Chalk.—There seems very
good reason to believe that few, if any, of the rocks known to geologists
correspond exactly to the deposits now forming at the bottom of our great
oceans. The white oceanic mud, or Globigerina-ooze, found in all the great
oceans at depths varying from 250 to nearly 3,000 fathoms, and almost
constantly in depths under 2,000 fathoms, has, however, been supposed to
be an exception, and to correspond exactly to our white and grey chalk.
Hence some naturalists have maintained that there has probably been one
continuous formation of chalk in the Atlantic from the Cretaceous epoch to
the present day. This view has been adopted chiefly on account of the
similarity of the minute organisms found to compose a considerable
proportion of both deposits, more especially the pelagic Foraminifera, of
which several species of Globigerina appear to be identical in the chalk and
the modern Atlantic mud. Other extremely minute organisms whose nature
is doubtful, called coccoliths and discoliths, are also found in both
formations, while there is a considerable general resemblance between the
higher forms of life. Sir Wyville Thomson tells us, that—"Sponges are
abundant in both, and the recent chalk-mud has yielded a large number of
examples of the group porifera vitrea, which find their nearest
representatives among the Ventriculites of the white chalk. The echinoderm
fauna of the deeper parts of the Atlantic basin is very characteristic, and
yields an assemblage of forms which represent in a remarkable degree the
corresponding group in the white chalk. Species of the genus Cidaris are
numerous; some remarkable flexible forms of the Diademidæ seem to
approach Echinothuria."[21] Now, as some explanation of the origin of chalk
had long been desired by geologists, it is not surprising that the amount of
resemblance shown to exist between it and some kinds of oceanic mud
should have been at once seized upon, and the conclusion arrived at that
chalk is a deep-sea oceanic formation exactly analogous to that which has

been shown to cover large areas of the Atlantic, Pacific and Southern
oceans.
But there are several objections to this view which seem fatal to its
acceptance. In the first place, no specimens of Globigerina-ooze from the
deep ocean-bed yet examined agree even approximately with chalk in
chemical composition, only containing from 44 to 79 per cent. of carbonate
of lime, with from 5 to 11 per cent of silica, and from 8 to 33 per cent. of
alumina and oxide of iron.[22] Chalk, on the other hand, contains usually
from 94 to 99 per cent. of carbonate of lime, and a very minute quantity of
alumina and silica. This large proportion of carbonate of lime implies some
other source of this mineral, and it is probably to be found in the
excessively fine mud produced by the decomposition and denudation of
coral reefs. Mr. Dana, the geologist of the United States Exploring
Expedition, found in the elevated coral reef of Oahu, one of the Sandwich
Islands, a deposit closely resembling chalk in colour, texture, &c.; while in
several growing reefs a similar formation of modern chalk
undistinguishable from the ancient, was observed.[23] Sir Charles Lyell well
remarks that the pure calcareous mud produced by the decomposition of the
shelly coverings of mollusca and zoophytes would be much lighter than
argillaceous or arenaceous mud, and being thus transported to greater
distances would be completely separated from all impurities.
Now the Globigerinæ have been shown by the Challenger explorations to
abound in all moderately warm seas; living both at the surface, at various
depths in the water, and at the bottom. It was long thought that they were
surface-dwellers only, and that their dead tests sank to the bottom,
producing the Globigerina-ooze in those areas where other deposits were
absent or scanty. But the examination of the whole of the dredgings and
surface-gatherings of the Challenger by Mr. H. B. Brady has led him to a
different conclusion; for he finds numerous forms at the bottom quite
distinct from those which inhabit the surface, while, when the same species
live both at surface and bottom, the latter are always larger and have thicker
and stronger cell-walls. This view is also supported by the fact that in many
stations not far from our own shores Globigerinæ are abundant in bottom
dredgings, but are never found on the surface in the towing-nets.[24] These
organisms then exist almost universally where the waters are pure and are

not too cold, and they would naturally abound most where the diffusion of
carbonate of lime both in suspension and solution afforded them an
abundant supply of material for their shelly coverings. Dr. Wallich believes
that they flourish best where the warm waters of the Gulf Stream bring
organic matter from which they derive nutriment, since they are wholly
wanting in the course of the Arctic current between Greenland and
Labrador. Dr. Carpenter also assures us that they are rigorously limited to
warm areas; but Mr. Brady says that a dwarf variety of Globigerina was
found in the soundings of the North Polar Expedition in Lat. 83° 19′ N.
Now with regard to the depth at which our chalk was formed, we have
evidence of several distinct kinds to show that it was not profoundly
oceanic. Mr. J. Murray, in the report already referred to, says: "The
Globigerina-oozes which we get in shallow water resemble the chalk much
more than those in deeper water, say over 1,000 fathoms."[25] This is
important and weighty evidence, and it is supported in a striking manner by
the nature of the molluscan fauna of the chalk. Dr. Gwyn Jeffreys, one of
our greatest authorities on shells, who has himself dredged largely both in
deep and shallow water and who has no theory to support, has carefully
examined this question. Taking the whole series of genera which are found
in the Chalk formation, seventy-one in number, he declared that they are all
comparatively shallow-water forms, many living at depths not exceeding 40
to 50 fathoms, while some are confined to still shallower waters. Even more
important is the fact that the genera especially characteristic of the deep
Atlantic ooze—Leda, Verticordia, Neæra, and the Bulla family—are either
very rare or entirely wanting in the ancient Cretaceous deposits.[26]
Let us now see how the various facts already adduced will enable us to
explain the peculiar characteristics of the chalk formation. Sir Charles Lyell
tells us that "pure chalk, of nearly uniform aspect and composition, is met
with in a north-west and south-east direction, from the north of Ireland to
the Crimea, a distance of about 1,500 geographical miles; and in an
opposite direction it extends from the south of Sweden to the south of
Bordeaux, a distance of about 840 geographical miles." This marks the
extreme limits within which true chalk is found, though it is by no means
continuous. It probably implies, however, the existence across Central
Europe of a sea somewhat larger than the Mediterranean. It may have been
much larger, because this pure chalk formation would only be formed at a

considerable distance from land, or in areas where there was no other shore
deposit. This sea was probably bounded on the north by the old
Scandinavian highlands, extending to Northern Germany and North-
western Russia, where Palæozoic and ancient Secondary rocks have a wide
extension, though now partially concealed by late Tertiary deposits; while
on the south it appears to have been limited by land extending through
Austria, South Germany, and the south of France, as shown in the map of
Central Europe during the Cretaceous period in Professor Heer's Primeval
World of Switzerland, p. 175. To the north the sea may have had an outlet to
the Arctic Ocean between the Ural range and Finland. South of the Alps
there was probably another sea, which may have communicated with the
northern one just described, and there was also a narrow strait across
Switzerland, north of the Alps, but, as might be expected, in this only marls,
clays, sandstones, and limestones were deposited instead of true chalk. It is
also a suggestive fact that both above and below the true chalk, in almost all
the countries where it occurs, are extensive deposits of marls, clays, and
even pure sands and sandstones, characterised by the same general types of
fossil remains as the chalk itself. These beds imply the vicinity of land, and
this is even more clearly proved by the occurrence, both in the Upper and
Lower Cretaceous, of deposits containing the remains of land-plants in
abundance, indicating a rich and varied flora.
Now all these facts are totally opposed to the idea of anything like oceanic
conditions having prevailed in Europe during the Cretaceous period; but
they are quite consistent with the existence of a great Mediterranean sea of
considerable depth in its central portions, and occupying either at one or
successive periods, the whole area of the Cretaceous formation. We may
also note that the Maestricht beds in Belgium and the Faxoe chalk in
Denmark are both highly coralline, the latter being, in fact, as completely
composed of corals as a modern coral-reef; so that we have here a clear
indication of the source whence the white calcareous mud was derived
which forms the basis of chalk. If we suppose that during this period the
comparatively shallow sea-bottom between Scandinavia and Greenland was
elevated, forming a land connection between these countries, the result
would be that a large portion of the Gulf Stream would be diverted into the
inland European sea, and would bring with it that abundance of
Globigerinæ, and other Foraminifera, which form such an important

constituent of chalk. This sea was probably bordered with islands and coral-
reefs, and if no very large rivers flowed into it we should have all the
conditions for the production of the true chalk, as well as the other members
of the Cretaceous formation. The products of the denudation of its shores
and islands would form the various sandstones, marls, and clays, which
would be deposited almost wholly within a few miles of its coasts; while
the great central sea, perhaps at no time more than a few thousand feet deep
and often much less, would receive only the impalpable mud of the coral-
reefs and the constantly falling tests of Foraminifera. These would imbed
and preserve for us the numerous echinoderms, sponges, and mollusca,
which lived upon the bottom, the fishes and turtles which swam in its
waters, and sometimes the winged reptiles that flew overhead. The
abundance of ammonites, and other cephalopods, in the chalk, is another
indication that the water in which they lived was not very deep, since Dr.
S. P. Woodward thinks that these organisms were limited to a depth of about
thirty fathoms.
The best example of the modern formation of chalk is perhaps to be found
on the coasts of sub-tropical North America, as described in the following
passage:—
"The observations of Pourtales show that the steep banks of Bahama are
covered with soft white lime mud. The lime-bottom, which consists almost
entirely of Polythalamia, covers in greater depths the entire channel of
Florida. This formation extends without interruption over the whole bed of
the Gulf Stream in the Gulf of Mexico, and is continued along the Atlantic
coast of America. The commonest genera met with in this deposit are
Globigerina, Rotalia cultrata in large numbers, several Textilariæ,
Marginulinæ, &c. Beside these, small free corals, Alcyonidæ, Ophiuræ,
Mollusca, Crustacea, small fishes, &c., are found living in these depths. The
whole sea-bottom appears to be covered with a vast deposit of white chalk
still in formation."[27]
There is yet another consideration which seems to have been altogether
overlooked by those who suppose that a deep and open island-studded
ocean occupied the place of Europe in Cretaceous times. No fact is more
certain than the considerable break, indicative of a great lapse of time,
intervening between the Cretaceous and Tertiary formations. A few deposits

of intermediate age have indeed been found, but these have been generally
allocated either with the Chalk or the Eocene, leaving the gap almost as
pronounced as before. Now, what does this gap mean? It implies that when
the deposition of the various Cretaceous beds of Europe came to an end
they were raised above the sea-level and subject to extensive denudation,
and that for a long but unknown period no extensive portion of what is now
European land was below the sea-level. It was only when this period
terminated that large areas in several parts of Europe became submerged
and received the earliest Tertiary deposits known as Eocene. If, therefore,
Europe at the close of the Cretaceous period was generally identical with
what it is now, and perhaps even more extensive, it is absurd to suppose that
it was all, or nearly all, under water during that period; or in fact, that any
part of it was submerged, except those areas on which we actually find
Cretaceous deposits, or where we have good reason to believe they have
existed; and even these need not have been all under water at the same time.
The several considerations now adduced are, I think, sufficient to show that
the view put forth by some naturalists (and which has met with a somewhat
hasty acceptance by geologists) that our white chalk is an oceanic formation
strictly comparable with that now forming at depths of a thousand fathoms
and upwards in the centre of the Atlantic, gives a totally erroneous idea of
the actual condition of Europe during that period. Instead of being a wide
ocean, with a few scattered islands, comparable to some parts of the Pacific,
it formed as truly a portion of the great northern continent as it does now,
although the inland seas of that epoch may have been more extensive and
more numerous than they are at the present day.[28]
Fresh-water and Shore Deposits as Proving the Permanence of Continents.
—The view here maintained, that all known marine deposits have been
formed near the coasts of continents and islands, and that our actual
continents have been in continuous existence under variously modified
forms during the whole period of known geological history, is further
supported by another and totally distinct series of facts. In almost every
period of geology, and in all the continents which have been well examined,
there are found lacustrine, estuarine, or shore deposits, containing the
remains of land animals or plants, thus demonstrating the continuous
existence of extensive land areas on or adjoining the sites of our present
continents. Beginning with the Miocene, or Middle Tertiary period, we have

such deposits with remains of land-animals, or plants, in Devonshire and
Scotland, in France, Switzerland, Germany, Croatia, Vienna, Greece, North
India, Central India, Burmah, North America, both east and west of the
Rocky Mountains, Greenland, and other parts of the Arctic regions. In the
older Eocene period similar formations are widely spread in the south of
England, in France, and to an enormous extent on the central plateau of
North America; while in the eastern states, from Maryland to Alabama,
there are extensive marine deposits of the same age, which, from the
abundance of fossil remains of a large cetacean (Zeuglodon), must have
been formed in shallow gulfs or estuaries where these huge animals were
stranded. Going back to the Cretaceous formation we have the same
indications of persisting lands in the rich plant-beds of Aix-la-Chapelle, and
a few other localities on the Continent, as well as in coniferous fruits from
the Gault of Folkestone; while in North America cretaceous plant-beds
occur in New Jersey, Alabama, Kansas, the sources of the Missouri, the
Rocky Mountains from New Mexico to the Arctic Ocean, Alaska,
California, and in Greenland and Spitzbergen; while birds and land reptiles
are found in the Cretaceous deposits of Colorado and other districts near the
centre of the Continent. Fresh-water deposits of this age are also found on
the coast of Brazil. In the lower part of this formation we have the fresh-
water Wealden deposits of England, extending into France, Hanover, and
Westphalia. In the older Oolite or Jurassic formation we have abundant
proofs of continental conditions in the fresh-water and "dirt"-beds of the
Purbecks in the south of England, with plants, insects and mammals; the
Bavarian lithographic stone, with fossil birds and insects; the earlier "forest
marble" of Wiltshire, with ripple-marks, wood, and broken shells, indicative
of an extensive beach; the Stonesfield slate, with plants, insects, and
marsupials; and the Oolitic coal of Yorkshire and Sutherlandshire. Beds of
the same age occur in the Rocky Mountains of North America, containing
abundance of Dinosaurians and other reptiles, among which is the
Atlantosaurus, the largest land-animal yet known to have existed on the
earth. Professor O. C. Marsh describes it as having been between fifty and
sixty feet long, and when standing erect at least thirty feet high![29] Such
monsters could hardly have been developed except in an extensive land
area. A small mammal, Dryolestes, has been discovered in the same
deposits. A rich Jurassic flora has also been found in East Siberia and the
Amur valley. The older Triassic deposits are very extensively developed in

America, and both in the Connecticut valley and the Rocky Mountains
show tracks or remains of land reptiles, amphibians and mammalia, while
coalfields of the same age in Virginia and Carolina produce abundance of
plants. Here too are found the ancient mammal, Microlestes, of
Wurtemberg, with the ferns, conifers, and Labyrinthodonts of the Bunter
Sandstone in Germany; while the beds of rock-salt in this formation, both in
England and in many parts of the Continent, could only have been formed
in inland seas or lakes, and thus equally demonstrate continental conditions.
We now pass into the oldest or Palæozoic formations, but find no
diminution in the proofs of continental conditions. The Permian formation
has a rich flora often producing coal in England, France, Saxony, Thuringia,
Silesia, and Eastern Russia. Coalfields of the same age occur in Ohio in
North America. In the still more ancient Carboniferous formation we find
the most remarkable proofs of the existence of our present land masses at
that remote epoch, in the wonderful extension of coal beds in all the known
continents. We find them in Ireland, England, and Scotland; in France,
Spain, Belgium, Saxony, Prussia, Bohemia, Hungary, Sweden, Spitzbergen,
Siberia, Russia, Greece, Turkey, and Persia; in many parts of continental
India, extensively in China, and in Australia, Tasmania, and New Zealand.
In North America there are immense coal fields, in Nova Scotia and New
Brunswick, from Pennsylvania southward to Alabama, in Indiana and
Illinois, in Missouri, and even so far west as Colorado; and there is also a
true coal formation in South Brazil. This wonderfully wide distribution of
coal, implying, as it does, a rich vegetation and extensive land areas, carries
back the proof of the persistence and general identity of our continents to a
period so remote that none of the higher animal types had probably been
developed. But we can go even further back than this, to the preceding
Devonian formation, which was almost certainly an inland deposit often
containing remains of fresh-water shells, plants, and even insects; while
Professor Ramsay believes that he has found "sun-cracks and rain-pittings"
in the Longmynd beds of the still earlier Cambrian formation.[30] If now, in
addition to the body of evidence here adduced, we take into consideration
the fresh-water deposits that still remain to be discovered, and those
extensive areas where they have been destroyed by denudation or remain
deeply covered up by later marine or volcanic formations, we cannot but be
struck by the abounding proofs of the permanence of the great features of

land and sea as they now exist; and we shall see how utterly gratuitous, and
how entirely opposed to all the evidence at our command, are the
hypothetical continents bridging over the deep oceans, by the help of which
it is so often attempted to cut the Gordian knot presented by some
anomalous fact in geographical distribution.
Oceanic Islands as Indications of the Permanence of Continents and
Oceans.—Coming to the question from the other side, Mr. Darwin has
adduced an argument of considerable weight in favour of the permanence of
the great oceans. He says (Origin of Species, 6th Ed. p. 288): "Looking to
existing oceans, which are thrice as extensive as the land, we see them
studded with many islands; but hardly one truly oceanic island (with the
exception of New Zealand, if this can be called a truly oceanic island) is as
yet known to afford even a fragment of any Palæozoic or Secondary
formation. Hence we may perhaps infer that during the Palæozoic and
Secondary periods neither continents nor continental islands existed where
our oceans now extend; for had they existed, Palæozoic and Secondary
formations would in all probability have been accumulated from sediment
derived from their wear and tear; and these would have been at least
partially upheaved by the oscillations of level, which must have intervened
during these enormously long periods. If then we may infer anything from
these facts, we may infer that, where our oceans now extend, oceans have
extended from the remotest period of which we have any record; and, on
the other hand, that where continents now exist, large tracts of land have
existed, subjected no doubt to great oscillations of level, since the Cambrian
period." This argument standing by itself has not received the attention it
deserves, but coming in support of the long series of facts of an altogether
distinct nature, going to show the permanence of continents, the cumulative
effect of the whole must, I think, be admitted to be irresistible.[31]
General Stability of Continents with Constant Change of Form.—It will be
observed that the very same evidence which has been adduced to prove the
general stability and permanence of our continental areas also goes to prove
that they have been subjected to wonderful and repeated changes in detail.
Every square mile of their surface has been again and again under water,
sometimes a few hundred feet deep, sometimes perhaps several thousands.
Lakes and inland seas have been formed, have been filled up with sediment,
and been subsequently raised into hills or even mountains. Arms of the sea

have existed crossing the continents in various directions, and thus
completely isolating the divided portions for varying intervals. Seas have
been changed into deserts and deserts into seas. Volcanoes have grown into
mountains, have been degraded and sunk beneath the ocean, have been
covered with sedimentary deposits, and again raised up into mountain
ranges; while other mountains have been formed by the upraised coral reefs
of inland seas. The mountains of one period have disappeared by
denudation or subsidence, while the mountains of the succeeding period
have been rising from beneath the waves. The valleys, the ravines, and the
mountain peaks, have been carved out and filled up again; and all the
vegetable forms which clothe the earth and furnish food for the various
classes of animals have been completely changed again and again.
Effect of Continental Changes on the Distribution of Animals.—It is
impossible to exaggerate, or even adequately to conceive, the effect of these
endless mutations on the animal world. Slowly but surely the whole
population of living things must have been driven backward and forward
from east to west, or from north to south, from one side of a continent or a
hemisphere to the other. Owing to the remarkable continuity of all the land
masses, animals and plants must have often been compelled to migrate into
other continents, where in the struggle for existence under new conditions
many would succumb; while such as were able to survive would constitute
those wide-spread groups whose distribution often puzzles us. Owing to the
repeated isolation of portions of continents for long periods, special forms
of life would have time to be developed, which, when again brought into
competition with the fauna from which they had been separated, would
cause fresh struggles of ever increasing complexity, and thus lead to the
development and preservation of every weapon, every habit, and every
instinct, which could in any way conduce to the safety and preservation of
the several species.
Changed Distribution proved by the Extinct Animals of Different Epochs.—
We thus find that, while the inorganic world has been in a state of continual
though very gradual change, the species of the organic world have also been
slowly changing in form and in the localities they inhabit; and the records
of these changes and these migrations are everywhere to be found, in the
actual distribution of the species no less than in the fossil remains which are
preserved in the rocks. Everywhere the animals which have most recently

become extinct resemble more or less closely those which now live in the
same country; and where there are exceptions to the rule, we can generally
trace them to some changed conditions which have led to the extinction of
certain types. But when we go a little further back, to the late or middle
Tertiary deposits, we almost always find, along with forms which might
have been the ancestors of some now living, others which are only now
found in remote regions and often in distinct continents—clear indications
of those extensive migrations which have ever been going on. Every large
island contains in its animal inhabitants a record of the period when it was
last separated from the adjacent continent, while some portions of existing
continents still show by the comparative poverty and speciality of their
animals that at no distant epoch they were cut off by arms of the sea and
formed islands. If the geological record were more perfect, or even if we
had as good a knowledge of that record in all parts of the world as we have
in Europe and North America, we could arrive at much more accurate
results than we are able to do with our present very imperfect knowledge of
extinct forms of life; but even with our present scanty information we are
able to throw much light upon the past history of our globe and its
inhabitants, and can sketch out with confidence many of the changes they
must have undergone.
Summary of Evidence for the General Permanence of Continents and
Oceans.—As this question of the permanence of our continents or, rather, of
the continental areas, lies at the root of all our inquiries into the past
changes of the earth and its inhabitants, and as it is at present completely
ignored by many writers, and even by naturalists of eminence, it will be
well to summarise the various kinds of evidence which go to establish it.[32]
We know as a fact that all sedimentary deposits have been formed under
water, but we also know that they were largely formed in lakes or inland
seas, or near the coasts of continents or great islands, and that deposits
uniform in character and more than 150 or 200 miles wide were rarely, if
ever, formed at the same time. The further we go from the land the less
rapidly deposition takes place, hence the great bulk of all the strata must
have been formed near land. Some deposits are, it is true, continually
forming in the midst of the great oceans, but these are chiefly organic and
increase very slowly, and there is no proof that any part of the series of
known geological formations exactly resembles them. Chalk, which is still

believed to be such a deposit by many naturalists, has been shown, by its
contained fossils, to be a comparatively shallow water formation—that is,
one formed at a depth measured by hundreds rather than by thousands of
fathoms. The nature of the formations composing all our continents also
proves the continuity of those continents. Everywhere we find clearly
marked shore and estuarine deposits, showing that every part of the existing
land has in turn been on the sea-shore; and we also find in all periods
lacustrine formations of considerable extent with remains of plants and land
animals, proving the existence of continents or extensive lands, in which
such lakes or estuaries could be formed. These lacustrine deposits can be
traced back through every period, from the newer Tertiary to the Devonian
and Cambrian, and in every continent which has been geologically
explored; and thus complete the proof that our continents have been in
existence under ever changing forms throughout the whole of that
enormous lapse of time.
On the side of the oceans we have also a great weight of evidence in favour
of their permanence and stability. In addition to their enormous depths and
great extent, and the circumstance that the deposits now forming in them
are distinct from anything found upon the land-surface, we have the
extraordinary fact that the countless islands scattered over their whole area
(with one or two exceptions only and those comparatively near to
continental areas) never contain any Palæozoic or Secondary rocks—that is,
have not preserved any fragments of the supposed ancient continents, nor of
the deposits which must have resulted from their denudation during the
whole period of their existence! The supposed exceptions are New Zealand
and the Seychelles Islands, both situated near to continents and not really
oceanic, leaving almost the whole of the vast areas of the Atlantic, Pacific,
Indian, and Southern oceans, without a solitary relic of the great islands or
continents supposed to have sunk beneath their waves.

CHAPTER VII

CHANGES OF CLIMATE WHICH HAVE INFLUENCED THE
DISPERSAL OF ORGANISMS: THE GLACIAL EPOCH

Proofs of the Recent Occurrence of a Glacial Epoch—Moraines—
Travelled Blocks—Glacial Deposits of Scotland: the "Till"—
Inferences from the Glacial Phenomena of Scotland—Glacial
Phenomena of North America—Effects of the Glacial Epoch on
Animal Life—Warm and Cold Periods—Palæontological Evidence of
Alternate Cold and Warm Periods—Evidence of Interglacial Warm
Periods on the Continent and in North America—Migrations and
Extinctions of Organisms caused by the Glacial Epoch.

We have now to consider another set of physical revolutions which have
profoundly affected the whole organic world. Besides the wonderful
geological changes to which, as we have seen, all continents have been
exposed, and which must, with extreme slowness, have brought about the
greater features of the dispersal of animals and plants throughout the world,
there has been also a long succession of climatal changes, which, though
very slow and gradual when measured by centuries, may have sometimes
been rapid as compared with the slow march of geological mutations.
These climatal changes may be divided into two classes, which have been
thought to be the opposite phases of the same great phenomenon—cold or
even glacial epochs in the Temperate zones on the one hand, and mild or
even warm periods extending into the Arctic regions on the other. The
evidence for both these changes having occurred is conclusive; and as they
must be taken account of whenever we endeavour to explain the past
migrations and actual distribution of the animal world, a brief outline of the
more important facts and of the conclusions they lead to must be here
given.
Proofs of the Recent Occurrence of a Glacial Epoch.—The phenomena that
prove the recent occurrence of glacial epochs in the temperate regions are
exceedingly varied, and extend over very wide areas. It will be well
therefore to state, first, what those facts are as exhibited in our own country,
referring afterwards to similar phenomena in other parts of the world.
Perhaps the most striking of all the evidences of glaciation are the grooved,
scratched, or striated rocks. These occur abundantly in Scotland,
Cumberland, and North Wales, and no rational explanation of them has ever
been given except that they were formed by glaciers. In many valleys, as,

for instance, that of Llanberris in North Wales, hundreds of examples may
be seen, consisting of deep grooves several inches wide, smaller furrows,
and striæ of extreme fineness wherever the rock is of sufficiently close and
hard texture to receive such marks. These grooves or scratches are often
many yards long, they are found in the bed of the valley as well as high up
on its sides, and they are almost all without exception in one general
direction—that of the valley itself, even though the particular surface they
are upon slopes in another direction. When the native covering of turf is
cleared away from the rock the grooves and striæ are often found in great
perfection, and there is reason to believe that such markings cover, or have
once covered, a large part of the surface. Accompanying these markings we
find another, hardly less curious phenomenon, the rounding off or planing
down of the hardest rocks to a smooth undulating surface. Hard crystalline
schists with their strata nearly vertical, and which one would expect to find
exposing jagged edges, are found ground off to a perfectly smooth but
never to a flat surface. These rounded surfaces are found not only on single
rocks but over whole valleys and mountain sides, and form what are termed
roches moutonnées, from their often having the appearance at a distance of
sheep lying down.
Now these two phenomena are actually produced by existing glaciers, while
there is no other known or even conceivable cause that could have produced
them. Whenever the Swiss glaciers retreat a little, as they sometimes do, the
rocks in the bed of the valley they have passed over are found to be
rounded, grooved, and striated just as are those of Wales and Scotland. The
two sets of phenomena are so exactly identical that no one who has ever
compared them can doubt that they are due to the same causes. But we have
further and even more convincing evidence. Glaciers produce many other
effects besides these two, and whatever effects they produce in Switzerland,
in Norway, or in Greenland, we find examples of similar effects having
been produced in our own country. The most striking of these are moraines
and travelled blocks.
Moraines.—Almost every existing glacier carries down with it great masses
of rock, stones, and earth, which fall on its surface from the precipices and
mountain slopes which hem it in, or the rocky peaks which rise above it. As
the glacier slowly moves downward, this débris forms long lines on each
side, or on the centre whenever two glacier-streams unite, and is deposited

at its termination in a huge mound called the terminal moraine. The
decrease of a glacier may often be traced by successive old moraines across
the valley up which it has retreated. When once seen and examined, these
moraines can always be distinguished almost at a glance. Their position is
most remarkable, having no apparent natural relation to the form of the
valley or the surrounding slopes, so that they look like huge earthworks
formed by man for purposes of defence. Their composition is equally
peculiar, consisting of a mixture of earth and rocks of all sizes, usually
without any arrangement, the rocks often being huge angular masses just as
they had fallen from the surrounding precipices. Some of these rock masses
often rest on the very top of the moraine in positions where no other natural
force but that of ice could have placed them. Exactly similar mounds are
found in the valleys of North Wales and Scotland, and always where the
other evidences of ice-action occur abundantly.

A GLACIER WITH MORAINES.

Travelled Blocks.—The phenomenon of travelled or perched blocks is also
a common one in all glacier countries, marking out very clearly the former
extent of the ice. When a glacier fills a lateral valley, its foot will sometimes
cross over the main valley and abut against its opposite slope, and it will
deposit there some portion of its terminal moraine. But in these
circumstances the end of the glacier not being confined laterally will spread

out, and the moraine matter will be distributed over a large surface, so that
the only well-marked token of its presence will be the larger masses of rock
that may have been brought down. Such blocks are found abundantly in
many of the districts of our own country where other marks of glaciation
exist, and they often rest on ridges or hillocks over which the ice has
passed, these elevations consisting sometimes of loose material and
sometimes of rock different from that of which the blocks are composed.
These are called travelled blocks, and can almost always be traced to their
source in one of the higher valleys from which the glacier descended. Some
of the most remarkable examples of such travelled blocks are to be found
on the southern slopes of the Jura. These consist of enormous angular
blocks of granite, gneiss, and other crystalline rocks, quite foreign to the
Jura mountains, but exactly agreeing with those of the Alpine range fifty
miles away across the great central valley of Switzerland. One of the largest
of these blocks is forty feet diameter, and is situated 900 feet above the
level of the Lake of Neufchatel. These blocks have been proved by Swiss
geologists to have been brought by the ancient glacier of the Rhone which
was fed by the whole Alpine range from Mont Blanc to the Furka Pass. This
glacier must have been many thousand feet thick at the mouth of the Rhone
valley near the head of the Lake of Geneva, since it spread over the whole
of the great valley of Switzerland, extending from Geneva to Neufchatel,
Berne, and Soleure, and even on the flanks of the Jura, reached a maximum
height of 2,015 feet above the valley. The numerous blocks scattered over
the Jura for a distance of about a hundred miles vary considerably in the
material of which they are composed, but they are found to be each
traceable to a part of the Alps corresponding to their position, on the theory
that they have been brought by a glacier spreading out from the Rhone
valley. Thus, all the blocks situated to the east of a central point G (see
map) can be traced to the eastern side of the Rhone valley (l e d), while
those found towards Geneva have all come from the west side (p h). It is
also very suggestive that the highest blocks on the Jura at G have come
from the eastern shoulder of Mont Blanc in the direct line h B F G. Here the
glacier would naturally preserve its greatest thickness, while as it spread out
eastward and westward it would become thinner. We accordingly find that
the travelled blocks on either side of the central point become lower and
lower, till near Soleure and Geneva they are not more than 500 feet above
the valley. The evidence is altogether so conclusive that, after personal

examination of the district in company with eminent Swiss geologists, Sir
Charles Lyell gave up the view he had first adopted—that the blocks had
been carried by floating ice during a period of submergence—as altogether
untenable.[33]

MAP SHOWING THE COURSE OF THE
ANCIENT GLACIER OF THE RHONE AND
THE DISTRIBUTION OF ERRATIC BLOCKS
ON THE JURA.

The phenomena now described demonstrate a change of climate sufficient
to cover all our higher mountains with perpetual snow, and fill the adjacent
valleys with huge glaciers at least as extensive as those now found in
Switzerland. But there are other phenomena, best developed in the northern
part of our islands, which show that even this state of things was but the
concluding phase of the glacial period, which, during its maximum
development, must have reduced the northern half of our island to a
condition only to be paralleled now in Greenland and the Antarctic regions.
As few persons besides professed geologists are acquainted with the weight
of evidence for this statement, and as it is most important for our purpose to
understand the amount of the climatal changes the northern hemisphere has
undergone, I will endeavour to make the evidence intelligible, referring my

readers for full details to Dr. James Geikie's descriptions and illustrations.
[34]

Glacial Deposits of Scotland: the "Till."—Over almost all the lowlands and
in most of the highland valleys of Scotland there are immense superficial
deposits of clay, sand, gravel, or drift, which can be traced more or less
directly to glacial action. Some of these are moraine matter, others are
lacustrine deposits, while others again have been formed or modified by the
sea during periods of submergence. But below them all, and often resting
directly on the rock-surface, there are extensive layers of a very tough
clayey deposit known as "till." The till is very fine in texture, very
tenacious, and often of a rock-like hardness. It is always full of stones, all of
which are of rude form, but with the angles rubbed off, and almost always
covered with scratches and striæ often crossing each other in various
directions. Sometimes the stones are so numerous that there seems to be
only just enough clay to unite them into a solid mass, and they are of all
sizes, from mere grit up to rocks many feet in diameter. The "till" is found
chiefly in the low-lying districts, where it covers extensive areas sometimes
to a depth of a hundred feet; while in the highlands it occurs in much
smaller patches, but in some of the broader valleys forms terraces which
have been cut through by the streams. Occasionally it is found as high as
two thousand feet above the sea, in hollows or hill-sides, where it seems to
have been protected from denudation.
The "till" is totally unstratified, and the rock-surfaces on which it almost
always rests are invariably worn smooth, and much grooved and striated
when the rock is hard; but when it is soft or jointed, it frequently shows a
greatly broken surface. Its colour and texture, and the nature of the stones it
contains, all correspond to the character of the rock of the district where it
occurs, so that it is clearly a local formation. It is often found underneath
moraines, drift, and other late glacial deposits, but never overlies them
(except in special cases to be hereafter referred to), so that it is certainly an
earlier deposit.
Throughout Scotland, where "till" is found, the glacial striæ, perched
blocks, roches moutonnées, and other marks of glacial action, occur very
high up the mountains to at least 3,000 and often even to 3,500 feet above
the sea, while all lower hills and mountains are rounded and grooved on

their very summits; and these grooves always radiate outwards from the
highest peaks and ridges towards the valleys or the sea.
Inferences from the Glacial Phenomena of Scotland.—Now all these
phenomena taken together render it certain that the whole of Scotland was
once buried in a vast sea of ice, out of which only the highest mountains
raised their summits. There is absolutely no escape from this conclusion; for
the facts which lead to it are not local—found only in one spot or one valley
—but general throughout the entire length and breadth of Scotland; and are
besides supported by such a mass of detailed corroborative evidence as to
amount to absolute demonstration. The weight of this vast ice-sheet, at least
three thousand feet in maximum thickness, and continually moving seaward
with a slow grinding motion like that of all existing glaciers, must have
ground down the whole surface of the country, especially all the
prominences, leaving the rounded rocks as well as the grooves and striæ we
still see marking the direction of its motion. All the loose stones and rock-
masses which lay on the surface would be pressed into the ice; the harder
blocks would serve as scratching and grinding tools, and would thus
themselves become rounded, scratched, and striated, as we see them, while
all the softer masses would be ground up into impalpable mud along with
the material planed off the rocky projections of the country, leaving them in
the condition of roches moutonnées.
The peculiar characters of the "till," its fineness and tenacity, correspond
closely with the fine matter which now issues from under all glaciers,
making the streams milky white, yellow, or brown, according to the nature
of the rock. The sediment from such water is a fine unctuous, sticky deposit,
only needing pressure to form it into a tenacious clay; and when "till" is
exposed to the action of water, it dissolves into a similar soft, sticky,
unctuous mud. The present glaciers of the Alps, being confined to valleys
which carry off a large quantity of drainage water, lose this mud perhaps as
rapidly as it is formed; but when the ice covered the whole country, there
was comparatively little drainage water, and thus the mud and stones
collected in vast compact masses in all the hollows, and especially in the
lower flat valleys, so that, when the ice retreated, the whole country was
more or less covered with it. It was then, no doubt, rapidly denuded by rain
and rivers, but, as we have seen, great quantities remain to the present day
to tell the tale of its wonderful formation.[35]

There is good evidence that, when the ice was at its maximum, it extended
not only over the land, but far out to sea, covering all the Scottish islands,
and stretching in one connected sheet to Ireland and Wales, where all the
evidences of glaciation are as well marked as in Scotland, though the ice
did not of course attain quite so great a thickness.[36]
It is evident that the change of climate requisite to produce such marvellous
effects in the British Isles could not have been local, and we accordingly
find strikingly similar proofs that Scandinavia and all northern Europe have
also been covered with a huge ice-sheet; while we have already seen that a
similar gigantic glacier buried the Alps, carrying granitic blocks to the Jura,
where it deposited them at a height of 3,450 feet above the sea; while to the
south, in the plains of Italy, the terminal moraines left by the retreating
glaciers have formed extensive hills, those of Ivrea the work of the great
glacier from the Val d'Aosta being fifteen miles across and from 700 to
1,500 feet high.
Glacial Phenomena in North America.—In North America the marks of
glaciation are even more extensive and striking than in Europe, stretching
over the whole of Canada and to the south of the great lakes as far as
latitude 39°. There is, in all these countries, a wide-spread deposit like the
"till" of Scotland, produced by the grinding of the great ice-sheet when it
was at its maximum thickness; and also extensive beds of moraine-matter,
true moraines, and travelled blocks, left by the glaciers as they retreated
towards the mountains and finally withdrew into the upland valleys. There
are, also, in Britain, Scandinavia, and North America, proofs of the
submersion of the land beneath the sea to a depth of upwards of a thousand
feet; but this is a subject we need not here enter upon, as our special object
is to show the reality and amount of that wonderful and comparatively
recent change of climate termed the glacial epoch.
Many persons, even among scientific men, who have not given much
attention to the question, look upon the whole subject of the glacial epoch
as a geological theory made to explain certain phenomena which are
otherwise a puzzle; and they would not be much surprised if they were
some day told that it was all a delusion, and that Mr. So-and-so had
explained the whole thing in a much more simple way. It is to prevent my
readers being imposed upon by any such statements or doubts, that I have

given this very brief and imperfect outline of the nature, extent, and
completeness of the evidence on which the existence of the glacial epoch
depends. There is perhaps no great conclusion in any science which rests
upon a surer foundation than this; and if we are to be guided by our reason
at all in deducing the unknown from the known, the past from the present,
we cannot refuse our assent to the reality of the glacial epoch of the
northern hemisphere in all its more important features.
Effects of the Glacial Epoch on Animal Life: Warm and Cold Periods. —It
is hardly necessary to point out what an important effect this great climatal
cycle must have had upon all living things. When an icy mantle crept
gradually over much of the northern hemisphere till large portions of
Europe and North America were reduced to the condition of Greenland
now, the greater part of the animal life must have been driven southward,
causing a struggle for existence which must have led to the extermination of
many forms, and the migration of others into new areas. But these effects
must have been greatly multiplied and intensified if, as there is very good
reason to believe, the glacial epoch itself—or at least the earlier and later
phases of it—consisted of two or more alternations of warm and cold
periods.
The evidence that such was the case is very remarkable. The "till," as we
have seen, could only have been formed when the country was entirely
buried under a large ice-sheet of enormous thickness, and when it must
therefore have been, in all the parts so covered, almost entirely destitute of
animal and vegetable life. But in several places in Scotland fine layers of
sand and gravel with beds of peaty matter, have been found resting on "till"
and again covered by "till." Sometimes these intercalated beds are very thin,
but in other cases they are twenty or thirty feet thick, and in them have been
found remains of the extinct ox, the Irish elk, the horse, reindeer and
mammoth. Here we have evidence of two distinct periods of intense cold,
and an intervening milder period sufficiently prolonged for the country to
become covered with vegetation and stocked with animal life. In some
districts borings have proved the existence of no less than four distinct
formations of "till" separated from each other by beds of sand from two to
twenty feet in thickness.[37] Facts of a similar nature have been observed in
other parts of our islands. In the east of England, Mr. Skertchly (of the
Geological Survey) enumerates four distinct boulder clays with intervening

deposits of gravels and sands.[38] Mr. Searles V. Wood, Jun., classes the
most recent (Hessle) boulder clay as "post-glacial," but he admits an
intervening warmer period, characterised by southern forms of mollusca
and insects, after which glacial conditions again prevailed with northern
types of mollusca.[39] Elsewhere he says: "Looking at the presence of such
fluviatile mollusca as Cyrena fluminalis and Unio littoralis and of such
mammalia as the hippopotamus and other great pachyderms, and of such a
littoral Lusitanian fauna as that of the Selsea bed where it is mixed up with
the remains of some of those pachyderms, as well as of some other features,
it has seemed to me that the climate of the earlier part of the post-glacial
period in England was possibly even warmer than our present climate; and
that it was succeeded by a refrigeration sufficiently severe to cause ice to
form all round our coasts, and glaciers to accumulate in the valleys of the
mountain districts; and that this increased severity of climate was preceded,
and partially accompanied, by a limited submergence, which nowhere
apparently exceeded 300 feet, and reached that amount only in the northern
counties of England."[40] This decided admission of an alternation of warm
and cold climates since the height of the glacial epoch by so cautious a
geologist as Mr. Wood is very important, as is his statement of an
accompanying depression of the land, accompanying the increased cold,
because many geologists maintain that a greater elevation of the land is the
true and sufficient explanation of glacial periods.
Further evidence of this alternation is found both in the Isle of Man and in
Ireland, where two distinct boulder clays have been described with
intervening beds of gravels and sands.
Palæontological Evidence of Alternate Cold and Warm Periods.—
Especially suggestive of a period warmer than the present, immediately
following glacial conditions, is the occurrence of the hippopotamus in
caves, brick-earths, and gravels of palæolithic age. Entire skeletons of this
animal have been found at Leeds in a bed of dark blue clay overlaid by
gravel. Further north at Kirkdale cave, in N. Lat. 54° 15′, remains of the
hippopotamus occur abundantly along with those of the Elephas antiquus,
Rhinoceros hemitœchus, reindeer, bear, horse, and other quadrupeds, and
with countless remains of the hyænas which devoured them; while it has

also been found in cave deposits in Glamorganshire, at Durdham Down
near Bristol, and in the post-Pliocene drifts of England and France.
The fact of the hippopotamus having lived at 54° N. Lat. in England
immediately after the glacial period seems quite inconsistent with a mere
gradual amelioration of climate from that time till the present day. The
entirely tropical distribution of the existing animal and the large quantity of
vegetable food which it requires both indicate a much warmer climate than
now prevails in any part of Europe. The problem, however, is complicated
by the fact that, both in the cave-deposits and river gravels, its remains are
often found associated with those of animals that imply a cold climate, such
as the reindeer, the mammoth, or the woolly rhinoceros. At this time the
British Isles were joined to the Continent, and a great river formed by the
union of the Rhine, the Elbe and all the eastern rivers of England, flowed
northward through what is now the German Ocean. The hippopotamus
appears to have been abundant in Central Europe before the glacial epoch,
but during the height of the cold was probably driven to the south of France,
whence it may have returned by way of the Rhone valley, some of the
tributaries of that river approaching those of the Rhine within a mile or two
a little south-west of Mulhausen, whence it would easily reach Yorkshire.
Professor Boyd Dawkins supposes that at this time our summers were
warm, as in Middle Asia and the United States, while the winters were cold,
and that the southern and northern animals migrated to and fro over the
great plains which extended from Britain to the Continent. The following
extract indicates how such a migration was calculated to bring about the
peculiar association of sub-tropical and arctic forms.
"It must not, however, be supposed that the southern animals migrated from
the Mediterranean area as far north as Yorkshire in the same year, or the
northern as far south as the Mediterranean. There were, as we shall see
presently, secular changes of climate in Pleistocene Europe, and while the
cold was at its maximum the arctic animals arrived at the southern limit,
and while it was at its minimum the spotted hyæna and hippopotamus and
other southern animals roamed to their northern limit. Thus every part of
the middle zone has been successively the frontier between the northern and
southern groups, and consequently their remains are mingled together in the
caverns and river-deposits, under conditions which prove them to have been
contemporaries in the same region. In some of the caverns, such as that of

Kirkdale, the hyæna preyed upon the reindeer at one time of the year and
the hippopotamus at another. In this manner the association of northern and
southern animals may be explained by their migration according to the
seasons; and their association over so wide an area as the middle zone, by
the secular changes of climate by which each part of the zone in turn was
traversed by the advancing and retreating animals."[41]
When we consider that remains of the hippopotamus have been found in the
caves of North Wales and Bristol as well as in those of Yorkshire, associated
in all with the reindeer and in some with the woolly rhinoceros or the
mammoth, and that the animal must have reached these localities by means
of slow-flowing rivers or flooded marshes by very circuitous routes, we
shall be convinced that these long journeys from the warmer regions of
South Europe could not have been made during the short summers of the
glacial period. Thus the very existence of such an animal in such remote
localities closely associated with those implying almost an arctic winter
climate appears to afford a strong support to the argument for the existence
of warm inter-glacial or post-glacial periods.
Evidence of Interglacial Warm Periods on the Continent and in North
America.—Besides the evidence already adduced from our own islands,
many similar facts have been noted in other countries. In Switzerland two
glacial periods are distinctly recognised, between which was a warm period
when vegetation was so luxuriant as to form beds of lignite sufficiently
thick to be worked for coal. The plants found in these deposits are similar to
those now inhabiting Switzerland—pines, oaks, birches, larch, etc., but
numerous animal remains are also found, showing that the country was then
inhabited by an elephant (Elephas antiquus), a rhinoceros (Rhinoceros
megarhinus), the urus (Bos primigenius), the red deer (Cervus elephas), and
the cave-bear, (Ursus spelœus); and there were also abundance of insects.
[42]

In Sweden also there are two "tills," the lower one having been in places
partly broken up and denuded before the upper one was deposited, but no
interglacial deposits have yet been found. In North America more complete
evidence has been obtained. On the shores of Lake Ontario sections are
exposed showing three separate beds of "till" with intervening stratified
deposits, the lower one of which has yielded many plant remains and fresh-

water organisms. These deposits are seen to extend continuously for more
than nine miles, and the fossiliferous interglacial beds attain a thickness of
140 feet. Similar beds have been discovered near Cleveland, Ohio,
consisting, first of "till" at the lake-level, secondly of about 48 feet of sand
and loam, and thirdly of unstratified "till" full of striated stones—six feet
thick.[43] On the other side of the continent, in British Columbia, Mr. G. M.
Dawson, geologist to the North American Boundary Commission, has
discovered similar evidence of two glaciations divided from each other by a
warm period.
This remarkable series of observations, spread over so wide an area, seems
to afford ample proof that the glacial epoch did not consist merely of one
process of change, from a temperate to a cold and arctic climate, which
having reached a maximum, then passed slowly and completely away; but
that there were certainly two, and probably several more alternations of
arctic and temperate climates.
It is evident, however, that if there have been, not two only, but a series of
such alternations of climate, we could not possibly expect to find more than
the most slender indications of them, because each succeeding ice-sheet
would necessarily grind down or otherwise destroy much of the superficial
deposits left by its predecessors, while the torrents that must always have
accompanied the melting of these huge masses of ice would wash away
even such fragments as might have escaped the ice itself. It is a fortunate
thing therefore, that we should find any fragments of these interglacial
deposits containing animal and vegetable remains; and just as we should
expect, the evidence they afford seems to show that the later phase of the
cold period was less severe than the earlier. Of such deposits as were
formed on land during the coming on of the glacial epoch when it was
continually increasing in severity hardly a trace has been preserved, because
each succeeding extension of the ice being greater and thicker than the last,
destroyed what had gone before it till the maximum was reached.
Migrations and Extinction of Organisms caused by the Glacial Epoch.—
Our last glacial epoch was accompanied by at least two considerable
submergences and elevations of the land, and there is some reason to think,
as we have already explained, that the two classes of phenomena are
connected as cause and effect. We can easily see how such repeated

submergences and elevations would increase and aggravate the migrations
and extinctions that a glacial epoch is calculated to produce. We can
therefore hardly fail to be right in attributing the wonderful changes in
animal and vegetable life that have occurred in Europe and N. America
between the Miocene Period and the present day, in part at least, to the two
or more cold epochs that have probably intervened. These changes consist,
first, in the extinction of a whole host of the higher animal forms, and
secondly, in a complete change of types due to extinction and migration,
leading to a much greater difference between the vegetable and animal
forms of the eastern and western hemisphere than before existed. Many
large and powerful mammalia lived in our own country in Pliocene times
and apparently survived a part of the glacial epoch; but when it finally
passed away they too had disappeared, some having become altogether
extinct while others continued to exist in more southern lands. Among the
first class are the sabre-toothed tiger, the extinct Siberian camel
(Merycotherium), three species of elephant, two of rhinoceros, two bears,
five species of deer, and the gigantic beaver; among the latter are the
hyæna, bear, and lion, which are considered to be only varieties of those
which once inhabited Britain. Down to Pliocene times the flora of Europe
was very similar to that which now prevails in Eastern Asia and Eastern
North America. The late Professor Asa Gray has pointed out that hundreds
of species of trees and shrubs of peculiar genera which still flourish in those
countries are now completely wanting in Europe, and there is good reason
to believe that these were exterminated during the glacial period, being cut
off from a southern migration, first by the Alps, and then by the
Mediterranean; whereas in eastern America and Asia the mountain chains
run in a north and south direction, and there is nothing to prevent the flora
from having been preserved by a southward migration into a milder region.
[44]

Our next two chapters will be devoted to a discussion of the causes which
brought about the glacial epoch, and that still more extraordinary climatic
phenomenon—the mild climate and luxuriant vegetation of the Arctic zone.
If my readers will follow me with the care and attention so difficult and
interesting a problem requires and deserves, they will find that I have
grappled with all the more important facts which have to be accounted for,

and have offered what I believe is the first complete and sufficient
explanation of them. The important influence of climatal changes on the
dispersal of animals and plants is a sufficient justification for introducing
such a discussion into the present volume.

CHAPTER VIII

THE CAUSES OF GLACIAL EPOCHS

Various Suggested Causes—Astronomical Causes of Changes of
Climate—Difference of Temperature caused by Varying Distance of
the Sun—Properties of Air and Water, Snow and Ice, in Relation to
Climate—Effects of Snow on Climate—High Land and Great
Moisture Essential to the Initiation of a Glacial Epoch—Perpetual
Snow nowhere Exists on Lowlands—Conditions Determining the
Presence or Absence of Perpetual Snow—Efficiency of Astronomical
Causes in Producing Glaciation—Action of Meteorological causes in
Intensifying Glaciation—Summary of Causes of Glaciation—Effect of
Clouds and Fog in cutting off the Sun's Heat—South Temperate
America as Illustrating the Influence of Astronomical Causes on
Climate—Geographical Changes how far a Cause of Glaciation—Land
acting as a Barrier to Ocean-currents—The theory of Interglacial
Periods and their Probable Character—Probable Effect of Winter in
Aphelion on the Climate of Britain—The Essential Principle of
Climatal Change Restated—Probable Date of the last Glacial Epoch—
Changes of the Sea-level dependent on Glaciation—The Planet Mars
as bearing on the Theory of Excentricity as a Cause of Glacial Epochs.

No less than seven different causes have been at various times advanced to
account for the glacial epoch and other changes of climate which the
geological record proves to have taken place. These, as enumerated by Mr.
Searles V. Wood, Jun., are as follows:—
1. A decrease in the original heat of our planet.

2. Changes in the obliquity of the ecliptic.
3. The combined effect of the precession of the equinoxes and of the
excentricity of the earth's orbit.
4. Changes in the distribution of land and water.
5. Changes in the position of the earth's axis of rotation.
6. A variation in the amount of heat radiated by the sun.
7. A variation in the temperature of space.
Of the above, causes (1) and (2) are undoubted realities; but it is now
generally admitted that they are utterly inadequate to produce the observed
effects. Causes (5) (6) and (7) are all purely hypothetical, for though such
changes may have occurred there is no evidence that they have occurred
during geological time; and it is besides certain that they would not, either
singly or combined, be adequate to explain the whole of the phenomena.
There remain causes (3) and (4), which have the advantage of being
demonstrated facts, and which are universally admitted to be capable of
producing some effect of the nature required, the only question being
whether, either alone or in combination, they are adequate to produce all the
observed effects. It is therefore to these two causes that we shall confine our
inquiry, taking first those astronomical causes whose complex and wide
reaching effects have been so admirably explained and discussed by Dr.
Croll in numerous papers and in his work—"Climate and Time in their
Geological Relations."

DIAGRAM SHOWING THE ALTERED
POSITION OF THE POLES AT INTERVALS
OF 10,500 YEARS PRODUCED BY THE
PRECESSION OF THE EQUINOXES AND
THE MOTION OF THE APHELION; AND

ITS EFFECT ON CLIMATE DURING A
PERIOD OF HIGH EXCENTRICITY.

Astronomical Causes of Changes of Climate.—The earth moves in an
elliptical orbit round the sun, which is situated in one of the foci of the
ellipse, so that the distance of the sun from us varies during the year to a
considerable amount. Strange to say we are now three millions of miles
nearer to the sun in winter than in summer, while the reverse is the case in
the southern hemisphere; and this must have some effect in making our
northern winters less severe than those of the south temperate zone. But the
earth moves more rapidly in that part of its orbit which is nearer to the sun,
so that our winter is not only milder, but several days shorter, than that of
the southern hemisphere. The distribution of land and sea and other local
causes prevent us from making any accurate estimate of the effects due to
these differences; but there can be no doubt that if our winter were as long
as our summer is now and we were also three million miles further from the
sun at the former period, a very decided difference of climate would result
—our winter would be colder and longer, our summer hotter and shorter.
Now there is a combination of astronomical revolutions (the precession of
the equinoxes and the motion of the aphelion) which actually brings this
change about every 10,500 years, so that after this interval the condition of
the two hemispheres is reversed as regards nearness to the sun in summer,
and comparative duration of summer and winter; and this change has been
going on throughout all geological periods. (See Diagram.) The influence of
the present phase of precession is perhaps seen in the great extension of the
antarctic ice-fields, and the existence of glaciers at the sea-level in the
southern hemisphere, in latitudes corresponding to that of England; but it is
not supposed that similar effects were produced with us at the last cold
period, 10,500 years ago, because we are exceptionally favoured, by the
Gulf-stream warming the whole North Atlantic ocean and by the prevalence
of westerly winds which convey that warmth to our shores; and also by the
comparatively small quantity of high land around the North Pole which
does not encourage great accumulations of ice. But besides this change in
the relation of our seasons to the earth's aphelion and perihelion there is
another and still more important astronomical factor in the change of
magnitude of the excentricity itself. This varies very largely, though very
slowly, and it is now nearly at a minimum. It also varies very irregularly;

but its amount has been calculated for several million years back. Fifty
thousand years ago it was rather less than it is now, but it then increased,
and when we come to a hundred thousand years ago there is a difference of
eight and a half millions of miles between our distance from the sun in
aphelion and perihelion (as the most distant and nearest points of the earth's
orbit are termed). At a hundred and fifty thousand years back it had
decreased somewhat—to six millions of miles; but then it increased again,
till at two hundred thousand years ago it was ten and a quarter, and at two
hundred and ten thousand years ten and a half millions of miles. By
reference to the accompanying diagram, which includes the last great period
of excentricity, we find, that for the immense period of a hundred and sixty
thousand years (commencing about eighty thousand years ago) the
excentricity was very great, reaching a maximum of three and a half times
its present amount at almost the remotest part of this period, at which time
the length of summer in one hemisphere and of winter in the other would be
nearly twenty-eight days in excess. Now, during all this time, our position
would change, as above described (and as indicated on the diagram), every
ten thousand five hundred years; so that we should have alternate periods of
very long and cold winters with short hot summers, and short mild winters
with long cool summers. In order to understand the important effects which
this would produce we must ascertain two things—first, what actual
difference of temperature would be caused by varying distances of the sun,
and, secondly, what are the properties of snow and ice in regard to climate.

DIAGRAM OF EXCENTRICITY AND PRECESSION.
The dark and light bands mark the phases of precession, the dark showing short mild
winters, and the light long cold winters, the contrast being greater as the excentricity is
higher. The horizontal dotted line shows the amount of the present excentricity. The
figures show the maxima and minima of excentricity during the last 300,000 years from
Dr. Croll's Tables.

Differences of Temperature Caused by Varying Distances of the Sun.—On
this subject comparatively few persons have correct ideas owing to the
unscientific manner in which we reckon heat by our thermometers. The zero
of Fahrenheit's thermometer is thirty-two degrees below the freezing point
of water, and that of the centigrade thermometer, the freezing point itself,
both of which are equally misleading when applied to cosmical problems. If
we say that the mean temperature of a place is 50° F., or 10° C., these
figures tell us nothing of how much the sun warms that place, because if the
sun were withdrawn the temperature would fall far below either of the zero
points. In the last Arctic Expedition a temperature of -74° F. was registered,
or 106° below the freezing point of water; and as at the same time the earth,
at a depth of two feet, was only, -13° F. and the sea water +28° F., both
influencing the temperature of the air, we may be sure that even this intense
cold was not near the possible minimum temperature. By various
calculations and experiments which cannot be entered upon here, it has
been determined that the temperature of space, independent of solar (but not
of stellar) influence, is about -239° F., and physicists almost universally

adopt this quantity in all estimates of cosmical temperature. It follows, that
if the mean temperature of the earth's surface at any time is 50° F. it is really
warmed by the sun to an amount measured by 50 + 239 = 289° F., which is
hence termed its absolute temperature. Now during the time of the glacial
epoch the greatest distance of the sun in winter was 98¼ millions of miles,
whereas it is now, in winter, only 91½ millions of miles, the mean distance
being taken as 93 million miles. But the quantity of heat received from the
sun is inversely as the square of the distance, so that it would then be in the
proportion of 8,372 to 9,613 now, or nearly one seventh less than its present
amount. The mean temperature of England in January is about 37° F., which
equals 276° F. of absolute temperature. But the above-named fraction of
276° is 237, the difference, 39, representing the amount which must be
deducted to obtain the January temperature during the glacial epoch, which
will therefore be -2° F. But this is a purely theoretic result. The actual
temperature at that time might have been very different from this, because
the temperature of a place does not depend so much on the amount of heat it
receives directly from the sun, as on the amount brought to it or carried
away from it by warm or cold winds. We often have it bitterly cold in the
middle of May when we are receiving as much sun heat as many parts of
the tropics, but we get cold winds from the iceberg-laden North Atlantic,
and this largely neutralises the effect of the sun. So we often have it very
mild in December if south-westerly winds bring us warm moist air from the
Gulf-stream. But though the above method does not give correct results for
any one time or place, it will be more nearly correct for very large areas,
because all the sensible surface-heat which produces climates necessarily
comes from the sun, and its proportionate amount may be very nearly
calculated in the manner above described. We may therefore say, generally,
that during our winter, at the time of the glacial epoch, the northern
hemisphere was receiving so much less heat from the sun as was calculated
to lower its surface temperature on an average about 39° F., while during
the height of summer of the same period it would be receiving so much
more heat as would suffice, other conditions being equal, to raise its mean
temperature about 48° above what it is now. The winter, moreover, would
be long and the summer short, the difference being twenty-six days.
We have here certainly an amount of cold in winter amply sufficient to
produce a glacial period,[45] especially as this cold would be long

continued; but at the same time we should have almost tropical heat in
summer, although that season would be somewhat shorter. How then, it may
be asked, could such a climate have the effect supposed? Would not the
snow that fell in winter be all melted by the excessively hot summer? In
order to answer this question we must take account of certain properties of
water and air, snow and ice, to which due weight has not been given by
writers on this subject.
Properties of Air and Water, Snow and Ice, in Relation to Climate.—The
great aerial ocean which surrounds us has the wonderful property of
allowing the heat-rays from the sun to pass through it without its being
warmed by them; but when the earth is heated the air gets warmed by
contact with it, and also to a considerable extent by the heat radiated from
the warm earth, because, although pure dry air allows such dark heat-rays to
pass freely, yet the aqueous vapour and carbonic acid in the air intercept and
absorb them. But the air thus warmed by the earth is in continual motion
owing to changes of density. It rises up and flows off, owing to the greater
weight of the cooler air which forces it up and takes its place; and thus heat
can never accumulate in the atmosphere beyond a very moderate degree,
the excessive sun-heat of the tropics being much of it carried away to the
upper atmosphere and radiated into space. Water also is very mobile; and
although it receives and stores up a great deal of heat, it is for ever
dispersing it over the earth. The rain which brings down a certain portion of
heat from the atmosphere, and which often absorbs heat from the earth on
which it falls, flows away in streams to the ocean; while the ocean itself,
constantly impelled by the winds, forms great currents, which carry off the
surplus heated water of the tropics to the temperate and even to the polar
regions, while colder water flows from the poles to ameliorate the heat of
the tropics. An immense quantity of sun-heat is also used up in evaporating
water, and the vapour thus produced is conveyed by the aerial currents to
distant countries, where, on being condensed into rain, it gives up much of
this heat to the earth and atmosphere.
The power of water in carrying away heat is well exhibited by the fact of
the abnormally high temperature of arid deserts and of very dry countries
generally; while the still more powerful influence of moving air may be
appreciated, by considering the effects of even our northern sun in heating a
tightly-closed glass house to far above the temperature produced by the

vertical sun of the equator where the free air and abundance of moisture
exert their beneficial influence. Were it not for the large proportion of the
sun's heat carried away by air and water the tropics would become
uninhabitable furnaces—as would indeed any part of the earth where the
sun shone brightly throughout a summer's day.
We see, therefore, that the excess of heat derived from the sun at any place
cannot be stored up to an important amount owing to the wonderful
dispersing agency of air and water; and though some heat does penetrate the
ground and is stored up there, this is so little in proportion to the whole
amount received, and the larger part of it is so soon given out from the
surface layers, that any surplus heat that may be thus preserved during one
summer of the temperate zones rarely or never remains in sufficient
quantity to affect the temperature of the succeeding summer, so that there is
no such thing as an accumulation of earth-heat from year to year. But,
though heat cannot, cold can be stored up to an almost unlimited amount,
owing to the peculiar property water possesses of becoming solid at a
moderately low temperature; and as this is a subject of the very greatest
importance to our inquiry—the whole question of the possibility of glacial
epochs and warm periods depending on it—we must consider it in some
detail.
Effects of Snow on Climate.—Let us then examine the very different effects
produced by water falling as a liquid in the form of rain, or as a solid in the
form of snow, although the two may not differ from each other more than
two or three degrees in temperature. The rain, however much of it may fall,
runs off rapidly into streams and rivers, and soon reaches the ocean, a small
portion only sinking into the earth and another portion evaporating into the
atmosphere. If cold it cools the air and the earth somewhat while passing
through or over them, but produces no permanent effect on temperature,
because a few hours of sunshine restore to the air or the surface-soil all the
heat they had lost. But if snow falls for a long time, the effect, as we all
know, is very different, because it has no mobility. It remains where it fell
and becomes compacted into a mass, and it then keeps the earth below it
and the air above, at or near the freezing-point till it is all melted. If the
quantity is great it may take days or weeks to melt; and if snow continues
falling it goes on accumulating all over the surface of a country (which
water cannot do), and may thus form such a mass that the warmth of the

whole succeeding summer may not be able to melt it. It then produces
perpetual snow, such as we find above a certain altitude on all the great
mountains of the globe; and when this takes place cold is rendered
permanent, no amount of sun-heat warming the air or the earth much above
the freezing-point. This is illustrated by the often-quoted fact that, at 80° N.
Lat., Captain Scoresby had the pitch melted on one side of his ship by the
heat of the sun, while water was freezing on the other side owing to the
coldness of the air.
The quantity of heat required to melt ice or snow is very great, as we all
know by experience of the long time masses of snow will remain unmelted
even in warm weather. We shall however be better able to appreciate the
great effect this has upon climate, by a few figures showing what this
amount really is. In order to melt one cubic foot of ice, as much heat is
required as would heat a cubic foot of water from the freezing point to 176°
F., or two cubic feet to 88° F. To melt a layer of ice a foot thick will
therefore use up as much heat as would raise a layer of ice-cold water two
feet thick to the temperature of 88° F.; and the effect becomes still more
easily understood if we estimate it as applied to air, for to melt a layer of ice
only 1½ inches thick would require as much heat as would raise a stratum
of air 800 feet thick from the freezing point to the tropical heat of 88° F.!
We thus obtain a good idea, both of the wonderful power of snow and ice in
keeping down temperature, and also of the reason why it requires so long a
time to melt away, and is able to go on accumulating to such an extent as to
become permanent. These properties would, however, be of no avail if it
were liquid, like water; hence it is the state of solidity and almost complete
immobility of ice that enables it to produce by its accumulation such
extraordinary effects in physical geography and in climate, as we see in the
glaciers of Switzerland and the ice-capped interior of Greenland.
High Land and great Moisture Essential to the Initiation of a Glacial
Epoch.—Another point of great importance in connection with this subject,
is the fact, that this permanent storing up of cold depends entirely on the
annual amount of snow-fall in proportion to that of the sun and air-heat, and
not on the actual cold of winter, or even on the average cold of the year.[46]
A place may be intensely cold in winter and may have a short arctic
summer, yet, if so little snow falls that it is quickly melted by the returning
sun, there is nothing to prevent the summer being hot and the earth

producing a luxuriant vegetation. As an example of this we have great
forests in the extreme north of Asia and America where the winters are
colder and the summers shorter than in Greenland in Lat. 62° N., or than in
Heard Island and South Georgia, both in Lat. 53° S. in the Southern Ocean,
and almost wholly covered with perpetual snow and ice. At the "Jardin" on
the Mount Blanc range, above the line of perpetual snow, a thermometer in
an exposed situation marked -6° F. as the lowest winter temperature: while
in many parts of Siberia mercury freezes during several weeks in winter,
showing a temperature below -40° F.; yet here the summers are hot, all the
snow disappears, and there is a luxuriant vegetation. Even in the very
highest latitudes reached by our last Arctic Expedition there is very little
perpetual snow or ice, for Captain Nares tells us that north of Haye's Sound,
in Lat. 79° N., the mountains were remarkably free from ice-cap, while
extensive tracts of land were free from snow during summer, and covered
with a rich vegetation with abundance of bright flowers. The reason of this
is evidently the scanty snow-fall, which rendered it sometimes difficult to
obtain enough to form shelter-banks around the ships; and this was north of
80° N. Lat., where the sun was absent for 142 days.
Perpetual Snow Nowhere Exists on Lowland Areas.—It is a very
remarkable and most suggestive fact, that nowhere in the world at the
present time are there any extensive lowlands covered with perpetual snow.
The Tundras of Siberia and the barren grounds of N. America are all clothed
with some kind of summer vegetation;[47] and it is only where there are
lofty mountains or plateaus—as in Greenland, Spitzbergen, and Grinnell's
Land—that glaciers, accompanied by perpetual snow, cover the country,
and descend in places to the level of the sea. In the Antarctic regions there
are extensive highlands and lofty mountains, and these are everywhere
exposed to the influence of moist sea-air; and it is here, accordingly, that we
find the nearest approach to a true ice-cap covering the whole
circumference of the Antarctic continent, and forming a girdle of ice-cliffs
which almost everywhere descend to the sea. Such Antarctic islands as
South Georgia, South Shetland, and Heard Island, are often said to have
perpetual snow at sea-level; but they are all very mountainous, and send
down glaciers into the sea, and as they are exposed to moist sea-air on every
side, the precipitation, almost all of which takes the form of snow even in
summer, is of course unusually large.[48]

That high land in an area of great precipitation is the necessary condition of
glaciation, is well shown by the general state of the two polar areas at the
present time. The northern part of the north temperate zone is almost all
land, mostly low but with elevated borders; while the polar area is, with the
exception of Greenland and a few other considerable islands, almost all
water. In the southern hemisphere the temperate zone is almost all water,
while the polar area is almost all land, or is at least inclosed by a ring of
high and mountainous land. The result is that in the north the polar area is
free from any accumulation of permanent ice (except on the highlands of
Greenland and Grinnell's Land), while in the south a complete barrier of ice
of enormous thickness appears to surround the pole. Dr. Croll shows, from
the measured height of numerous Antarctic icebergs (often miles in length)
that the ice-sheet from which they are the broken outer fragments must be
from a mile to a mile and a half in thickness.[49] As this is the thickness of
the outer edge of the ice it must be far thicker inland; and we thus find that
the Antarctic continent is at this very time suffering glaciation to quite as
great an extent as we have reason to believe occurred in the same latitudes
of the northern hemisphere during the last glacial epoch.
The accompanying diagrams show the comparative state of the two polar
areas both as regards the distribution of land and sea, and the extent of the
ice-sheet and floating icebergs. The much greater quantity of ice at the
south pole is undoubtedly due to the presence of a large extent of high land,
which acts as a condenser, and an unbroken surrounding ocean, which
affords a constant supply of vapour; and the effect is intensified by winter
being there in aphelion, and thus several days longer than with us, while the
whole southern hemisphere is at that time farther from the sun, and
therefore receives less heat.
We see, however, that with less favourable conditions for the production
and accumulation of ice, Greenland is glaciated down to Lat. 61°. What,
then, would be the effect if the Antarctic continent, instead of being
confined almost wholly within the south polar circle, were to extend in one
or two great mountainous promontories far into the temperate zone? The
comparatively small Heard Island in S. Lat. 53° is even now glaciated down
to the sea. What would be its condition were it a northerly extension of a
lofty Antarctic continent? We may be quite sure that glaciation would then
be far more severe, and that an ice-sheet corresponding to that of Greenland

might extend to beyond the parallel of 50° S. Lat. Even this is probably
much too low an estimate, for on the west coast of New Zealand in S. Lat.
43° 35′ a glacier even now descends to within 705 feet of the sea-level; and
if those islands were the northern extension of an Antarctic continent, we
may be pretty sure that they would be nearly in the ice-covered condition of
Greenland, although situated in the latitude of Marseilles.

Diagram of the approximate extent of Permanent and Floating Ice around the North and the
South Poles. (After Petermann.)

Conditions Determining the Presence or Absence of Perpetual Snow.—It is
clear, then, that the vicinity of a sea or ocean to supply moisture, together
with high land to serve as a condenser of that moisture into snow, are the
prime essentials of a great accumulation of ice; and it is fully in accordance
with this view that we find the most undoubted signs of extensive glaciation
in the west of Europe and the east of North America, both washed by the
Atlantic and both having abundance of high land to condense the moisture
which it supplies. Without these conditions cold alone, however great, can
produce no glacial epoch. This is strikingly shown by the fact, that in the
very coldest portions of the two northern continents—Eastern Siberia and
the north-western shores of Hudson's Bay—there is no perennial covering
of snow or ice whatever. No less remarkable is the coincidence of the
districts of greatest glaciation with those of greatest rainfall at the present
time. Looking at a rain-map of the British Isles, we see that the greatest area
of excessive rainfall is the Highlands of Scotland, then follows the west of
Ireland, Wales, and the north of England; and these were glaciated pretty
nearly in proportion to the area of country over which there is an abundant
supply of moisture. So in Europe, the Alps and the Scandinavian mountains
have excessive rainfall, and have been areas of excessive glaciation, while
the Ural and Caucasian mountains, with less rain, never seem to have been
proportionally glaciated. In North America the eastern coast has an

abundant rainfall, and New England with North-eastern Canada seems to
have been the source of much of the glaciation of that continent.[50]
The reason why no accumulation of snow or ice ever takes place on Arctic
lowlands is explained by the observations of Lieut. Payer of the Austrian
Polar Expedition, who found that during the short Arctic summer of the
highest latitudes the ice-fields diminished four feet in thickness under the
influence of the sun and wind. To replace this would require a precipitation
of snow equivalent to about 45 inches of rain, an amount which rarely
occurs in lowlands out of the tropics. In Siberia, within and near the Arctic
circle, about six feet of snow covers the country all the winter and spring,
and is not sensibly diminished by the powerful sun so long as northerly
winds keep the air below the freezing-point and occasional snow-storms
occur. But early in June the wind usually changes to southerly, probably the
south-western anti-trades overcoming the northern inflow; and under its
influence the snow all disappears in a few days and the vegetable kingdom
bursts into full luxuriance. This is very important as showing the impotence
of mere sun-heat to get rid of a thick mass of snow so long as the air
remains cold, while currents of warm air are in the highest degree effective.
If, however, they are not of sufficiently high temperature or do not last long
enough to melt the snow, they are likely to increase it, from the quantity of
moisture they bring with them which will be condensed into snow by
coming into contact with the frozen surface. We may therefore expect the
transition from perpetual snow to a luxuriant arctic vegetation to be very
abrupt, depending as it must on a few degrees more or less in the summer
temperature of the air; and this is quite in accordance with the fact of corn
ripening by the sides of alpine glaciers.
Efficiency of Astronomical Causes in Producing Glaciation.—Having now
collected a sufficient body of facts, let us endeavour to ascertain what
would be the state to which the northern hemisphere would be reduced by a
high degree of excentricity and a winter in aphelion. When the glacial
epoch is supposed to have been at its maximum, about 210,000 years ago,
the excentricity was more than three times as great as it is now, and,
according to Dr. Croll's calculations, the mid-winter temperature of the
northern hemisphere would have been lowered 36° F., while the winter half
of the year would have been twenty-six days longer than the summer half.
This would bring the January mean temperature of England and Scotland

almost down to zero or about 30° F. of frost, a winter climate corresponding
to that of Labrador, or the coast of Greenland on the Arctic circle. But we
must remember that the summer would be very much hotter than it is now,
and the problem to be solved is, whether, supposing the geography of the
northern hemisphere to have been identical with what it is now, the snow
that fell in winter would accumulate to such an extent that it would not be
melted in summer, and so go on increasing year by year till it covered the
whole of Scotland, Ireland, and Wales, and much of England. Dr. Croll and
Dr. Geikie answer that it would. Sir Charles Lyell maintained that it would
only do so if geographical conditions were then more favourable than they
are now; while the late Mr. Belt has argued, that excentricity alone would
not produce the effect unless aided by increased obliquity of the ecliptic,
which, by extending the width of the polar regions, would increase the
duration and severity of the winter to such an extent that snow and ice
would be formed in the Arctic and Antarctic regions at the same time
whether the winter were in perihelion or aphelion.[51]
The problem we have now to solve is a very difficult one, because we have
no case at all parallel to it from which we can draw direct conclusions. It is,
however, clear from the various considerations we have already adduced,
that the increased cold of winter when the excentricity was great and the
sun in aphelion during that season, would not of itself produce a glacial
epoch unless the amount of vapour supplied for condensation was also
exceptionally great. The greatest quantity of snow falls in the Arctic regions
in summer and autumn, and with us the greatest quantity of rain falls in the
autumnal months. It seems probable, then, that in all northern lands
glaciation would commence when autumn occurred in aphelion. All the rain
which falls on our mountains at that season would then fall as snow, and,
being further increased by the snow of winter, would form accumulations
which the summer might not be able to melt. As time went on, and the
aphelion occurred in winter, the perennial snow on the mountains would
have accumulated to such an extent as to chill the spring and summer
vapours, so that they too would fall as snow, and thus increase the amount
of deposition; but it is probable that this would never in our latitudes have
been sufficient to produce glaciation, were it not for a series of climatal
reactions which tend still further to increase the production of snow.

Action of Meteorological Causes in intensifying Glaciation.—The trade-
winds owe their existence to the great difference between the temperature
of the equator and the poles, which causes a constant flow of air towards the
equator. The strength of this flow depends on the difference of temperature
and the extent of the cooled and heated masses of air, and this effect is now
greatest between the south pole and the equator, owing to the much greater
accumulation of ice in the Antarctic regions. The consequence is, that the
south-east trades are stronger than the north-east, the neutral zone or belt of
calms between them not being on the equator but several degrees to the
north of it. But just in proportion to the strength of the trade-winds is the
strength of the anti-trades, that is, the upper return current which carries the
warm moisture-laden air of the tropics towards the poles, descending in the
temperate zone as west and south-west winds. These are now strongest in
the southern hemisphere, and, passing everywhere over a wide ocean, they
supply the moisture necessary to produce the enormous quantity of snow
which falls in the Antarctic area. During the period we are now discussing,
however, this state of things would have been partially reversed. The south
polar area, having its winter in perihelion, would probably have had less
ice, while the north-temperate and Arctic regions would have been largely
ice-clad; and the north-east trades would therefore be stronger than they are
now. The south-westerly anti-trades would also be stronger in the same
proportion, and would bring with them a greatly increased quantity of
moisture, which is the prime necessity to produce a condition of glaciation.
But this is only one-half of the effect that would be produced, for the
increased force of the trades sets up another action which still further helps
on the accumulation of snow and ice. It is now generally admitted that we
owe much of our mild climate and our comparative freedom from snow to
the influence of the Gulf Stream, which also ameliorates the climate of
Scandinavia and Spitzbergen, as shown by the remarkable northward
curvature of the isothermal lines, so that Drontheim in N. Lat. 62° has the
same mean temperature as Halifax (Nova Scotia) in N. Lat. 45°. The
quantity of heat now brought into the North Atlantic by the Gulf Stream
depends mainly on the superior strength of the south-east trades. When the
north-east trades were the more powerful, the Gulf Stream would certainly
be of much less magnitude and velocity; while it is possible, as Dr. Croll
thinks, that a large portion of it might be diverted southward owing to the

peculiar form of the east coast of South America, and so go to swell the
Brazilian current and ameliorate the climate of the southern hemisphere.
That effects of this nature would follow from any increase of the Arctic, and
decrease of the Antarctic ice, may be considered certain; and Dr. Croll has
clearly shown that in this case cause and effect act and react on each other
in a remarkable way. The increase of snow and ice in the northern
hemisphere is the cause of an increased supply of moisture being brought
by the more powerful anti-trades, and this greater supply of moisture leads
to an extension of the ice, which reacts in still further increasing the supply
of moisture. The same increase of snow and ice, by causing the north-east
to be stronger than the south-east trade-winds, diminishes the force of the
Gulf Stream, and this diminution lowers the temperature of the North
Atlantic both in summer and winter, and thus helps on still further the
formation and perpetuation of the icy mantle. It must also be remembered
that these agencies are at the same time acting in a reverse way in the
southern hemisphere, diminishing the supply of the moisture carried by the
anti-trades, and increasing the temperature by means of more powerful
southward ocean-currents;—and all this again reacts on the northern
hemisphere, increasing yet further the supply of moisture by the more
powerful south-westerly winds, while still further lowering the temperature
by the southward diversion of the Gulf Stream.
Summary of Principal Causes of Glaciation.—I have now sufficiently
answered the question, why the short hot summer would not melt the snow
which accumulated during the long cold winter (produced by high
excentricity and winter in aphelion), although the annual amount of heat
received from the sun was exactly the same as it is now, and equal in the
two hemispheres. It may be well, before going further, briefly to summarise
the essential causes of this apparent paradox. These are—primarily, the fact
that solar heat cannot be stored up owing to its being continually carried
away by air and water, while cold can be so stored up owing to the
comparative immobility of snow and ice; and, in the second place, because
the two great heat-distributing agencies, the winds and the ocean-currents,
are so affected by an increase of the snow and ice towards one pole and its
diminution towards the other, as to help on the process when it has once
begun, and by their action and reaction produce a maximum of effect
which, without their aid, would be altogether unattainable.

But even this does not exhaust the causes at work, all tending in one
direction. Snow and ice reflect heat to a much greater degree than do land
or water. The heat, therefore, of the short summer would have far less effect
than is due to its calculated amount in melting the snow, because so much
of it would be lost by reflection. A portion of the reflected heat would no
doubt warm the vapour in the atmosphere, but this heat would be carried off
to other parts of the earth, while a considerable portion of the whole would
be lost in space. It must also be remembered that an enormous quantity of
heat is used up in melting snow and ice, without raising its temperature;
each cubic foot of ice requiring as much heat to melt it as would raise
nearly six cubic feet of water 30° F. It has, however, been argued that
because when water is frozen it evolves just as much heat as it requires to
melt it again, there is no loss of heat on the whole; and as this is adduced
over and over again as a valid argument in every criticism of Dr. Croll's
theory, it may be well to consider it a little more closely. In the act of
freezing no doubt water gives up some of its heat to the surrounding air; but
that air still remains below the freezing point or freezing would not take
place. The heat liberated by freezing is, therefore, what may be termed low-
grade heat—heat incapable of melting snow or ice; while the heat absorbed
while ice or snow is melting is high-grade heat, such as is capable of
melting snow and supporting vegetable growth. Moreover, the low-grade
heat liberated in the formation of snow is usually liberated high up in the
atmosphere, where it may be carried off by winds to more southern
latitudes, while the heat absorbed in melting the surface of snow and ice is
absorbed close to the earth and is thus prevented from warming the lower
atmosphere, which is in contact with vegetation. The two phenomena,
therefore, by no means counterbalance or counteract each other, as it is so
constantly and superficially asserted that they do.
Effect of Clouds and Fog in cutting off the Sun's Heat.—Another very
important cause of diminution of heat during summer in a glaciated country
would be the intervention of clouds and fogs, which would reflect or absorb
a large proportion of the sun-heat and prevent it reaching the surface of the
earth; and such a cloudy atmosphere would be a necessary result of large
areas of high land covered with snow and ice. That such a prevalence of
fogs and cloud is an actual fact in all ice-clad countries has been shown by
Dr. Croll most conclusively, and he has further shown that the existence of

perpetual snow often depends upon it. South Georgia in the latitude of
Yorkshire is almost, and Sandwich Land in the latitude of the north of
Scotland, is entirely covered with perpetual snow; yet in their summer the
sun is three million miles nearer the earth than it is in our summer, and the
heat actually received from the sun must be sufficient to raise the
temperature 20° F. higher than in the same latitudes in the northern
hemisphere, were the conditions equal—instead of which their summer
temperature is probably full 20° lower. The chief cause of this can only be
that the heat of the sun does not reach the surface of the earth; and that this
is the fact is testified by all Antarctic voyagers. Darwin notes the cloudy sky
and constant moisture of the southern part of Chile, and in his remarks on
the climate and productions of the Antarctic islands he says: "In the
Southern Ocean the winter is not so excessively cold, but the summer is far
less hot (than in the north), for the clouded sky seldom allows the sun to
warm the ocean, itself a bad absorbent of heat; and hence the mean
temperature of the year, which regulates the zone of perpetually congealed
under-soil, is low." Sir James Ross, Lieutenant Wilkes, and other Antarctic
voyagers speak of the snow-storms, the absence of sunshine, and the
freezing temperature in the height of summer; and Dr. Croll shows that this
is a constant phenomenon accompanying the presence of large masses of
ice in every part of the world.[52]
In reply to the objections of a recent critic Dr. Croll has given a new proof
of this important fact by comparing the known amount of snow-fall with the
equally well-known melting power of direct sun-heat in different latitudes.
He says: "The annual precipitation on Greenland in the form of snow and
rain, according to Dr. Rink, amounts to only twelve inches, and two inches
of this he considers is never melted, but is carried away in the form of
icebergs. The quantity of heat received at the equator from sunrise to sunset,
if none were cut off by the atmosphere, would melt 3⅓ inches of ice, or 100
feet in a year. The quantity received between latitude 60° and 80°, which is
that of Greenland, is, according to Meech, one-half that received at the
equator. The heat received by Greenland from the sun, if none were cut off
by the atmosphere, would therefore melt fifty feet of ice per annum, or fifty
times the amount of snow which falls on that continent. What then cuts off
the ninety-eight per cent of the sun's heat?" The only possible answer is,

that it is the clouds and fog during a great part of the summer, and reflection
from the surface of the snow and ice when these are absent.
South Temperate America as Illustrating the Influence of Astronomical
Causes on Climate.—Those persons who still doubt the effect of winter in
aphelion with a high degree of excentricity in producing glaciation, should
consider how the condition of south temperate America at the present day is
explicable if they reject this agency. The line of perpetual snow in the
Southern Andes is so low as 6,000 feet in the same latitude as the Pyrenees;
in the latitude of the Swiss Alps mountains only 6,200 feet high produce
immense glaciers which descend to the sea-level; while in the latitude of
Cumberland mountains only from 3,000 to 4,000 feet high have every
valley filled with streams of ice descending to the sea-coast and giving off
abundance of huge icebergs.[53] Here we have exactly the condition of
things to which England and Western Europe were subjected during the
latter portion of the glacial epoch, when every valley in Wales, Cumberland,
and Scotland had its glacier; and to what can this state of things be imputed
if not to the fact that there is now a moderate amount of excentricity, and
the winter of the southern hemisphere is in aphelion? The mere
geographical position of the southern extremity of America does not seem
especially favourable to the production of such a state of glaciation. The
land narrows from the tropics southwards and terminates altogether in about
the latitude of Edinburgh; the mountains are of moderate height; while
during summer the sun is three millions of miles nearer, and the heat
received from it is equivalent to a rise of 20° F. as compared with the same
season in the northern hemisphere. The only important differences are: the
open southern ocean, the longer and colder winter, and the general low
temperature caused by the south polar ice. But the great accumulation of
south polar ice is itself due to the great extent of high land within the
Antarctic circle acted upon by the long cold winter and furnished with
moisture by the surrounding wide ocean. These conditions of high land and
open ocean we know did not prevail to so great an extent in the northern
hemisphere during the glacial epoch, as they do in the southern hemisphere
at the present time; but the other acting cause—the long cold winter—
existed in a far higher degree, owing to the excentricity being about three
times as much as it is now. It is, so far as we know or are justified in
believing, the only efficient cause of glaciation which was undoubtedly

much more powerful at that time; and we are therefore compelled to accept
it as the most probable cause of the much greater glaciation which then
prevailed.
Geographical Changes, how far a Cause of Glaciation.—Messrs. Croll and
Geikie have both objected to the views of Sir Charles Lyell as to the
preponderating influence of the distribution of land and sea on climate; and
they maintain that if the land were accumulated almost wholly in the
equatorial regions, the temperature of the earth's surface as a whole would
be lowered, not raised, as Sir Charles Lyell maintained. The reason given is,
that the land being heated heats the air, which rises and thus gives off much
of the heat to space, while the same area covered with water would retain
more of the heat, and by means of currents carry it to other parts of the
earth's surface. But although the mean temperature of the whole earth might
be somewhat lowered by such a disposition of the land, there can be little
doubt that it would render all extremes of temperature impossible, and that
even during a period of high excentricity there would be no glacial epochs,
and perhaps no such thing as ice anywhere produced. This would result
from there being no land near the poles to retain snow, while the constant
interchange of water by means of currents between the polar and tropical
regions would most likely prevent ice from ever forming in the sea. On the
other hand, were all the land accumulated in the polar and temperate
regions there can be little doubt that a state of almost perpetual glaciation of
much of the land would result, notwithstanding that the whole earth should
theoretically be at a somewhat higher temperature. Two main causes would
bring about this glaciation. A very large area of elevated land in high
latitudes would act as a powerful condenser of the enormous quantity of
vapour produced by the whole of the equatorial and much of the temperate
regions being areas of evaporation, and thus a greater accumulation of snow
and ice would take place around both poles than would be possible under
any other conditions. In the second place there would be little or no check
to this accumulation of ice, because, owing to the quantity of land around
the polar areas, warm oceanic currents could not reach them, while the
warm winds would necessarily bring so much moisture that they would help
on instead of checking the process of ice-accumulation. If we suppose the
continents to be of the same total area and to have the same extent and
altitude of mountain ranges as the present ones, these mountains must

necessarily offer an almost continuous barrier to the vapour-bearing winds
from the south, and the result would probably be that three-fourths of the
land would be in the ice-clad condition of Greenland, while a comparatively
narrow belt of the more southern lowlands would alone afford habitable
surfaces or produce any woody vegetation.
Notwithstanding, therefore, the criticism above referred to, I believe that Sir
Charles Lyell was substantially right, and that the two ideal maps given in
the Principles of Geology (11th ed. Vol. i. p. 270), if somewhat modified so
as to allow a freer passage of currents in the tropics, do really exhibit a
condition of the earth which, by geographical changes alone, would bring
about a perpetual summer or an almost universal winter. But we have seen
in our sixth chapter that there is the strongest cumulative evidence, almost
amounting to demonstration, that for all known geological periods our
continents and oceans have occupied the same general position they do
now, and that no such radical changes in the distribution of sea and land as
imagined by way of hypothesis by Sir Charles Lyell, have ever occurred.
Such an hypothesis, however, is not without its use in our present inquiry,
for if we obtain thereby a clear conception of the influence of such great
changes on climate, we are the better able to appreciate the tendency of
lesser changes such as have undoubtedly often occurred.
Land as a Barrier to Ocean Currents.—We have seen already the great
importance of elevated land to serve as condensers and ice-accumulators;
but there is another and hardly less important effect that may be produced
by an extension of land in high latitudes, which is, to act as a barrier to the
flow of ocean currents. In the region with which we are more immediately
interested it is easy to see how a comparatively slight alteration of land and
sea, such as has undoubtedly occurred, would produce an enormous effect
on climate. Let us suppose, for instance, that the British Isles again became
continental, and that this continental land extended across the Färoe Islands
and Iceland to Greenland. The whole of the warm waters of the Atlantic,
with the Gulf Stream, would then be shut out from Northern Europe, and
the result would almost certainly be that snow would accumulate on the
high mountains of Scandinavia till they became glaciated to as great an
extent as Greenland, and the cold thus produced would react on our own
country and cover the Grampians with perpetual snow, like mountains of
the same height at even a lower latitude in South America.

If a similar change were to occur on the opposite side of the Atlantic very
different effects would be produced. Suppose, for instance, the east side of
Greenland were to sink considerably, while on the west the sea bottom were
to rise in Davis' Strait so as to unite Greenland with Baffin's Land, thus
stopping altogether the cold Arctic current with its enormous stream of
icebergs from the west coast of Greenland. Such a change might cause a
great accumulation of ice in the higher polar latitudes, but it would certainly
produce a wonderful ameliorating effect on the climate of the east coast of
North America, and might raise the temperature of Labrador to that of
Scotland. Now these two changes have almost certainly occurred, either
together or separately, during the Tertiary period, and they must have had a
considerable effect either in aiding or checking the action of the terrestrial
and astronomical causes affecting climate which were then in operation.
It would be easy to suggest other probable changes which would produce a
marked effect on climate; but we will only refer to the subsidence of the
Isthmus of Panama, which has certainly happened more than once in
Tertiary times. If this subsidence were considerable it would have allowed
much of the accumulated warm water which initiates the Gulf Stream to
pass into the Pacific; and if this occurred while astronomical causes were
tending to bring about a cold period in the northern hemisphere, the
resulting glaciation might be exceptionally severe. The effect of this change
would however be neutralised if at the same epoch the Lesser and Greater
Antilles formed a connected land.
Now, as such possible and even probable geographical changes are very
numerous, they must have produced important effects; and though we may
admit that the astronomical causes already explained were the most
important in determining the last glacial epoch, we must also allow that
geographical changes must often have had an equally important and
perhaps even a preponderating influence on climate. We must also
remember that changes of land and sea are almost always accompanied by
elevation or depression of the pre-existing land: and whereas the former
produces its chief effect by diverting the course of warm or cold oceanic
currents, the latter is of not less importance in adding to or diminishing
those areas of condensation and ice-accumulation which, as we have seen,
are the most efficient agents in producing glaciation.

If then Sir Charles Lyell may have somewhat erred in attaching too
exclusive an importance to geographical changes as bringing about
mutations of climate, his critics have, I think, attached far too little
importance to these changes. We know that they have always been in
progress to a sufficient extent to produce important climatal effects; and we
shall probably be nearest the truth if we consider, that great extremes of
cold have only occurred when astronomical and geographical causes were
acting in the same direction and thus produced a cumulative result, while,
through the agency of warm oceanic currents, the latter alone have been the
chief cause of mild climates in high latitudes, as we shall attempt to prove
in our next chapter.[54]
On the Theory of Inter-glacial Periods and their Probable Character.—The
theory by which the glacial epoch is here explained is one which apparently
necessitates repeated changes from glacial to warm periods, with all the
consequences and modifications both of climate and physical geography
which follow or accompany such changes. It is essentially a theory of
alternation; and it is certainly remarkable in how many cases geologists
have independently deduced some alternations of climate as probable. Such
are the interglacial deposits indicating a mild climate, both in Europe and
America; an early phase of very severe glaciation when the "till" was
deposited, with later less extensive glaciation when moraines were left in
the valleys; several successive periods of submergence and elevation, the
later ones becoming less and less in amount, as indicated by the raised
beaches slightly elevated above our present coast line; and lastly, the
occurrence in the same deposits of animal remains indicating both a warm
and a cold climate, and especially the existence of the hippopotamus in
Yorkshire not long after the period of extreme glaciation.
But although the evidence of some alternations of climate seems
indisputable, and no suggestion of any adequate cause for them other than
the alternating phases of precession during high excentricity has been made,
it by no means follows that these changes were always very great—that is
to say, that the ice completely disappeared and a warm climate prevailed
throughout the whole year. It is quite evident that during the height of the
glacial epoch there was a combination of causes at work which led to a
large portion of North-western Europe and Eastern America being buried in
ice to a greater extent even than Greenland is now, since it certainly

extended beyond the land and filled up all the shallow seas between our
islands and Scandinavia. Among these causes we must reckon a diminution
of the force of the Gulf Stream, or its being diverted from the north-western
coasts of Europe; and what we have to consider is, whether the alteration
from a long cold winter and short hot summer to a short mild winter and
long cool summer would greatly affect the amount of ice if the ocean
currents remained the same. The force of these currents is, it is true, by our
hypothesis, modified by the increase or diminution of the ice in the two
hemispheres alternately, and they then react upon climate; but they cannot
be thus changed till after the ice-accumulation has been considerably
affected by other causes. Their direction may, indeed, be greatly changed by
slight alterations in the outline of the land, while they may be barred out
altogether by other alterations of not very great amount; but such changes
as these have no relation to the alteration of climates caused by the
changing phases of precession.
Now, the existence at the present time of an ice-clad Greenland is an
anomaly in the northern hemisphere, only to be explained by the fact that
cold currents from the polar area flow down both sides of it. In Eastern Asia
we have the lofty Stanivoi Mountains in the same latitude as the southern
part of Greenland, which, though their summits are covered with perpetual
snow, give rise to no ice-sheet, and, apparently, even to no important
glaciers;—a fact undoubtedly connected with the warm Japan current
flowing partially into the Sea of Okhotsk. So in North-west America we
have the lofty coast range, culminating in Mt. St. Elias, nearly 15,000 feet
high, and an extensive tract of high land to the north and north-west, with
glaciers comparable in size with those of New Zealand, although situated in
Lat. 60° instead of in Lat. 45°. Here, too, we have the main body of the
Japan current turning east and south, and thus producing a mild climate,
little inferior to that of Norway, warmed by the Gulf Stream. We thus have
it made clear that could the two Arctic currents be diverted from Greenland,
that country would become free from ice, and might even be completely
forest-clad and inhabitable; while, if the Japan current were to be diverted
from the coast of North America and a cold current come out of Behring's
Strait, the entire northwestern extremity of America would even now
become buried in ice.

Now it is the opinion of the best American geologists that during the height
of the glacial epoch North-eastern America was considerably elevated.[55]
This elevation would bring the wide area of the banks of Newfoundland far
above water, causing the American coast to stretch out in an immense curve
to a point more than 600 miles east of Halifax; and this would certainly
divert much of the greatly reduced Gulf Stream straight across to the coast
of Spain. The consequence of such a state of things would probably be that
the southward flowing Arctic currents would be much reduced in velocity;
and the enormous quantity of icebergs continually produced by the ice-
sheets of all the lands bordering the North Atlantic would hang about their
shores and the adjacent seas, filling them with a dense ice-pack, equalling
that of the Antarctic regions, and chilling the atmosphere so as to produce
constant clouds and fog with almost perpetual snowstorms, even at
midsummer, such as now prevail in the worst portions of the Southern
Ocean.
But when such was the state of the North Atlantic (and, however caused,
such must have been its state during the height of the glacial epoch), can we
suppose that the mere change from the distant sun in winter and near sun in
summer, to the reverse, could bring about any important alteration—the
physical and geographical causes of glaciation remaining unchanged? For,
certainly, the less powerful sun of summer, even though lasting somewhat
longer, could not do more than the much more powerful sun did during the
phase of summer in perihelion, while during the less severe winters the sun
would have far less power than when it was equally near and at a very much
greater altitude in summer. It seems to me, therefore, quite certain that
whenever extreme glaciation has been brought about by high excentricity
combined with favourable geographical and physical causes (and without
this combination it is doubtful whether extreme glaciation would ever
occur), then the ice-sheet will not be removed during the alternate phases of
precession, so long as these geographical and physical causes remain
unaltered. It is true that the warm and cold oceanic currents, which are the
most important agents in increasing or diminishing glaciation, depend for
their strength and efficiency upon the comparative extents of the northern
and southern ice-sheets; but these ice-sheets cannot, I believe, increase or
diminish to any important extent unless some geographical or physical
change first occurs.[56]

If this argument is valid, then it would follow that, so long as excentricity
was high, whatever condition of climate was brought about by it in
combination with geographical causes, would persist through several phases
of precession; but this would not necessarily be the case when the
excentricity itself changed, and became more moderate. It would then
depend upon the proportionate effect of climatal and geographical causes in
producing glaciation as to what change would be produced by the changing
phases of precession; and we can best examine this question by considering
the probable effect of the change in precession during the next period of
10,500 years, with the present moderate degree of excentricity.
Probable Effect of Winter in Aphelion on the Climate of Britain.—Let us
then suppose the winters of the northern hemisphere to become longer and
much colder, the summers being proportionately shorter and hotter, without
any other change whatever. The long cold winter would certainly bring
down the snow-line considerably, covering large areas of high land with
snow during the winter months, and causing all glaciers and ice-fields to
become larger. This would chill the superincumbent atmosphere to such an
extent that the warm sun and winds of spring and early summer would bring
clouds and fog, so that the sun-heat would be cut off and much vapour be
condensed as snow. The greater sun-heat of summer would no doubt
considerably reduce the snow and ice; but it is, I think, quite certain that the
extra accumulation would not be all melted, and that therefore the snow-
line would be permanently lowered. This would be a necessary result,
because the greater part of the increased cold of winter would be stored up
in snow and ice, while the increased heat of summer could not be in any
way stored up, but would be largely prevented from producing any effect,
by reflection from the surface of the snow and by the intervention of clouds
and fog which would carry much of the heat they received to other regions.
It follows that 10,000 years hence, when our winter will occur in aphelion
(instead of, as now, in perihelion), there will be produced a colder climate,
independently of any change of land and sea, of heights of mountains, or in
the force of oceanic currents.
But if this is true, then the reverse change, bringing the sun back into
exactly the same position with regard to us as it is in now (all geographical
and physical conditions remaining unchanged), would certainly bring back
again our present milder climate. The change either way would not

probably be very great, but it might be sufficient to bring the snow-line
down to 3,000 feet in Scotland, so that all the higher mountains would have
their tops covered with perpetual snow. This perpetual snow, down to a
fixed line, would be kept up by the needful supply of snow falling during
autumn, winter, and spring, and this would, as we have seen, depend mainly
on the increased length and greatly increased cold of the winter. As both the
duration and the cold of winter decreased the amount of snow would
certainly decrease, and of this lesser quantity of snow a larger proportion
would be melted by the longer, though somewhat cooler summer. This
would follow because the total amount of sun-heat received during the
summer would be the same as before, while it would act on a less quantity
of snow; there would thus be a smaller surface to reflect the heat, and a
smaller condensing area to produce fogs, while the diminished intensity of
the sun would produce a less dense canopy of clouds, which have been
shown to be of prime importance in checking the melting of snow by the
sun. We have considered this case, for simplicity of reasoning, on the
supposition that all geographical and physical causes remained unchanged.
But if an alteration of the climate of the whole north temperate and Arctic
zones occurred, as here indicated, this would certainly affect both the winds
and currents, in the manner already explained (see p. 142), so as to react
upon climate and increase the differences produced by phases of precession.
How far that effect would be again increased by corresponding but opposite
changes in the southern hemisphere it is impossible to say. It may be that
existing geographical and physical conditions are there such potent agents
in producing a state of glaciation that no change in the phases of precession
would materially affect it. Still, as the climate of the whole southern
hemisphere is dominated by the great mass of ice within the Antarctic
circle, it seems probable that if the winter were shorter and the summer
longer the quantity of ice would slightly diminish; and this would again
react on the northern climate as already fully explained.
The Essential Principle of Climatal Change Restated.—The preceding
discussion has been somewhat lengthy, owing to the varied nature of the
facts and arguments adduced, and the extreme complexity of the subject.
But if, as I venture to urge, the principle here laid down is a sound one, it
will be of the greatest assistance in clearing away some of the many
difficulties that beset the whole question of geological climates. This

principle is, briefly, that the great features of climate are determined by a
combination of causes, of which geographical conditions and the degree of
excentricity of the earth's orbit are by far the most important; that, when
these combine to produce a severe glacial epoch, the changing phases of
precession every 10,500 years have very little, if any, effect on the character
of the climate, as mild or glacial, though it may modify the seasons; but
when the excentricity becomes moderate and the resulting glaciation less
severe, then the changing phases of precession bring about a considerable
alteration, and even a partial reversal of the glacial conditions.
The reason of this may perhaps be made clearer by considering the stability
of either extreme glacial conditions or the entire absence of perpetual ice
and snow, and the comparative instability of an intermediate state of
climate. When a country is largely covered with ice, we may look upon it as
possessing the accumulated or stored-up cold of a long series of preceding
winters; and however much heat is poured upon it, its temperature cannot
be raised above the freezing point till that store of cold is got rid of—that is,
till the ice is all melted. But the ice itself, when extensive, tends to its own
preservation, even under the influence of heat; for the chilled atmosphere
becomes filled with fog, and this keeps off the sun-heat, and then snow falls
even during summer, and the stored-up cold does not diminish during the
year. When, however, only a small portion of the surface is covered with
ice, the exposed earth becomes heated by the hot sun, this warms the air,
and the warm air melts the adjacent ice. It follows, that towards the
equatorial limits of a glaciated country alternations of climate may occur
during a period of high excentricity, while nearer the pole, where the
surface is almost completely ice-clad, no amelioration may take place. The
same argument will, to some extent apply, inversely, with mild Arctic
climates; but this is a subject which will be discussed in the next chapter.
This view of the character of the last glacial epoch appears to correspond
very closely with the facts adduced by geologists. The inter-glacial deposits
never exhibit any indication of a climate whose warmth corresponded to the
severity of the preceding cold, but rather of a partial amelioration of that
cold; while it is only the very latest of them, which we may suppose to have
occurred when the excentricity was considerably diminished, that exhibit
any indications of a climate at all warmer than that which now prevails.[57]

Probable Date of the Glacial Epoch.—The state of extreme glaciation in
the northern hemisphere, of which we gave a general description at the
commencement of the preceding chapter, is a fact of which there can be no
doubt whatever, and it occurred at a period so recent geologically that all
the mollusca were the same as species still living. There is clear geological
proof, however, that considerable changes of sea and land, and a large
amount of valley denudation, took place during and since the glacial epoch,
while on the other hand the surface markings produced by the ice have been
extensively preserved; and taking all these facts into consideration, the
period of about 200,000 years since it reached its maximum, and about
80,000 years since it passed away, is generally considered by geologists to
be ample. There seems, therefore, to be little doubt that in increased
excentricity we have found one of the chief exciting causes of the glacial
epoch, and that we are therefore able to fix its date with a considerable
probability of being correct. The enormous duration of the glacial epoch
itself (including its interglacial mild or warm phases), as compared with the
lapse of time since it finally passed away, is a consideration of the greatest
importance, and has not yet been taken fully into account in the
interpretation given by geologists of the physical and biological changes
that were coincident with, and probably dependent on, it.
Changes of the Sea-level Dependent on Glaciation.—It has been pointed
out by Dr. Croll, that many of the changes of level of sea and land which
occurred about the time of the glacial epoch may be due to an alteration of
the sea-level caused by a shifting of the earth's centre of gravity; and
physicists have generally admitted that the cause is a real one, and must
have produced some effect of the kind indicated. It is evident that if ice-
sheets several miles in thickness were removed from one polar area and
placed on the other, the centre of gravity of the earth would shift towards
the heavier pole, and the sea would necessarily follow it, and would rise
accordingly. Extreme glacialists have maintained that during the height of
the glacial epoch, an ice-cap extended from about 50° N. Lat. in Europe,
and 40° N. Lat. in America, continually increasing in thickness, till it
reached at least six miles thick at the pole; but this view is now generally
given up. A similar ice-cap is however believed to exist on the Antarctic
pole at the present day, and its transference to the northern hemisphere
would, it is calculated, produce a rise of the ocean to the extent of 800 or

1,000 feet. We have, however, shown that the production of any such ice-
cap is improbable if not impossible, because snow and ice can only
accumulate where precipitation is greater than melting and evaporation, and
this is never the case except in areas exposed to the full influence of the
vapour-bearing winds. The outer rim of the ice-sheet would inevitably
exhaust the air of so much of its moisture that what reached the inner parts
would produce far less snow than would be melted by the long hot days of
summer.[58] The accumulations of ice were therefore probably confined, in
the northern hemisphere, to the coasts exposed to moist winds, and where
elevated land and mountain ranges afforded condensers to initiate the
process of glaciation; and we have already seen that the evidence strongly
supports this view. Even with this limitation, however, the mass of
accumulated ice would be enormous, as indeed we have positive evidence
that it was, and might have caused a sufficient shifting of the centre of
gravity of the earth to produce a submergence of about 150 or 200 feet.
But this would only be the case if the accumulation of ice on one pole was
accompanied by a diminution on the other, and this may have occurred to a
limited extent during the earlier stages of the glacial epoch, when
alternations of warmer and colder periods would be caused by winter
occurring in perihelion or aphelion. If, however, as is here maintained, no
such alternations occurred when the excentricity was near its maximum,
then the ice would accumulate in the southern hemisphere at the same time
as in the northern, unless changed geographical conditions, of which we
have no evidence whatever, prevented such accumulations. That there was
such a greater accumulation of ice is shown by the traces of ancient glaciers
in the Southern Andes and in New Zealand, and also, according to several
writers, in South Africa; and the indications in all these localities point to a
period so recent that it must almost certainly have been contemporaneous
with the glacial period of the northern hemisphere.[59] This greater
accumulation of ice in both hemispheres would lower the whole ocean by
the quantity of water abstracted from it, while any want of perfect
synchronism between the decrease of the ice at the two poles would cause a
movement of the centre of gravity of the earth, and a slight rise of the sea-
level at one pole and depression at the other. It is also generally believed
that a great accumulation of ice would cause subsidence by its pressure on
the flexible crust of the earth, and we thus have a very complex series of

agents leading to elevations and subsidences of limited amount, such as
seem always to have accompanied glaciation. This complexity of the causes
at work may explain the somewhat contradictory evidence as to rise and fall
of land, some authors maintaining that it stood higher, and others lower,
during the glacial period.
The State of the Planet Mars, as Bearing on the Theory of Excentricity as a
Cause of Glacial Periods.—It is well known that the polar regions of the
planet Mars are covered with white patches or discs, which undergo
considerable alterations of size according as they are more or less exposed
to the sun's rays. They have therefore been generally considered to be snow
or ice-caps, and to prove that Mars is now undergoing something like a
glacial period. It must always be remembered, however, that we are very
ignorant of the exact physical conditions of the surface of Mars. It appears
to have a cloudy atmosphere like our own, but the gaseous composition of
that atmosphere may be different, and the clouds may be formed of other
matter besides aqueous vapour. Its much smaller mass and attractive power
must have an effect on the nature and extent of these clouds, and the heat of
the sun may consequently be modified in a way quite different from
anything that obtains upon our earth. Bearing these difficulties and
uncertainties in mind, let us see what are the actual facts connected with the
supposed polar snows of Mars.[60]
Mars offers an excellent subject for comparison with the Earth as regards
this question, because its excentricity is now a little greater than the
maximum excentricity of the Earth during the last million years,—(Mars
excentricity 0.0931, Earth excentricity, 850,000 years back, 0.0707); the
inclination of its axis is also a little greater than ours (Mars 28° 51′, Earth
23° 27′), and both Mars and the Earth are so situated that they now have the
winter of their northern hemispheres in perihelion, that of their southern
hemisphere being in aphelion. If, therefore, the physical condition of Mars
were the same or nearly the same as that of the Earth, all circumstances
combine, according to Dr. Croll's hypothesis, to produce a severe glacial
epoch in its southern, with a perpetual spring or summer in its northern,
hemisphere; while on the hypothesis here advocated we should expect
glaciation at both poles. As a matter of fact Mars has two snow-caps, of
nearly equal magnitude at their maximum in winter, but varying very

unequally. The northern cap varies slowly and little, the southern varies
rapidly and largely.
In the year 1830 the southern snow was observed, during the midsummer of
Mars, to diminish to half its former diameter in a fortnight (the duration of
such phenomena on Mars being reckoned in Martian months equivalent to
one-twelfth of a Martian year). Thus on June 23rd it was 11° 30′ in
diameter, and on July 9th had diminished to 5° 46′, after which it rapidly
increased again. In 1837 the same cap was observed near its maximum in
winter, and was found to be about 35° in diameter.
In the same year the northern snow-cap was observed during its summer,
and was found to vary as follows:—
May 4th. Diameter of spot 31° 24′
June 4th. ,, ,, 28° 0′
,, 17th. ,, ,, 22° 54′
July 4th. ,, ,, 18° 24′
,, 12th. ,, ,, 15° 20′
,, 20th. ,, ,, 18° 0′
We thus see that Mars has two permanent snow-caps, of nearly equal size in
winter but diminishing very unequally 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.[61]
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 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 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.

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
Palæozoic Epochs—Warm Arctic Climates in Early Secondary and
Palæozoic 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 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.
But if the last glacial epoch has coincided with, and has been to a
considerable extent caused by, a high excentricity of the earth's orbit, we are

naturally led to expect that earlier glacial epochs would have occurred
whenever the excentricity was unusually large. Dr. Croll has published
tables showing the varying amounts of excentricity for three million years
back; and from these it appears that there have been many periods of high
excentricity, which has often been far greater than at the time of the last
glacial epoch.[62] The accompanying diagram has been drawn from these
tables, and it will be seen that the highest excentricity occurred 850,000
years ago, at which time the difference between the sun's distance at
aphelion and perihelion was thirteen and a half millions of miles, whereas
during the last glacial period the maximum difference was ten and a half
million miles.

DIAGRAM SHOWING THE CHANGES OF EXCENTRICITY DURING THE LAST
THREE MILLION YEARS.

Now, judging by the amount of organic and physical change that occurred
during and since the glacial epoch, and that which has occurred since the
Miocene period, it is considered probable that this maximum of excentricity
coincided with some part of the latter period; and Dr. Croll maintains that a
glacial epoch must then have occurred surpassing in severity that of which
we have such convincing proofs, and consisting like it of alternations of
cold and warm phases every 10,500 years. The diagram also shows us
another long-continued period of high excentricity from 1,750,000 to
1,950,000 years ago, and yet another almost equal to the maximum
2,500,000 years back. These may perhaps have occurred during the Eocene
and Cretaceous epochs respectively, or all may have been included within
the limits of the Tertiary period. As two of these high excentricities greatly
exceed that which caused our glacial epoch, while the third is almost equal
to it and of longer duration, they seem to afford us the means of testing rival
theories of the causes of glaciation. If, as Dr. Croll argues, high excentricity
is the great and dominating agency in bringing on glacial epochs,

geographical changes being subordinate, then there must have been glacial
epochs of great severity at all these three periods; while if he is also correct
in supposing that the alternate phases of precession would inevitably
produce glaciation in one hemisphere, and a proportionately mild and
equable climate in the opposite hemisphere, then we should have to look for
evidence of exceptionally warm and exceptionally cold periods, occurring
alternately and with several repetitions, within a space of time which,
geologically speaking, is very short indeed.
Let us then inquire first into the character of the evidence we should expect
to find of such changes of climate, if they have occurred; we shall then be
in a better position to estimate at its proper value the evidence that actually
exists, and, after giving it due weight, to arrive at some conclusion as to the
theory that best explains and harmonises it.

Effects of Denudation in Destroying the Evidence of Remote Glacial
Epochs.—It may be supposed, that if earlier glacial epochs than the last did
really occur, we ought to meet with some evidence of the fact
corresponding to that which has satisfied us of the extensive recent
glaciation of the northern hemisphere; but Dr. Croll and other writers have
ably argued that no such evidence is likely to be found. It is now generally
admitted that sub-aërial denudation is a much more powerful agent in
lowering and modifying the surface of a country than was formerly
supposed. It has in fact been proved to be so powerful that the difficulty
now felt is, not to account for the denudation which can be proved to have
occurred, but to explain the apparent persistence of superficial features
which ought long ago to have been destroyed.
A proof of the lowering and eating away of the land-surface which every
one can understand, is to be found in the quantity of solid matter carried
down to the sea and to low grounds by rivers. This is capable of pretty
accurate measurement, and it has been carefully measured for several
rivers, large and small, in different parts of the world. The details of these
measurements will be given in a future chapter, and it is only necessary here
to state that the average of them all gives us this result—that one foot must,
on an average, be taken off the entire surface of the land each 3,000 years in

order to produce the amount of sediment and matter in solution which is
actually carried into the sea. To give an idea of the limits of variation in
different rivers it may be mentioned that the Mississippi is one which
denudes its valley at a slow rate, taking 6,000 years to remove one foot;
while the Po is the most rapid, taking only 729 years to do the same work in
its valley. The cause of this difference is very easy to understand. A large
part of the area of the Mississippi basin consists of the almost rainless
prairie and desert regions of the west, while its sources are in comparatively
arid mountains with scanty snow-fields, or in a low forest-clad plateau. The
Po, on the other hand, is wholly in a district of abundant rainfall, while its
sources are spread over a great amphitheatre of snowy Alps nearly 400
miles in extent, where the denuding forces are at a maximum. As Scotland
is a mountain region of rather abundant rainfall, the denuding power of its
rains and rivers is probably rather above than under the average, but to
avoid any possible exaggeration we will take it at a foot in 4,000 years.
Now if the end of the glacial epoch be taken to coincide with the
termination of the last period of high excentricity, which occurred about
80,000 years ago (and no geologist will consider this too long for the
changes which have since taken place), it follows that the entire surface of
Scotland must have been since lowered an average amount of twenty feet.
But over large areas of alluvial plains, and wherever the rivers have spread
during floods, the ground will have been raised instead of lowered; and on
all nearly level ground and gentle slopes there will have been comparatively
little denudation; so that proportionally much more must have been taken
away from mountain sides and from the bottoms of valleys having a
considerable downward slope. One of the very highest authorities on the
subject of denudation, Mr. Archibald Geikie, estimates the area of these
more rapidly denuded portions as only one-tenth of the comparatively level
grounds, and he further estimates that the former will be denuded about ten
times as fast as the latter. It follows that the valleys will be deepened and
widened on the average about five feet in the 4,000 years instead of one
foot; and thus many valleys must have been deepened and widened 100
feet, and some even more, since the glacial epoch, while the more level
portions of the country will have been lowered on the average only about
two feet.

Now Dr. Croll gives us the following account of the present aspect of the
surface of a large part of the country:—
"Go where one will in the lowlands of Scotland and he shall hardly find a
single acre whose upper surface bears the marks of being formed by the
denuding agents now in operation. He will observe everywhere mounds and
hollows which cannot be accounted for by the present agencies at work....
In regard to the general surface of the country the present agencies may be
said to be just beginning to carve a new line of features out of the old
glacially-formed surface. But so little progress has yet been made, that the
kames, gravel-mounds, knolls of boulder clay, &c., still retain in most cases
their original form."[63]
The facts here seem a little inconsistent, and we must suppose that Dr. Croll
has somewhat exaggerated the universality and complete preservation of the
glaciated surface. The amount of average denudation, however, is not a
matter of opinion but of measurement; and its consequences can in no way
be evaded. They are, moreover, strictly proportionate to the time elapsed;
and if so much of the old surface of the country has certainly been
remodelled or carried into the sea since the last glacial epoch, it becomes
evident that any surface-phenomena produced by still earlier glacial epochs
must have long since entirely disappeared.
Rise of the Sea-level Connected with Glacial Epochs, a Cause of Further
Denudation.—There is also another powerful agent that must have assisted
in the destruction of any such surface deposits or markings. During the last
glacial epoch itself there were several minor oscillations of the land,
without counting the great submergence of over 1,300 feet, supposed to be
indicated by patches of shelly clays and gravels in Wales and Ireland, and
also in a few localities in England and Scotland, since these are otherwise
explained by many geologists. Other subsidences have no doubt occurred in
the same areas during the Tertiary epoch, and some writers connect these
subsidences with the glacial period itself, the unequal amount of ice at the
two poles causing the centre of gravity of the earth to be displaced when, of
course, the surface of the ocean will conform to it and appear to rise in the
one hemisphere and sink in the other. If this is the case, subsidences of the
land are natural concomitants of a glacial period, and will powerfully aid in
removing all evidence of its occurrence. We have seen reason to believe,

however, that during the height of the glacial epoch the extreme cold
persisted through the successive phases of precession, and if so, both polar
areas would probably be glaciated at once. This would cause the abstraction
of a large quantity of water from the ocean, and a proportionate elevation of
the land, which would react on the accumulation of snow and ice, and thus
add another to that wonderful series of physical agents which act and react
on each other so as to intensify glacial epochs.
But whether or not these causes would produce any important fluctuations
of the sea-level is of comparatively little importance to our present inquiry,
because the wide extent of marine Tertiary deposits in the northern
hemisphere and their occurrence at considerable elevations above the
present sea-level, afford the most conclusive proofs that great changes of
sea and land have occurred throughout the entire Tertiary period; and these
repeated submergences and emergences of the land combined with sub-
aërial and marine denudation, would undoubtedly destroy all those
superficial evidences of ice-action on which we mainly depend for proofs of
the occurrence of the last glacial epoch.
What Evidence of Early Glacial Epochs may be Expected.—Although we
may admit the force of the preceding argument as to the extreme
improbability of our finding any clear evidence of the superficial action of
ice during remote glacial epochs, there is nevertheless one kind of evidence
that we ought to find, because it is both wide-spread and practically
indestructible.
One of the most constant of all the phenomena of a glaciated country is the
abundance of icebergs produced by the breaking off of the ends of glaciers
which terminate in arms of the sea, or of the terminal face of the ice-sheet
which passes beyond the land into the ocean. In both these cases abundance
of rocks and débris, such as form the terminal moraines of glaciers on land,
are carried out to sea and deposited over the sea-bottom of the area
occupied by icebergs. In the case of an ice-sheet it is almost certain that
much of the ground-moraine, consisting of mud and imbedded stones,
similar to that which forms the "till" when deposited on land, will be carried
out to sea with the ice and form a deposit of marine "till" near the shore.

It has indeed been objected that when an ice-sheet covered an entire country
there would be no moraines, and that rocks or débris are very rarely seen on
icebergs. But during every glacial epoch there will be a southern limit to the
glaciated area, and everywhere near this limit the mountain-tops will rise
far above the ice and deposit on it great masses of débris; and as the ice-
sheet spreads, and again as it passes away, this moraine-forming area will
successively occupy the whole country. But even such an ice-clad country
as Greenland is now known to have protruding peaks and rocky masses
which give rise to moraines on its surface;[64] and, as rocks from
Cumberland and Ireland were carried by the ice-sheet to the Isle of Man,
there must have been a very long period during which the ice-sheets of
Britain and Ireland terminated in the ocean and sent off abundance of rock-
laden bergs into the surrounding seas; and the same thing must have
occurred along all the coasts of Northern Europe and Eastern America.
We cannot therefore doubt that throughout the greater part of the duration of
a glacial epoch the seas adjacent to the glaciated countries would receive
continual deposits of large rocks, rock-fragments, and gravel, similar to the
material of modern and ancient moraines, and analogous to the drift and the
numerous travelled blocks which the ice has undoubtedly scattered
broadcast over every glaciated country; and these rocks and boulders would
be imbedded in whatever deposits were then forming, either from the matter
carried down by rivers or from the mud ground off the rocks and carried out
to sea by the glaciers themselves. Moreover, as icebergs float far beyond the
limits of the countries which gave them birth, these ice-borne materials
would be largely imbedded in deposits forming from the denudation of
countries which had never been glaciated, or from which the ice had already
disappeared.
But if every period of high excentricity produced a glacial epoch of greater
or less extent and severity, then, on account of the frequent occurrence of a
high phase of excentricity during the three million years for which we have
the tables, these boulder and rock-strewn deposits would be both numerous
and extensive. Four hundred thousand years ago the excentricity was almost
exactly the same as it is now, and it continually increased from that time up
to the glacial epoch. Now if we take double the present excentricity as
being sufficient to produce some glaciation in the temperate zone, we find
(by drawing out the diagram at p. 171 on a larger scale) that during

1,150,000 years out of the 2,400,000 years immediately preceding the last
glacial epoch, the excentricity reached or exceeded this amount, consisting
of sixteen separate epochs, divided from each other by periods varying from
30,000 to 200,000 years. But if the last glacial epoch was at its maximum
200,000 years ago, a space of three million years will certainly include
much, if not all, of the Tertiary period; and even if it does not, we have no
reason to suppose that the character of the excentricity would suddenly
change beyond the three million years.
It follows, therefore, that if periods of high excentricity, like that which
appears to have been synchronous with our last glacial epoch and is
generally admitted to have been one of its efficient causes, always produced
glacial epochs (with or without alternating warm periods), then the whole of
the Tertiary deposits in the north temperate and Arctic zones should exhibit
frequent alternations of boulder and rock-bearing beds, or coarse rock-
strewn gravels analogous to our existing glacial drift, and with some
corresponding change of organic remains. Let us then see what evidence
can be adduced of the existence of such deposits, and whether it is adequate
to support the theory of repeated glacial epochs during the Tertiary period.
Evidences of Ice-action during the Tertiary Period.—The Tertiary fossils
both of Europe and North America indicate throughout warm or temperate
climates, except those of the more recent Pliocene deposits which merge
into the earlier glacial beds. The Miocene deposits of Central and Southern
Europe, for example, contain marine shells of some genera now only found
farther south, while the fossil plants often resemble those of Madeira and
the southern states of North America. Large reptiles, too, abounded, and
man-like apes lived in the south of France and in Germany. Yet in Northern
Italy, near Turin, there are beds of sandstone and conglomerate full of
characteristic Miocene shells, but containing in an intercalated deposit
angular blocks of serpentine and greenstone often of enormous size, one
being fourteen feet long, and another twenty-six feet. Some of the blocks
were observed by Sir Charles Lyell to be faintly striated and partly polished
on one side, and they are scattered through the beds for a thickness of
nearly 150 feet. It is interesting that the particular bed in which the blocks
occur yields no organic remains, though these are plentiful both in the
underlying and overlying beds, as if the cold of the icebergs, combined with
the turbidity produced by the glacial mud, had driven away the organisms

adapted to live only in a comparatively warm sea. Rock similar in kind to
these erratics occurs about twenty miles distant in the Alps.
The Eocene period is even more characteristically tropical in its flora and
fauna, since palms and Cycadaceæ, turtles, snakes, and crocodiles then
inhabited England. Yet on the north side of the Alps, extending from
Switzerland to Vienna, and also south of the Alps near Genoa, there is a
deposit of finely-stratified sandstone several thousand feet in thickness,
quite destitute of organic remains, but containing in several places in
Switzerland enormous blocks either angular or partly rounded, and
composed of oolitic limestone or of granite. Near the Lake of Thun some of
the granite blocks found in this deposit are of enormous size, one of them
being 105 feet long, ninety feet wide, and forty-five feet thick! The granite
is red, and of a peculiar kind which cannot be matched anywhere in the
Alps, or indeed elsewhere. Similar erratics have also been found in beds of
the same age in the Carpathians and in the Apennines, indicating probably
an extensive inland European sea into which glaciers descended from the
surrounding mountains, depositing these erratics, and cooling the water so
as to destroy the mollusca and other organisms which had previously
inhabited it. It is to be observed that wherever these erratics occur they are
always in the vicinity of great mountain ranges; and although these can be
proved to have been in great part elevated during the Tertiary period, we
must also remember that they must have been since very much lowered by
denudation, of the amount of which, the enormously thick Eocene and
Miocene beds now forming portions of them is in some degree a measure as
well as a proof. It is not therefore at all improbable that during some part of
the Tertiary period these mountains may have been far higher than they are
now, and this we know might be sufficient for the production of glaciers
descending to the sea-level, even were the climate of the lowlands
somewhat warmer than at present.[65]
The Weight of the Negative Evidence.—But when we proceed to examine
the Tertiary deposits of other parts of Europe, and especially of our own
country, for evidence of this kind, not only is such evidence completely
wanting, but the facts are of so definite a character as to satisfy most
geologists that it can never have existed; and the same maybe said of
temperate North America and of the Arctic regions generally.

In his carefully written paper on "The Climate Controversy" the late Mr.
Searles V. Wood, Jun., remarks on this point as follows: "Now the Eocene
formation is complete in England, and is exposed in continuous section
along the north coast of the Isle of Wight from its base to its junction with
the Oligocene (or Lower Miocene according to some), and along the
northern coast of Kent from its base to the Lower Bagshot Sand. It has been
intersected by railway and other cuttings in all directions and at all
horizons, and pierced by wells innumerable; while from its strata in
England, France, and Belgium, the most extensive collections of organic
remains have been made of any formation yet explored, and from nearly all
its horizons, for at one place or another in these three countries nearly every
horizon may be said to have yielded fossils of some kind. These fossils,
however, whether they be the remains of a flora such as that of Sheppey, or
of a vertebrate fauna containing the crocodile and alligator, such as is
yielded by beds indicative of terrestrial conditions, or of a molluscan
assemblage such as is present in marine or fluvio-marine beds of the
formation, are of unmistakably tropical or sub-tropical character
throughout; and no trace whatever has appeared of the intercalation of a
glacial period, much less of successive intercalations indicative of more
than one period of 10,500 years' glaciation. Nor can it be urged that the
glacial epochs of the Eocene in England were intervals of dry land, and so
have left no evidence of their existence behind them, because a large part of
the continuous sequence of Eocene deposits in this country consists of
alternations of fluviatile, fluvio-marine, and purely marine strata; so that it
seems impossible that during the accumulation of the Eocene formation in
England a glacial period could have occurred without its evidences being
abundantly apparent. The Oligocene of Northern Germany and Belgium,
and the Miocene of those countries and of France, have also afforded a rich
molluscan fauna, which, like that of the Eocene, has as yet presented no
indication of the intrusion of anything to interfere with its uniformly sub-
tropical character."[66]
This is sufficiently striking; but when we consider that this enormous series
of deposits, many thousand feet in thickness, consists wholly of alternations
of clays, sands, marls, shales, or limestones, with a few beds of pebbles or
conglomerate, not one of the whole series containing irregular blocks of
foreign material, boulders or gravel, such as we have seen to be the

essential characteristic of a glacial epoch; and when we find that this same
general character pervades all the extensive Tertiary deposits of temperate
North America, we shall, I think, be forced to the conclusion that no general
glacial epochs could have occurred during their formation. It must be
remembered that the "imperfection of the geological record" will not help
us here, because the series of Tertiary deposits is unusually complete, and
we must suppose some destructive agency to have selected all the
intercalated glacial beds and to have so completely made away with them
that not a fragment remains, while preserving all or almost all the
interglacial beds; and to have acted thus capriciously, not in one limited
area only, but over the whole northern hemisphere, with the local
exceptions on the flanks of great mountain ranges already referred to.
Temperate Climates in the Arctic Regions.—As we have just seen, the
geological evidence of the persistence of sub-tropical or warm climates in
the north temperate zone during the greater part of the Tertiary period is
almost irresistible, and we have now to consider the still more extraordinary
series of observations which demonstrate that this amelioration of climate
extended into the Arctic zone, and into countries now almost wholly buried
in snow and ice. These warm Arctic climates have been explained by Dr.
Croll as due to periods of high excentricity with winter in perihelion, a
theory which implies alternating epochs of glaciation far exceeding what
now prevails; and it is therefore necessary to examine the evidence pretty
closely in order to see if this view is more tenable in the case of the north
polar regions than we have found it to be in that of the north temperate
zone.
The most recent of these milder climates is perhaps indicated by the
abundant remains of large mammalia—such as the mammoth, woolly
rhinoceros, bison and horse, in the icy alluvial plains of Northern Siberia,
and especially in the Liakhov Islands in the same latitude as the North Cape
of Asia. These remains occur not in one or two spots only, as if collected by
eddies at the mouth of a river, but along the whole borders of the Arctic
Ocean; and it is generally admitted that the animals must have lived upon
the adjacent plains, and that a considerably milder climate than now
prevails could alone have enabled them to do so. How long ago this
occurred we do not know, but one of the last intercalated mild periods of the
glacial epoch itself seems to offer all the necessary conditions. Again, Sir

Edward Belcher discovered on the dreary shores of Wellington Channel in
75½° N. Lat. the trunk and root of a fir tree which had evidently grown
where it was found. It appeared to belong to the species Abies alba, or
white fir, which now reaches 68° N. Lat. and is the most northerly conifer
known. Similar trees, one four feet in circumference and thirty feet long,
were found by Lieut. Mecham in Prince Patrick's Island in Lat. 76° 12′ N.,
and other Arctic explorers have found remains of trees in high latitudes.[67]
Similar indications of a recent milder climate are found in Spitzbergen.
Professor Nordenskjöld says: "At various places on Spitzbergen, at the
bottom of Lomme Bay, at Cape Thordsen, in Blomstrand's strata in Advent
Bay, there are found large and well-developed shells of a bivalve, Mytilus
edulis, which is not now found living on the coast of Spitzbergen, though
on the west coast of Scandinavia it everywhere covers the rocks near the
sea-shore. These shells occur most plentifully in the bed of a river which
runs through Reindeer Valley at Cape Thordsen. They are probably washed
out of a thin bed of sand at a height of about twenty or thirty feet above the
present sea-level, which is intersected by the river. The geological age of
this bed cannot be very great, and it has clearly been formed since the
present basin of the Ice Sound, or at least the greater part of it, has been
hollowed out by glacial action."[68]
The Miocene Arctic Flora.—One of the most startling and important of the
scientific discoveries of the last forty years has been that of the relics of a
luxuriant Miocene flora in various parts of the Arctic regions. It is a
discovery that was totally unexpected, and is even now considered by many
men of science to be completely unintelligible; but it is so thoroughly
established, and it has such a direct and important bearing on the subjects
we are discussing in the present volume, that it is necessary to lay a
tolerably complete outline of the facts before our readers.
The Miocene flora of temperate Europe was very like that of Eastern Asia,
Japan, and the warmer part of Eastern North America of the present day. It
is very richly represented in Switzerland by well preserved fossil remains,
and after a close comparison with the flora of other countries Professor
Heer concludes that the Swiss Lower Miocene flora indicates a climate
corresponding to that of Louisiana, North Africa, and South China, while
the Upper Miocene climate of the same country would correspond to that of

the south of Spain, Southern Japan, and Georgia (U.S. of America). Of this
latter flora, found chiefly at Œninghen in the northern extremity of
Switzerland, 465 species are known, of which 166 species are trees or
shrubs, half of them being evergreens. They comprise sequoias like the
Californian giant trees, camphor-trees, cinnamons, sassafras, bignonias,
cassias, gleditschias, tulip-trees, and many other American genera, together
with maples, ashes, planes, oaks, poplars, and other familiar European trees
represented by a variety of extinct species. If we now go to the west coast
of Greenland in 70° N. Lat. we find abundant remains of a flora of the same
general type as that of Œninghen but of a more northern character. We have
a sequoia identical with one of the species found at Œninghen, a chestnut,
salisburia, liquidambar, sassafras, and even a magnolia. We have also seven
species of oaks, two planes, two vines, three beeches, four poplars, two
willows, a walnut, a plum, and several shrubs supposed to be evergreens;
altogether 137 species, mostly well and abundantly preserved!
But even further north, in Spitzbergen, in 78° and 79° N. Lat. and one of the
most barren and inhospitable regions on the globe, an almost equally rich
fossil flora has been discovered including several of the Greenland species,
and others peculiar, but mostly of the same genera. There seem to be no
evergreens here except coniferæ, one of which is identical with the swamp-
cypress (Taxodium distichum) now found living in the Southern United
States! There are also eleven pines, two Libocedrus, two sequoias, with
oaks, poplars, birches, planes, limes, a hazel, an ash, and a walnut; also
water-lilies, pond-weeds, and an iris—altogether about a hundred species of
flowering plants. Even in Grinnell Land, within 8¼ degrees of the pole, a
similar flora existed, twenty-five species of fossil plants having been
collected by the last Arctic expedition, of which eighteen were identical
with the species from other Arctic localities. This flora comprised poplars,
birches, hazels, elms, viburnums, and eight species of conifers including the
swamp cypress and the Norway spruce (Pinus abies) which last does not
now extend beyond 69½° N.
Fossil plants closely resembling those just mentioned have been found at
many other Arctic localities, especially in Iceland, on the Mackenzie River
in 65° N. Lat. and in Alaska. As an intermediate station we have, in the
neighbourhood of Dantzic in Lat. 55° N., a similar flora, with the swamp-
cypress, sequoias, oaks, poplars, and some cinnamons, laurels, and figs. A

little further south, near Breslau, north of the Carpathians, a rich flora has
been found allied to that of Œninghen, but wanting in some of the more
tropical forms. Again, in the Isle of Mull in Scotland, in about 56½° N. Lat.,
a plant-bed has been discovered containing a hazel, a plane, and a sequoia,
apparently identical with a Swiss Miocene species.
We thus find one well-marked type of vegetation spread from Switzerland
and Vienna to North Germany, Scotland, Iceland, Greenland, Alaska, and
Spitzbergen, some few of the species even ranging over the extremes of
latitude between Œninghen and Spitzbergen, but the great majority being
distinct, and exhibiting decided indications of a decrease of temperature
according to latitude, though much less in amount than now exists. Some
writers have thought that the great similarity of the floras of Greenland and
Œninghen is a proof that they were not contemporaneous, but successive;
and that of Greenland has been supposed to be as old as the Eocene. But the
arguments yet adduced do not seem to prove such a difference of age,
because there is only that amount of specific and generic diversity between
the two which might be produced by distance and difference of temperature,
under the exceptionally equable climate of the period. We have even now
examples of an equally wide range of well-marked types; as in temperate
South America, where many of the genera and some of the species range
from the Straits of Magellan to Valparaiso—places differing as much in
latitude as Switzerland and West Greenland; and the same may be said of
North Australia and Tasmania, where, at a greater latitudinal distance apart,
closely allied forms of Eucalyptus, Acacia, Casuarina, Stylidium, Goodenia,
and many other genera would certainly form a prominent feature in any
fossil flora now being preserved.
Mild Arctic Climates of the Cretaceous Period.—In the Upper Cretaceous
deposits of Greenland (in a locality not far from those of the Miocene age
last described) another remarkable flora has been discovered, agreeing
generally with that of Europe and North America of the same geological
age. Sixty-five species of plants have been identified, of which there are
fifteen ferns, two cycads, eleven coniferæ, three monocotyledons, and
thirty-four dicotyledons. One of the ferns is a tree-fern with thick stems,
which has also been found in the Upper Greensand of England. Among the
conifers the giant sequoias are found, and among the dicotyledons the
genera Populus, Myrica, Ficus, Sassafras, Andromeda, Diospyros, Myrsine,

Panax, as well as magnolias, myrtles, and leguminosæ. Several of these
groups occur also in the much richer deposits of the same age in North
America and Central Europe; but all of them evidently afford such
fragmentary records of the actual flora of the period, that it is impossible to
say that any genus found in one locality was absent from the other merely
because it has not yet been found there. On the whole, there seems to be
less difference between the floras of Arctic and temperate latitudes in Upper
Cretaceous than in Miocene times.
In the same locality in Greenland (70° 33′ N. Lat. and 52° W. Long.), and
also in Spitzbergen, a more ancient flora, of Lower Cretaceous age, has
been found; but it differs widely from the other in the great abundance of
cycads and conifers and the scarcity of exogens, which latter are
represented by a single poplar. Of the thirty-eight ferns, fifteen belong to
the genus Gleichenia now almost entirely tropical. There are four genera of
cycads, and three extinct genera of conifers, besides Glyptostrobus and
Torreya now found only in China and California, six species of true pines,
and five of the genus Sequoia, one of which occurs also in Spitzbergen. The
European deposits of the same age closely agree with these in their general
character, conifers, cycads, and ferns forming the mass of the vegetation,
while exogens are entirely absent, the above-named Greenland poplar being
the oldest known dicotyledonous plant.[69]
If we take these facts as really representing the flora of the period, we shall
be forced to conclude that, measured by the change effected in its plants,
the lapse of time between the Lower and Upper Cretaceous deposits was far
greater than between the Upper Cretaceous and the Miocene—a conclusion
quite opposed to the indications afforded by the mollusca and the higher
animals of the two periods. It seems probable, therefore, that these Lower
Cretaceous plants represent local peculiarities of vegetation such as now
sometimes occur in tropical countries. On sandy or coralline islands in the
Malay Archipelago there will often be found a vegetation consisting almost
wholly of cycads, pandani, and palms, while a few miles off, on moderately
elevated land, not a single specimen of either of these families may be seen,
but a dense forest of dicotyledonous trees covering the whole country. A
lowland vegetation, such as that above described, might be destroyed and
its remains preserved by a slight depression, allowing it to be covered up by
the detritus of some adjacent river, while not only would the subsidence of

high land be a less frequent occurrence, but when it did occur the steep
banks would be undermined by the waves, and the trees falling down would
be floated away, and would either be cast on some distant shore or slowly
decay on the surface or in the depths of the ocean.
From the remarkable series of facts now briefly summarized, we learn, that
whenever plant-remains have been discovered within the Arctic regions,
either in Tertiary or Cretaceous deposits, they show that the climate was one
capable of supporting a rich vegetation of trees, shrubs, and herbaceous
plants, similar in general character to that which prevailed in the temperate
zone at the same periods, but showing the influence of a less congenial
climate. These deposits belong to at least four distinct geological horizons,
and have been found widely scattered within the Arctic circle, yet nowhere
has any proof been obtained of intercalated cold periods, such as would be
indicated by the remains of a stunted vegetation, or a molluscan fauna
similar to that which now prevails there.
Stratigraphical Evidence of Long-Continued Mild Arctic Conditions.—Let
us now turn to the stratigraphical evidence, which, as we have already
shown, offers a crucial test of the occurrence or non-occurrence of
glaciation during any extensive geological period; and here we have the
testimony of perhaps the greatest living authority on Arctic geology—
Professor Nordenskjöld. In his lecture on "The Former Climate of the Polar
Regions," he says: "The character of the coasts in the Arctic regions is
especially favourable to geological investigations. While the valleys are for
the most part filled with ice, the sides of the mountains in summer, even in
the 80th degree of latitude, and to a height of 1,000 or 1,500 feet above the
level of the sea, are almost wholly free from snow. Nor are the rocks
covered with any amount of vegetation worth mentioning; and, moreover,
the sides of the mountains on the shore itself frequently present
perpendicular sections, which everywhere expose their bare surfaces to the
investigator. The knowledge of a mountain's geognostic character, at which
one, in the more southerly countries, can only arrive after long and
laborious researches, removal of soil and the like, is here gained almost at
the first glance; and as we have never seen in Spitzbergen nor in Greenland,
in these sections often many miles in length, and including one may say all
formations from the Silurian to the Tertiary, any boulders even as large as a
child's head, there is not the smallest probability that strata of any

considerable extent, containing boulders, are to be found in the polar tracts
previous to the middle of the Tertiary period. Since, then, both an
examination of the geognostic condition, and an investigation of the fossil
flora and fauna of the polar lands, show no signs of a glacial era having
existed in those parts before the termination of the Miocene period, we are
fully justified in rejecting, on the evidence of actual observation, the
hypotheses founded on purely theoretical speculations, which assume the
many times repeated alternation of warm and glacial climates between the
present time and the earliest geological ages."[70] And again, in his Sketch of
the Geology of Spitzbergen, after describing the various formations down to
the Miocene, he says: "All the fossils found in the foregoing strata show
that Spitzbergen, during former geological ages, enjoyed a magnificent
climate, which indeed was somewhat colder during the Miocene period, but
was still favourable for an extraordinarily abundant vegetation, much more
luxuriant than that which now occurs even in the southern part of
Scandinavia: and I have in those strata sought in vain for any sign, that, as
some geologists have of late endeavoured to render probable, these
favourable climatic conditions have been broken off by intervals of ancient
glacial periods. The profiles I have had the opportunity to examine during
my various Spitzbergen expeditions would certainly, if laid down on a line,
occupy an extent of a thousand English miles; and if any former glacial
period had existed in this region, there ought to have been some trace to be
observed of erratic blocks, or other formations which distinguish glacial
action. But this has not been the case. In the strata, whose length I have
reckoned alone, I have not found a single fragment of a foreign rock so
large as a child's head."[71]
Now it is quite impossible to ignore or evade the force of this testimony as
to the continuous warm climates of the north temperate and polar zones
throughout Tertiary times. The evidence extends over a vast area, both in
space and time, it is derived from the work of the most competent living
geologists, and it is absolutely consistent in its general tendency. We have in
the Lower Cretaceous period an almost tropical climate in France and
England, a somewhat lower temperature in the United States, and a mild
insular climate in the Arctic regions. In each successive period the climate
becomes somewhat less tropical; but down to the Upper Miocene it remains
warm temperate in Central Europe, and cold temperate within the polar

area, with not a trace of any intervening periods of Arctic cold. It then
gradually cools down and merges through the Pliocene into the glacial
epoch in Europe, while in the Arctic zone there is a break in the record
between the Miocene and the recent glacial deposits.[72]
Accepting this as a substantially correct account of the general climatic
aspect of the Tertiary period in the northern hemisphere, let us see whether
the principles we have already laid down will enable us to give a
satisfactory explanation of its causes.
The Causes of mild Arctic Climates.—In his remarkable series of papers on
"Ocean Currents," the late Dr. James Croll has proved, with a wealth of
argument and illustration whose cogency is irresistible, that the very
habitability of our globe is due to the equalizing climatic effects of the
waters of the ocean; and that it is to the same cause that we owe, either
directly or indirectly, almost all the chief diversities of climate between
places situated in the same latitude. Owing to the peculiar distribution of
land and sea upon the globe, more than its fair proportion of the warm
equatorial waters is directed towards the western shores of Europe, the
result being that the British Isles, Norway, and Spitzbergen, have all a
milder climate than any other parts of the globe in corresponding latitudes.
A very small portion of the Arctic regions, however, obtains this benefit,
and it thus remains, generally speaking, a land of snow and ice, with too
short a summer to nourish more than a very scanty and fugitive vegetation.
The only other opening than that between Iceland and Britain by which
warm water penetrates within the Arctic circle, is through Behring's Straits;
but this is both shallow and limited in width, and the consequence is that the
larger part of the warm currents of the Pacific turns back along the shores of
the Aleutian Islands and North-west America, while a very small quantity
enters the icy ocean.
But if there were other and wider openings into the Arctic Ocean, a vast
quantity of the heated water which is now turned backward would enter it,
and would produce an amelioration of the climate of which we can hardly
form a conception. A great amelioration of climate would also be caused by
the breaking up or the lowering of such Arctic highlands as now favour the
accumulation of ice; while the interpenetration of the sea into any part of
the great continents in the tropical or temperate zones would again tend to

raise the winter temperature, and render any long continuance of snow in
their vicinity almost impossible.
Now geologists have proved, quite independently of any such questions as
we are here discussing, that changes of the very kinds above referred to
have occurred during the Tertiary period; and that there has been, speaking
broadly, a steady change from a comparatively fragmentary and insular
condition of the great north temperate lands in early Tertiary times, to that
more compact and continental condition which now prevails. It is, no doubt,
difficult and often impossible to determine how long any particular
geographical condition lasted, or whether the changes in one country were
exactly coincident with those in another; but it will be sufficient for our
purpose briefly to indicate those more important changes of land and sea
during the Tertiary period, which must have produced a decided effect on
the climate of the northern hemisphere.
Geographical Changes Favouring Mild Northern Climates in Tertiary
Times.—The distribution of the Eocene and Miocene formations shows, that
during a considerable portion of the Tertiary period, an inland sea, more or
less occupied by an archipelago of islands, extended across Central Europe
between the Baltic and the Black and Caspian Seas, and thence by narrower
channels south-eastward to the valley of the Euphrates and the Persian Gulf,
thus opening a communication between the North Atlantic and the Indian
Oceans. From the Caspian also a wide arm of the sea extended during some
part of the Tertiary epoch northwards to the Arctic Ocean, and there is
nothing to show that this sea may not have been in existence during the
whole Tertiary period. Another channel probably existed over Egypt[73] into
the eastern basin of the Mediterranean and the Black Sea; while it is
probable that there was a communication between the Baltic and the White
Sea, leaving Scandinavia as an extensive island. Turning to India, we find
that an arm of the sea of great width and depth extended from the Bay of
Bengal to the mouths of the Indus; while the enormous depression indicated
by the presence of marine fossils of Eocene age at a height of 10,500 feet in
Western Tibet, renders it not improbable that a more direct channel across
Afghanistan may have opened a communication between the West Asiatic
and Polar seas.

It may be said that the changes here indicated are not warranted by an
actual knowledge of continuous Tertiary deposits over the situations of the
alleged marine channels; but it is no less certain that the seas in which any
particular strata were deposited were always more extensive than the
fragments of those strata now existing, and often immensely more
extensive. The Eocene deposits of Europe, for example, have certainly
undergone enormous denudation both marine and subaërial, and may have
once covered areas where we now find older deposits (as the chalk once
covered the weald), while a portion of them may lie concealed under
Miocene, Pliocene, or recent beds. We find them widely scattered over
Europe and Asia, and often elevated into lofty mountain ranges; and we
should certainly err far more seriously in confining the Eocene seas to the
exact areas where we now find Eocene rocks, than in liberally extending
them, so as to connect the several detached portions of the formation
whenever there is no valid argument against our doing so. Considering then,
that some one or more of the sea-communications here indicated almost
certainly existed during Eocene and Miocene times, let us endeavour to
estimate the probable effect such communications would have upon the
climate of the northern hemisphere.
The Indian Ocean as a Source of Heat in Tertiary Times.—If we compare
the Indian Ocean with the South Atlantic we shall see that the position and
outline of the former are very favourable for the accumulation of a large
body of warm water moving northwards. Its southern opening between
South Africa and Australia is very wide, and the tendency of the trade-
winds would be to concentrate the currents towards its north-western
extremity, just where the two great channels above described formed an
outlet to the northern seas. As will be shown in our nineteenth chapter, there
was probably, during the earlier portion of the Tertiary period at least,
several large islands in the space between Madagascar and South India; but
these had wide and deep channels between them, and their existence may
have been favourable to the conveyance of heated water northward, by
concentrating the currents, and thus producing massive bodies of moving
water analogous to the Gulf Stream of the Atlantic.[74] Less heat would thus
be lost by evaporation and radiation in the tropical zone, and an impulse
would be acquired which would carry the warm water into the north polar
area. About the same period Australia was probably divided into two

islands, separated by a wide channel in a north and south direction (see
Chapter XXII.), and through this another current would almost certainly set
northwards, and be directed to the north-west by the southern extension of
Malayan Asia. The more insular condition at this period of Australia, India,
and North Africa, with the depression and probable fertility of the Central
Asiatic plateau, would lead to the Indian Ocean being traversed by regular
trade-winds instead of by variable monsoons, and thus the constant vis a
tergo, which is so efficient in the Atlantic, would keep up a steady and
powerful current towards the northern parts of the Indian Ocean, and thence
through the midst of the European archipelago to the northern seas.
Now it is quite certain that such a condition as we have here sketched out
would produce a wonderful effect on the climate of Central Europe and
Western and Northern Asia. Owing to the warm currents being concentrated
in inland seas instead of being dispersed over a wide ocean like the North
Atlantic, much more heat would be conveyed into the Arctic Ocean, and
this would altogether prevent the formation of ice on the northern shores of
Asia, which continent did not then extend nearly so far north and was
probably deeply inter-penetrated by the sea. This open ocean to the north,
and the warm currents along all the northern lands, would so equalise
temperature, that even the northern parts of Europe might then have
enjoyed a climate fully equal to that of the warmer parts of New Zealand at
the present day, and might have well supported the luxuriant vegetation of
the Miocene period, even without any help from similar changes in the
western hemisphere.[75]
Condition of North America during the Tertiary Period.—But changes of a
somewhat similar character have also taken place in America and the
Pacific. An enormous area west of the Mississippi, extending over much of
the Rocky Mountains, consists of marine Cretaceous beds 10,000 feet thick,
indicating great and long-continued subsidence, and an insular condition of
Western America with a sea probably extending northwards to the Arctic
Ocean. As marine Tertiary deposits are found conformably overlying these
Cretaceous strata, Professor Dana is of opinion that the great elevation of
this part of America did not begin till early Tertiary times. Other Tertiary
beds in California, Alaska, Kamschatka, the Mackenzie River, the Parry
Islands, and Greenland, indicate partial submergence of all these lands with
the possible influx of warm water from the Pacific; and the considerable

elevation of some of the Miocene beds in Greenland and Spitzbergen
renders it probable that these countries were then much less elevated, in
which case only their higher summits would be covered with perpetual
snow, and no glaciers would descend to the sea.
In the Pacific there was probably an elevation of land counterbalancing, to
some extent, the great depression of so much of the northern continents.
Our map in Chapter XV. shows the islands that would be produced by an
elevation of the great shoals under a thousand fathoms deep, and it is seen
that these all trend in a south-east and north-west direction, and would thus
facilitate the production of definite currents impelled by the south-east
trades towards the north-west Pacific, where they would gain access to the
polar seas through Behring's Straits, which were, perhaps, sometimes both
wider and deeper than at present.
Effect of these Changes on the Climate of the Arctic Regions.—These
various changes of sea and land, all tending towards a transference of heat
from the equator to the north temperate zone, were not improbably still
further augmented by the existence of a great inland South American sea
occupying what are now the extensive valleys of the Amazon and Orinoco,
and forming an additional reservoir of super-heated water to add to the
supply poured into the North Atlantic.
It is not of course supposed that all the modifications here indicated co-
existed at the same time. We have good reason to believe, from the known
distribution of animals in the Tertiary period, that land-communications
have at times existed between Europe or Asia and North America, either by
way of Behring's Straits, or by Iceland, Greenland, and Labrador. But the
same evidence shows that these land-communications were the exception
rather than the rule, and that they occurred only at long intervals and for
short periods, so as at no time to bring about anything like a complete
interchange of the productions of the two continents.[76] We may therefore
admit that the communication between the tropical and Arctic oceans was
occasionally interrupted in one or other direction; but if we look at a globe
instead of a Mercator's chart of the world, we shall see that the
disproportion between the extent of the polar and tropical seas is so
enormous that a single wide opening, with an adequate impulse to carry in a
considerable stream of warm water, would be amply sufficient for the

complete abolition of polar snow and ice, when aided by the absence of any
great areas of high land within the polar circle, such high land being, as we
have seen, essential to the production of perpetual snow even at the present
time.
Those who wish to understand the effect of oceanic currents in conveying
heat to the north temperate and polar regions, should study the papers of Dr.
Croll already referred to. But the same thing is equally well shown by the
facts of the actual distribution of heat due to the Gulf Stream. The
difference between the mean annual temperatures of the opposite coasts of
Europe and America is well known and has been already quoted, but the
difference of their mean winter temperature is still more striking, and it is
this which concerns us as more especially affecting the distribution of
vegetable and animal life. Our mean winter temperature in the west of
England is the same as that of the Southern United States, as well as that of
Shanghai in China, both about twenty degrees of latitude further south; and
as we go northward the difference increases, so that the winter climate of
Nova Scotia in Lat. 45° is found within the Arctic circle on the coast of
Norway; and if the latter country did not consist almost wholly of
precipitous snow-clad mountains, it would be capable of supporting most of
the vegetable products of the American coast in the latitude of Bordeaux.[77]
With these astounding facts before us, due wholly to the transference of a
portion of the warm currents of the Atlantic to the shores of Europe, even
with all the disadvantages of an icy sea to the north-east and ice-covered
Greenland to the north-west, how can we doubt the enormously greater
effect of such a condition of things as has been shown to have existed
during the Tertiary epoch? Instead of one great stream of warm water
spreading widely over the North Atlantic and thus losing the greater part of
its store of heat before it reaches the Arctic seas, we should have several
streams conveying the heat of far more extensive tropical oceans by
comparatively narrow inland channels, thus being able to transfer a large
proportion of their heat into the northern and Arctic seas. The heat that they
gave out during the passage, instead of being widely dispersed by winds
and much of it lost in the higher atmosphere, would directly ameliorate the
climate of the continents they passed through, and prevent all accumulation
of snow except on the loftiest mountains. The formation of ice in the Arctic
seas would then be impossible; and the mild winter climate of the latitude

of North Carolina, which by the Gulf Stream is transferred 20° northwards
to our islands, might certainly, under the favourable conditions which
prevailed during the Cretaceous, Eocene, and Miocene periods, have been
carried another 20° north to Greenland and Spitzbergen; and this would
bring about exactly the climate indicated by the fossil Arctic vegetation. For
it must be remembered that the Arctic summers are, even now, really hotter
than ours, and if the winter's cold were abolished and all ice-accumulation
prevented, the high northern lands would be able to support a far more
luxuriant summer vegetation than is possible in our unequal and cloudy
climate.[78]
Effect of High Excentricity on the Warm Polar Climates.—If the
explanation of the cause of the glacial epoch given in the last chapter is a
correct one, it will, I believe, follow that changes in the amount of
excentricity will produce no important alteration of the climates of the
temperate and Arctic zones so long as favourable geographical conditions,
such as have been now sketched out, render the accumulation of ice
impossible. The effect of a high excentricity in producing a glacial epoch
was shown to be due to the capacity of snow and ice for storing up cold,
and its singular power (when in large masses) of preserving itself unmelted
under a hot sun by itself causing the interposition of a protective covering
of cloud and vapour. But mobile currents of water have no such power of
accumulating and storing up heat or cold from one year to another, though
they do in a pre-eminent degree possess the power of equalising the
temperature of winter and summer and of conveying the superabundant heat
of the tropics to ameliorate the rigour of the Arctic winters. However great
was the difference between the amount of heat received from the sun in
winter and summer in the Arctic zone during a period of high excentricity
and winter in aphelion, the inequality would be greatly diminished by the
free ingress of warm currents to the polar area; and if this was sufficient to
prevent any accumulation of ice, the summers would be warmed to the full
extent of the powers of the sun during the long polar day, which is such as
to give the pole at midsummer actually more heat during the twenty-four
hours than the equator receives during its day of twelve hours. The only
difference, then, that would be directly produced by the changes of
excentricity and precession would be, that the summers would be at one
period almost tropical, at the other of a more mild and uniform temperate

character; while the winters would be at one time somewhat longer and
colder, but never, probably, more severe than they are now in the west of
Scotland.
But though high excentricity would not directly modify the mild climates
produced by the state of the northern hemisphere which prevailed during
Cretaceous, Eocene, and Miocene times, it might indirectly affect it by
increasing the mass of Antarctic ice, and thus increasing the force of the
trade-winds and the resulting northward-flowing warm currents. Now there
are many peculiarities in the distribution of plants and of some groups of
animals in the southern hemisphere, which render it almost certain that
there has sometimes been a greater extension of the Antarctic lands during
Tertiary times; and it is therefore not improbable that a more or less
glaciated condition may have been a long persistent feature of the southern
hemisphere, due to the peculiar distribution of land and sea which favours
the production of ice-fields and glaciers. And as we have seen that during
the last three million years the excentricity has been almost always much
higher than it is now, we should expect that the quantity of ice in the
southern hemisphere will usually have been greater, and will thus have
tended to increase the force of those oceanic currents which produce the
mild climates of the northern hemisphere.
Evidences of Climate in the Secondary and Palæozoic Epochs.—We have
already seen, that so far back as the Cretaceous period there is the most
conclusive evidence of the prevalence of a very mild climate not only in
temperate but also in Arctic lands, while there is no proof whatever, or even
any clear indication, of early glacial epochs at all comparable in extent and
severity with that which has so recently occurred; and we have seen reason
to connect this state of things with a distribution of land and sea highly
favourable to the transference of warm water from equatorial to polar
latitudes. So far as we can judge by the plant-remains of our own country,
the climate appears to have been almost tropical in the Lower Eocene
period; and as we go further back we find no clear indications of a higher,
but often of a lower temperature, though always warmer or more equable
than our present climate. The abundant corals and reptiles of the Oolite and
Lias indicate equally tropical conditions; but further back, in the Trias, the
flora and fauna, in the British area, become poorer, and there is nothing
incompatible with a climate no warmer than that of the Upper Miocene.

This poverty is still more marked in the Permian formation, and it is here
that some indications of ice-action are found in the Lower Permian
conglomerates of the west of England. These beds contain abundant
fragments of various rocks, often angular and sometimes weighing half a
ton, while others are partially rounded, and have polished and striated
surfaces, just like the stones of the "till." They lie confusedly bedded in a
red unstratified marl, and some of them can be traced to the Welsh hills
from twenty to fifty miles distant. This remarkable formation was first
pointed out as proving a remote glacial period, by Professor Ramsay; and
Sir Charles Lyell agreed that this is the only possible explanation that, with
our present knowledge, we can give of them.
Permian breccias are also found in Ireland, containing blocks of Silurian
and Old Red sandstone rocks which Professor Hull believes could only
have been carried by floating ice. Similar breccias occur in the south of
Scotland, and these are stated to be "overlain by a deposit of glacial age, so
similar to the breccia below as to be with difficulty distinguished from it."
[79]

These numerous physical indications of ice-action over a considerable area
during the same geological period, coinciding with just such a poverty of
organic remains as might be produced by a very cold climate, are very
important, and seem clearly to indicate that at this remote period
geographical conditions were such as to bring about a glacial epoch, or
perhaps only local glaciation, in our part of the world.
Boulder-beds also occur in the Carboniferous formation, both in Scotland,
on the continent of Europe, and in North America; and Professor Dawson
considers that he has detected true glacial deposits of the same age in Nova
Scotia. Boulder-beds also occur in the Silurian rocks of Scotland and North
America, and according to Professor Dawson, even in the Huronian, older
than our Cambrian. None of these indications are however so satisfactory as
those of Permian age, where we have the very kind of evidence we looked
for in vain throughout the whole of the Tertiary and Secondary periods. Its
presence in several localities in such ancient rocks as the Permian is not
only most important as indicating a glacial epoch of some kind in
Palæozoic times, but confirms us in the validity of our conclusion, that the
total absence of any such evidence throughout the Tertiary and Secondary

epochs demonstrates the absence of recurring glacial epochs in the northern
hemisphere, notwithstanding the frequent recurrence of periods of high
excentricity.
Warm Arctic Climates in Early Secondary and Palæozoic Times.—The
evidence we have already adduced of the mild climates prevailing in the
Arctic regions throughout the Miocene, Eocene, and Cretaceous periods is
supplemented by a considerable body of facts relating to still earlier epochs.
In the Jurassic period, for example, we have proofs of a mild Arctic climate,
in the abundant plant-remains of East Siberia and Amurland, with less
productive deposits in Spitzbergen, and at Ando in Norway just within the
Arctic circle. But even more remarkable are the marine remains found in
many places in high northern latitudes, among which we may especially
mention the numerous ammonites and the vertebræ of huge reptiles of the
genera Ichthyosaurus and Teleosaurus found in the Jurassic deposits of the
Parry Islands in 77° N. Lat.
In the still earlier Triassic age, nautili and ammonites inhabited the seas of
Spitzbergen, where their fossil remains are now found.
In the Carboniferous formation we again meet with plant-remains and beds
of true coal in the Arctic regions. Lepidodendrons and Calamites, together
with large spreading ferns, are found at Spitzbergen, and at Bear Island in
the extreme north of Eastern Siberia; while marine deposits of the same age
contain abundance of large stony corals.
Lastly, the ancient Silurian limestones, which are widely spread in the high
Arctic regions, contain abundance of corals and cephalopodous mollusca
resembling those from the same deposits in more temperate lands.
Conclusions as to the Climates of Tertiary and Secondary Periods.—If now
we look at the whole series of geological facts as to the animal and
vegetable productions of the Arctic regions in past ages, it is certainly
difficult to avoid the conclusion that they indicate a climate of a uniformly
temperate or warm character. Whether in Miocene, Upper or Lower
Cretaceous, Jurassic, Triassic, Carboniferous or Silurian times, and in all
the numerous localities extending over more than half the polar regions, we
find one uniform climatic aspect in the fossils. This is quite inconsistent

with the theory of alternate cold and mild epochs during phases of high
excentricity, and persistent cold epochs when the excentricity was as low as
it is now or lower, for that would imply that the duration of cold conditions
was greater than that of warm. Why then should the fauna and flora of the
cold epochs never be preserved? Mollusca and many other forms of life are
abundant in the Arctic seas, and there is often a luxuriant dwarf woody
vegetation on the land, yet in no one case has a single example of such a
fauna or flora been discovered of a date anterior to the last glacial epoch.
And this argument is very much strengthened when we remember that an
exactly analogous series of facts is found over all the temperate zones.
Everywhere we have abundant floras and faunas indicating warmer
conditions than such as now prevail, but never in a single instance one
which as clearly indicates colder conditions. The fact that drift with Arctic
shells was deposited during the last glacial epoch, as well as gravels and
crag with the remains of arctic animals and plants, shows us that there is
nothing to prevent such deposits being formed in cold as well as in warm
periods; and it is quite impossible to believe that in every place and at all
epochs all records of the former have been destroyed, while in a
considerable number of instances those of the latter have been preserved.
When to this uniform testimony of the palæontological evidence we add the
equally uniform absence of any indication of those ice-borne rocks,
boulders, and drift, which are the constant and necessary accompaniment of
every period of glaciation, and which must inevitably pervade all the
marine deposits formed over a wide area so long as the state of glaciation
continues, we are driven to the conclusion that the last glacial epoch of the
northern hemisphere was exceptional, and was not preceded by numerous
similar glacial epochs throughout Tertiary and Secondary time.
But although glacial epochs (with the one or two exceptions already
referred to) were certainly absent, considerable changes of climate may
have frequently occurred, and these would lead to important changes in the
organic world. We can hardly doubt that some such change occurred
between the Lower and Upper Cretaceous periods, the floras of which
exhibit such an extraordinary contrast in general character. We have also the
testimony of Mr. J. S. Gardner, who has long worked at the fossil floras of
the Tertiary deposits, and who states, that there is strong negative and some
positive evidence of alternating warmer and colder conditions, not glacial,

contained not only in English Eocene, but all Tertiary beds throughout the
world.[80] In the case of marine faunas it is more difficult to judge, but the
numerous changes in the fossil remains from bed to bed only a few feet and
sometimes a few inches apart, may be sometimes due to change of climate;
and when it is recognised that such changes have probably occurred at all
geological epochs and their effects are systematically searched for, many
peculiarities in the distribution of organisms through the different members
of one deposit may be traced to this cause.
General View of Geological Climates as dependent on the Physical
Features of the Earth's Surface.—In the preceding chapters I have earnestly
endeavoured to arrive at an explanation of geological climates in the
temperate and Arctic zones, which should be in harmony with the great
body of geological facts now available for their elucidation. If my
conclusions as here set forth diverge considerably from those of Dr. Croll, it
is not from any want of appreciation of his facts and arguments, since for
many years I have upheld and enforced his views to the best of my ability.
But a careful re-examination of the whole question has now convinced me
that an error has been made in estimating the comparative effect of
geographical and astronomical causes on changes of climate, and that,
while the latter have undoubtedly played an important part in bringing
about the glacial epoch, it is to the former that the mild climates of the
Arctic regions are almost entirely due. If I have now succeeded in
approaching to a true solution of this difficult problem, I owe it mainly to
the study of Dr. Croll's writings, since my theory is entirely based on the
facts and principles so clearly set forth in his admirable papers on "Ocean
Currents in relation to the Distribution of Heat over the Globe." The main
features of this theory as distinct from that of Dr. Croll I will now
endeavour to summarise.
Looking at the subject broadly, we see that the climatic condition of the
northern hemisphere is the result of the peculiar distribution of land and
water upon the globe; and the general permanence of the position of the
continental and oceanic areas—which we have shown to be proved by so
many distinct lines of evidence—is also implied by the general stability of
climate throughout long geological periods. The land surface of our earth
appears to have always consisted of three great masses in the north
temperate zone, narrowing southward, and terminating in three

comparatively narrow extremities represented by Southern America, South
Africa, and Australia. Towards the north these masses have approached
each other, and have sometimes become united; leaving beyond them a
considerable area of open polar sea. Towards the south they have never
been much further prolonged than at present, but far beyond their
extremities an extensive mass of land has occupied the south polar area.
This arrangement is such as would cause the northern hemisphere to be
always (as it is now) warmer than the southern, and this would lead to the
preponderance of northward winds and ocean currents, and would bring
about the concentration of the latter in three great streams carrying warmth
to the north-polar regions. These streams would, as Dr. Croll has so well
shown, be greatly increased in power by the glaciation of the south polar
land; and whenever any considerable portion of this land was elevated, such
a condition of glaciation would certainly be brought about, and would be
heightened whenever a high degree of excentricity prevailed.
It is now the general opinion of geologists that the great continents have
undergone a process of development from earlier to later times. Professor
Dana appears to have been the first who taught it explicitly in the case of
the North American continent, and he has continued the development of his
views from 1856, when he discussed the subject in the American Journal,
to the later editions of his Manual of Geology in which the same views are
extended to all the great continents. He says:—
"The North American continent, which since early time had been gradually
expanding in each direction from the northern Azoic, eastward, westward,
and southward, and which, after the Palæozoic, was finished in its rocky
foundation, excepting on the borders of the Atlantic and Pacific and the area
of the Rocky Mountains, had reached its full expansion at the close of the
Tertiary period. The progress from the first was uniform and systematic: the
land was at all times simple in outline; and its enlargement took place with
almost the regularity of an exogenous plant."[81]
A similar development undoubtedly took place in the European area, which
was apparently never so compact and so little interpenetrated by the sea as
it is now, while Europe and Asia have only become united into one
unbroken mass since late Tertiary times.

If, however, the greater continents have become more compact and massive
from age to age, and have received their chief extensions northward at a
comparatively recent period, while the Antarctic lands had a corresponding
but somewhat earlier development, we have all the conditions requisite to
explain the persistence, with slight fluctuations, of warm climates far into
the north-polar area throughout Palæozoic, Mesozoic, and Tertiary times. At
length, during the latter part of the Tertiary epoch, a considerable elevation
took place, closing up several of the water passages to the north, and raising
up extensive areas in the Arctic regions to become the receptacle of snow
and ice-fields. This elevation is indicated by the abundance of Miocene and
the absence of Pliocene deposits in the Arctic zone and the considerable
altitude of many Miocene rocks in Europe and North America; and the
occurrence at this time of a long-continued period of high excentricity
necessarily brought on the glacial epoch in the manner already described in
our last chapter. A depression seems to have occurred during the glacial
period itself in North America as in Britain, but this may have been due
partly to the weight of the ice and partly to a rise of the ocean level caused
by the earth's centre of gravity being shifted towards the north.
We thus see that the last glacial epoch was the climax of a great process of
continental development which had been going on throughout long
geological ages; and that it was the direct consequence of the north
temperate and polar land having attained a great extension and a
considerable altitude just at the time when a phase of very high excentricity
was coming on. Throughout earlier Tertiary and Secondary times an equally
high excentricity often occurred, but it never produced a glacial epoch,
because the north temperate and polar areas had less high land, and were
more freely open to the influx of warm oceanic currents. But wherever great
plateaux with lofty mountains occurred in the temperate zone a
considerable local glaciation might be produced, which would be specially
intense during periods of high excentricity; and it is to such causes we must
impute the indications of ice-action in the vicinity of the Alps during the
Tertiary period. The Permian glaciation appears to have been more
extensive, and it is quite possible that at this remote epoch a sufficient mass
of high land existed in our area and northwards towards the pole, to have
brought on a true glacial period comparable with that which has so recently
passed away.

Estimate of the comparative effects of Geographical and Astronomical
Causes in producing Changes of Climate.—It appears then, that while
geographical and physical causes alone, by their influence on ocean
currents, have been the main agents in producing the mild climates which
for such long periods prevailed in the Arctic regions, the concurrence of
astronomical causes—high excentricity with winter in aphelion—was
necessary to the production of the great glacial epoch. If we reject this latter
agency, we shall be obliged to imagine a concurrence of geographical
changes at a very recent period of which we have no evidence. We must
suppose, for example, that a large part of the British Isles—Scotland,
Ireland, and Wales at all events—were simultaneously elevated so as to
bring extensive areas above the line of perpetual snow; that about the same
time Scandinavia, the Alps, and the Pyrenees received a similar increase of
altitude; and that, almost simultaneously, Eastern North America, the Sierra
Nevada of California, the Caucasus, Lebanon, the southern mountains of
Spain, the Atlas range, and the Himalayas, were each some thousands of
feet higher than they are now; for all these mountains present us with
indications of a recent extension of their glaciers, in superficial phenomena
so similar to those which occur in our own country and in Western Europe,
that we cannot suppose them to belong to a different epoch. Such a
supposition is rendered more difficult by the general concurrence of
scientific testimony to a partial submergence during the glacial epoch, not
only in all parts of Britain, but in North America, Scandinavia, and, as
shown by the wide extension of the drift, in Northern Europe; and when to
this we add the difficulty of understanding how any probable addition to the
altitude of our islands could have brought about the extreme amount of
glaciation which they certainly underwent, and when, further, we know that
a phase of very high excentricity did occur at a period which is generally
admitted to agree well with physical evidence of the time elapsed since the
cold passed away, there seems no sufficient reason why such an agency
should be ignored.
No doubt a prejudice has been excited against it in the minds of many
geologists, by its being thought to lead necessarily to frequently recurring
glacial epochs throughout all geological time. But I have here endeavoured
to show that this is not a necessary consequence of the theory, because a
concurrence of favourable geographical conditions is essential to the

initiation of a glaciation, which when once initiated has a tendency to
maintain itself throughout the varying phases of precession occurring
during a period of high excentricity. When, however, geographical
conditions favour warm Arctic climates—as it has been shown they have
done throughout the larger portion of geological time—then changes of
excentricity, to however great an extent, have no tendency to bring about a
state of glaciation, because warm oceanic currents have a preponderating
influence, and without very large areas of high northern land to act as
condensers, no perpetual snow is possible, and hence the initial process of
glaciation does not occur.
The theory as now set forth should commend itself to geologists, since it
shows the direct dependence of climate on physical processes, which are
guided and modified by those changes in the earth's surface which geology
alone can trace out. It is in perfect accord with the most recent teachings of
the science as to the gradual and progressive development of the earth's
crust from the rudimentary formations of the Azoic age, and it lends support
to the view that no inportant[**important] departure from the great lines of
elevation and depression originally marked out on the earth's surface has
ever taken place.
It also shows us how important an agent in the production of a habitable
globe with comparatively small extremes of climates over its whole area, is
the great disproportion between the extent of the land and the water
surfaces. For if these proportions had been reversed, large areas of land
would necessarily have been removed from the beneficial influence of
aqueous currents or moisture-laden winds; and slight geological changes
might easily have led to half the land surface becoming covered with
perpetual snow and ice, or being exposed to extremes of summer heat and
winter cold, of which our water-permeated globe at present affords no
example. We thus see that what are usually regarded as geographical
anomalies—the disproportion of land and water, the gathering of the land
mainly into one hemisphere, and the singular arrangement of the land in
three great southward-pointing masses—are really facts of the greatest
significance and importance, since it is to these very anomalies that the
universal spread of vegetation and the adaptability of so large a portion of
the earth's surface for human habitation is directly due.

CHAPTER X

THE EARTH'S AGE, AND THE RATE OF DEVELOPMENT OF
ANIMALS AND PLANTS

Various Estimates of Geological Time—Denudation and Deposition of
Strata as a Measure of Time—How to Estimate the Thickness of the
Sedimentary Rocks—How to Estimate the Average Rate of Deposition
of the Sedimentary Rocks—The Rate of Geological Change Probably
greater in very Remote Times—Value of the Preceding Estimate of
Geological Time—Organic Modification Dependent on Change of
Conditions—Geographical Mutations as a Motive Power in bringing
about Organic Changes—Climatal Revolutions as an Agent in
Producing Organic Changes—Present Condition of the Earth one of
Exceptional Stability as Regards Climate—Date of last Glacial Epoch
and its Bearing on the Measurement of Geological Time—Concluding
Remarks.

The subjects discussed in the last three chapters introduce us to a difficulty
which has hitherto been considered a very formidable one—that the
maximum age of the habitable earth, as deduced from physical
considerations, does not afford sufficient time either for the geological or
the organic changes of which we have evidence. Geologists continually
dwell on the slowness of the processes of upheaval and subsidence, of
denudation of the earth's surface, and of the formation of new strata; while
on the theory of development, as expounded by Mr. Darwin, the variation
and modification of organic forms is also a very slow process, and has
usually been considered to require an even longer series of ages than might
satisfy the requirements of physical geology alone.
As an indication of the periods usually contemplated by geologists, we may
refer to Sir Charles Lyell's calculation in the tenth edition of his Principles
of Geology (omitted in later editions), by which he arrived at 240 millions
of years as having probably elapsed since the Cambrian period—a very
moderate estimate in the opinion of most geologists. This calculation was

founded on the rate of modification of the species of mollusca; but much
more recently Professor Haughton has arrived at nearly similar figures from
a consideration of the rate of formation of rocks and their known maximum
thickness, whence he deduces a maximum of 200 millions of years for the
whole duration of geological time, as indicated by the series of stratified
formations.[82] But in the opinion of all our first naturalists and geologists,
the period occupied in the formation of the known stratified rocks only
represents a portion, and perhaps a small portion, of geological time. In the
sixth edition of the Origin of Species (p. 286), Mr. Darwin says:
"Consequently, if the theory be true, it is indisputable that before the lowest
Cambrian stratum was deposited long periods elapsed, as long as, or
probably far longer than, the whole interval from the Cambrian age to the
present day; and that during these vast periods the world swarmed with
living creatures." Professor Huxley, in his anniversary address to the
Geological Society in 1870, adduced a number of special cases showing
that, on the theory of development, almost all the higher forms of life must
have existed during the Palæozoic period. Thus, from the fact that almost
the whole of the Tertiary period has been required to convert the ancestral
Orohippus into the true horse, he believes that, in order to have time for the
much greater change of the ancestral Ungulata into the two great odd-toed
and even-toed divisions (of which change there is no trace even among the
earliest Eocene mammals), we should require a large portion, if not the
whole, of the Mesozoic or Secondary period. Another case is furnished by
the bats and whales, both of which strange modifications of the mammalian
type occur perfectly developed in the Eocene formation. What countless
ages back must we then go for the origin of these groups, the whales from
some ancestral carnivorous animal, and the bats from the insectivora! And
even then we have to seek for the common origin of carnivora, insectivora,
ungulata, and marsupials at a far earlier period; so that, on the lowest
estimate, we must place the origin of the mammalia very far back in
Palæozoic times. Similar evidence is afforded by reptiles, of which
Professor Huxley says: "If the very small differences which are observable
between the crocodiles of the older Secondary formations and those of the
present day furnish any sort of an approximation towards an estimate of the
average rate of change among reptiles, it is almost appalling to reflect how
far back in Palæozoic times we must go before we can hope to arrive at that
common stock from which the crocodiles, lizards, Ornithoscelida, and

Plesiosauria, which had attained so great a development in the Triassic
epoch, must have been derived." Professor Ramsay has expressed similar
views, derived from a general study of the whole series of geological
formations and their contained fossils. He says, speaking of the abundant,
varied, and well-developed fauna of the Cambrian period: "In this earliest
known varied life we find no evidence of its having lived near the
beginning of the zoological series. In a broad sense, compared with what
must have gone before, both biologically and physically, all the phenomena
connected with this old period seem, to my mind, to be of quite a recent
description; and the climates of seas and lands were of the very same kind
as those the world enjoys at the present day."[83]
These opinions, and the facts on which they are founded, are so weighty,
that we can hardly doubt that, if the time since the Cambrian epoch is
correctly estimated at 200 millions of years, the date of the commencement
of life on the earth cannot be much less than 500 millions; while it may not
improbably have been longer, because the reaction of the organism under
changes of the environment is believed to have been less active in low and
simple, than in high and complex forms of life, and thus the processes of
organic development may for countless ages have been excessively slow.
But according to the physicists, no such periods as are here contemplated
can be granted. From a consideration of the possible sources of the heat of
the sun, as well as from calculations of the period during which the earth
can have been cooling to bring about the present rate of increase of
temperature as we descend beneath the surface, Sir William Thomson
concludes that the crust of the earth cannot have been solidified much
longer than 100 million years (the maximum possible being 400 millions),
and this conclusion is held by Dr. Croll and other men of eminence to be
almost indisputable.[84] It will therefore be well to consider on what data the
calculations of geologists have been founded, and how far the views here
set forth, as to frequent changes of climate throughout all geological time,
may affect the rate of biological change.
Denudation and Deposition of Strata as a Measure of Time.—The materials
of all the stratified rocks of the globe have been obtained from the dry land.
Every point of the surface is exposed to the destructive influences of sun
and wind, frost, snow, and rain, which break up and wear away the hardest

rocks as well as the softer deposits, and by means of rivers convey the worn
material to the sea. The existence of a considerable depth of soil over the
greater part of the earth's surface; of vast heaps of rocky débris at the foot
of every inland cliff; of enormous deposits of gravel, sand, and loam; as
well as the shingle, pebbles, sand or mud, of every sea-shore, alike attest the
universality of this destructive agency. It is no less clearly shown by the
way in which almost every drop of running water—whether in gutter,
brooklet, stream or large river—becomes discoloured after each heavy
rainfall, since the matter which causes this discolouration must be derived
from the surface of the country, must always pass from a higher to a lower
level, and must ultimately reach the sea, unless it is first deposited in some
lake, or by the overflowing of a river goes to form an alluvial plain. The
universality of this subaërial denudation, both as regards space and time,
renders it certain that its cumulative effects must be very great; but no
attempt seems to have been made to determine the magnitude of these
effects till Mr. Alfred Tylor, in 1853,[85] pointed out that by measuring the
quantity of solid matter brought down by rivers (which can be done with
considerable accuracy), we may obtain the amount of lowering of the land-
area, and also the rise of the ocean level, owing to the quantity of matter
deposited on its floor. A few years later Dr. Croll applied the same method
in more detail to an estimate of the amount by which the land is lowered in
a given period; and the validity of this method has been upheld by Sir A.
Geikie, Sir Charles Lyell, and all our best geologists, as affording a means
of actually determining with some approach to accuracy, the time occupied
by one important phase of geological change.
The quantity of matter carried away from the land by a river is greater than
at first sight appears, and is more likely to be under- than over-estimated.
By taking samples of water near the mouth of a river (but above the
influence of the tide) at a sufficient number of points in its channel and at
different depths, and repeating this daily or at other short intervals
throughout the year, it is easy to determine the quantity of solid matter held
in suspension and solution; and if corresponding observations determine the
quantity of water that is discharged, the total amount of solid matter brought
down annually may be calculated. But besides this, a considerable quantity
of sand or even gravel is carried along the bottom or bed of the river, and
this has rarely been estimated, so that the figures hitherto obtained are

usually under the real quantities. There is also another source of error
caused by the quantity of matter the river may deposit in lakes or in flooded
lands during its course, for this adds to the amount of denudation performed
by the river, although the matter so deposited does not come down to the
sea. After a careful examination of all the best records, Sir A. Geikie arrives
at the following results, as to the quantity of matter removed by seven rivers
from their basins, estimated by the number of years required to lower the
whole surface an average of one foot:
The Mississippi removes one foot in 6,000 years.
,, Ganges ,, ,, 2,358 ,,
,, Hoang Ho ,, ,, 1,464 ,,
,, Rhone ,, ,, 1,528 ,,
,, Danube ,, ,, 6,846 ,,
,, Po ,, ,, 729 ,,
,, Nith ,, ,, 4,723 ,,
Here we see an intelligible relation between the character of the river basin
and the amount of denudation. The Mississippi has a large portion of its
basin in an arid country, and its sources are either in forest-clad plateaux or
in mountains free from glaciers and with a scanty rainfall. The Danube
flows through Eastern Europe where the rainfall is considerably less than in
the west, while comparatively few of its tributaries rise among the loftiest
Alps. The proportionate amounts of denudation being then what we might
expect, and as all are probably under rather than over the truth, we may
safely take the average of them all as representing an amount of denudation
which, if not true for the whole land surface of the globe, will certainly be
so for a very considerable proportion of it. This average is almost exactly
one foot in three thousand years.[86] The mean altitude of the several
continents has been recently estimated by Mr. John Murray,[87] to be as
follows: Europe 939 feet, Asia 3,189 feet, Africa 2020 feet, North America
1,888 feet, and South America 2,078 feet. At the rate of denudation above
given, it results that, were no other forces at work, Europe would be planed
down to the sea-level in about two million eight hundred thousand years;
while if we take a somewhat slower rate for North America, that continent
might last about four or five million years.[88] This also implies that the
mean height of these continents would have been about double what it is

now three million and five million years ago respectively: and as we have
no reason to suppose this to have been the case, we are led to infer the
constant action of that upheaving force which the presence of sedimentary
formations even on the highest mountains also demonstrates.
We have already discussed the unequal rate of denudation on hills, valleys,
and lowlands, in connection with the evidence of remote glacial epochs (p.
173); what we have now to consider is, what becomes of all this denuded
matter, and how far the known rate of denudation affords us a measure of
the rate of deposition, and thus gives us some indication of the lapse of
geological time from a comparison of this rate with the observed thickness
of stratified rocks on the earth's surface.
How to Estimate the Thickness of the Sedimentary Rocks.—The
sedimentary rocks of which the earth's crust is mainly composed consist,
according to Sir Charles Lyell's classification, of fourteen great formations,
of which the most ancient is the Laurentian, and the most recent the Post-
Tertiary or Pleistocene; with thirty important subdivisions, each of which
again consists of a more or less considerable number of distinct beds or
strata. Thus, the Silurian formation is divided into Upper and Lower
Silurian, each characterized by a distinct set of fossil remains, and the
Upper Silurian again consists of a large number of separate beds, such as
the Wenlock Limestone, the Upper Llandovery Sandstone the Lower
Llandovery Slates, &c., each usually characterised by a difference of
mineral composition or mechanical structure, as well as by some peculiar
fossils. These beds and formations vary greatly in extent, both above and
beneath the surface, and are also of very various thicknesses in different
localities. A thick bed or series of beds often thins out in a given direction,
and sometimes disappears altogether, so that two beds which were
respectively above and beneath it may come into contact. As an example of
this thinning out, American geologists adduce the Palæozoic formations of
the Appalachian Mountains, which have a total thickness of 42,000 feet, but
as they are traced westward thin out till they become only 4,000 feet in total
thickness. In like manner the Carboniferous grits and shales are 18,000 feet
thick in Yorkshire and Lancashire, but they thin out southwards, so that in
Leicestershire they are only 3,000 feet thick; and similar phenomena occur
in all strata and in every part of the world. It must be observed that this
thinning out has nothing to do with denudation (which acts upon the surface

of a country so as to produce great irregularities of contour), but is a regular
attenuation of the layers of rock, due to a deficiency of sediment in certain
directions at the original formation of the deposit. Owing to this thinning
out of stratified rocks, they are on the whole of far less extent than is
usually supposed. When we see a geological map showing successive
formations following each other in long irregular belts across the country
(as is well seen in the case of the Secondary rocks of England), and a
corresponding section showing each bed dipping beneath its predecessor,
we are apt to imagine that beneath the uppermost bed we should find all the
others following in succession like the coats of an onion. But this is far
from being the case, and a remarkable proof of the narrow limitation of
these formations has been recently obtained by a boring at Ware through the
Chalk and Gault Clay, which latter immediately rests on the Upper Silurian
Wenlock Limestone full of characteristic fossils, at a depth of only 800 feet.
Here we have an enormous gap, showing that none of earlier Secondary or
late Palæozoic formations extend to this part of England, unless indeed they
had been all once elevated and entirely swept away by denudation.[89]
But if we consider how such deposits are now forming, we shall find that
the thinning out of the beds of each formation, and their restriction to
irregular bands and patches, is exactly what we should expect. The
enormous quantity of sediment continually poured into the sea by rivers,
gradually subsides to the bottom as soon as the motion of the water is
checked. All the heavier material must be deposited near the shore or in
those areas over which it is first spread by the tides or currents of the ocean;
while only the very fine mud and clay is carried out to considerable
distances. Thus all stratified deposits will form most quickly near the
shores, and will thin out rapidly at greater distances, little or none being
formed in the depths of the great oceans. This important fact was
demonstrated by the specimens of sea-bottom examined during the voyage
of the Challenger, all the "shore deposits" being usually confined within a
distance of 100 or 150 miles from the coast; while the "deep-sea deposits"
are either purely organic, being formed of the calcareous or siliceous
skeletons of globigerinæ, radiolarians, and diatomaceæ, or are clays formed
of undissolved portions of these, together with the disintegrated or
dissolved materials of pumice and volcanic dust, which being very light are
carried by wind or by water over the widest oceans.

From the preceding considerations we shall be better able to appreciate the
calculations as to the thickness of stratified deposits made by geologists.
Professor Ramsay has calculated that the sedimentary rocks of Britain alone
have a total maximum thickness of 72,600 feet; while Professor Haughton,
from a survey of the whole world, estimates the maximum thickness of the
known stratified rocks at 177,200 feet. Now these maximum thicknesses of
each deposit will have been produced only where the conditions were
exceptionally favourable, either in deep water near the mouths of great
rivers, or in inland seas, or in places to which the drainage of extensive
countries was conveyed by ocean currents; and this great thickness will
necessarily be accompanied by a corresponding thinness, or complete
absence of deposit, elsewhere. How far the series of rocks found in any
extensive area, as Europe or North America, represents the whole series of
deposits which have been made there we cannot tell; but there is no reason
to think that it is a very inadequate representation of their maximum
thickness, though it undoubtedly is of their extent and bulk. When we see in
how many distinct localities patches of the same formation occur, it seems
improbable that the whole of the deposits formed during any one period
should have been destroyed, even in such an area as Europe, while it is still
more improbable that they should be so destroyed over the whole world;
and if any considerable portion of them is left, that portion may give a fair
idea of their average, or even of their maximum, thickness. In his admirable
paper on "The Mean Thickness of the Sedimentary Rocks,"[90] Dr. James
Croll has dwelt on the extent of denudation in diminishing the mean
thickness of the rocks that have been formed, remarking, "Whatever the
present mean thickness of all the sedimentary rocks of our globe may be, it
must be small in comparison to the mean thickness of all the sedimentary
rocks which have been formed. This is obvious from the fact that the
sedimentary rocks of one age are partly formed from the destruction of the
sedimentary rocks of former ages. From the Laurentian age down to the
present day the stratified rocks have been undergoing constant denudation."
This is perfectly true, and yet the mean thickness of that portion of the
sedimentary rocks which remains may not be very different from that of the
entire mass, because denudation acts only on those rocks which are exposed
on the surface of a country, and most largely on those that are upheaved;
while, except in the rare case of an extensive formation being quite
horizontal, and wholly exposed to the sea or to the atmosphere, denudation

can have no tendency to diminish the thickness of any entire deposit.[91]
Unless, therefore, a formation is completely destroyed by denudation in
every part of the world (a thing very improbable), we may have in existing
rocks a not very inadequate representation of the mean thickness of all that
have been formed, and even of the maximum thickness of the larger portion.
This will be the more likely because it is almost certain that many rocks
contemporaneously formed are counted by geologists as distinct formations,
whenever they differ in lithological character or in organic remains. But we
know that limestones, sandstones, and shales, are always forming at the
same time; while a great difference in organic remains may arise from
comparatively slight changes of geographical features, or from difference in
the depth or purity of the water in which the animals lived.[92]
How to Estimate the Average Rate of Deposition of the Sedimentary Rocks.
—But if we take the estimate of Professor Haughton (177,200 feet), which,
as we have seen, is probably excessive, for the maximum thickness of the
sedimentary rocks of the globe of all known geological ages, can we arrive
at any estimate of the rate at which they were formed? Dr. Croll has
attempted to make such an estimate, but he has taken for his basis the mean
thickness of the rocks, which we have no means whatever of arriving at,
and which he guesses, allowing for denudation, to be equal to the maximum
thickness as measured by geologists. The land-area of the globe is,
according to Dr. Croll, 57,000,000[93] square miles, and he gives the coast-
line as 116,000 miles. This, however, is, for our purpose, rather too much,
as it allows for bays, inlets, and the smaller islands. An approximate
measurement on a globe shows that 100,000 miles will be nearer the mark,
and this has the advantage of being an easily remembered even number. The
distance from the coast, to which shore-deposits usually extend, may be
reckoned at about 100 or 150 miles, but by far the larger portion of the
matter brought down from the land will be deposited comparatively close to
the shore; that is, within twenty or thirty miles. If we suppose the portion
deposited beyond thirty miles to be added to the deposits within that
distance, and the whole reduced to a uniform thickness in a direction at
right angles to the coast, we should probably include all areas where
deposits of the maximum thickness are forming at the present time, along
with a large but unknown proportion of surface where the deposits were far
below the maximum thickness. This follows, if we consider that deposit

must go on very unequally along different parts of a coast, owing to the
distance from each other of the mouths of great rivers and the limitations of
ocean currents; and because, compared with the areas over which a thick
deposit is forming annually, those where there is little or none are probably
at least twice as extensive. If, therefore, we take a width of thirty miles
along the whole coast-line of the globe as representing the area over which
deposits are forming, corresponding to the maximum thickness as measured
by geologists, we shall certainly over rather than under-estimate the
possible rate of deposit.[94]
Now a coast line of 100,000 miles with a width of 30 gives an area of
3,000,000 square miles, on which the denuded matter of the whole land-
area of 57,000,000 square miles is deposited. As these two areas are as 1 to
19, it follows that deposition, as measured by maximum thickness, goes on
at least nineteen times as fast as denudation—probably very much faster.
But the mean rate of denudation over the whole earth is about one foot in
three thousand years; therefore the rate of maximum deposition will be at
least 19 feet in the same time; and as the total maximum thickness of all the
stratified rocks of the globe is, according to Professor Haughton, 177,200
feet, the time required to produce this thickness of rock, at the present rate
of denudation and deposition, is only 28,000,000 years.[95]
The Rate of Geological Change Probably Greater in very Remote Times.—
The opinion that denudation and deposition went on more rapidly in earlier
times owing to the frequent occurrence of vast convulsions and cataclysms
was strenuously opposed by Sir Charles Lyell, who so well showed that
causes of the very same nature as those now in action were sufficient to
account for all the phenomena presented by the rocks throughout the whole
series of geological formations. But while upholding the soundness of the
views of the "uniformitarians" as opposed to the "convulsionists," we must
yet admit that there is reason for believing in a gradually increasing
intensity of all telluric action as we go back into past time. This subject has
been well treated by Mr. W. J. Sollas,[96] who shows that, if, as all physicists
maintain, the sun gave out perceptibly more heat in past ages than now, this
alone would cause an increase in almost all the forces that have brought
about geological phenomena. With greater heat there would be a more
extensive aqueous atmosphere, and, perhaps, a greater difference between

equatorial and polar temperatures; hence more violent winds, heavier rains
and snows, and more powerful oceanic currents, all producing more rapid
denudation. At the same time, the internal heat of the earth being greater, it
would be cooling more rapidly, and thus the forces of contraction—which
cause the upheaving of mountains, the eruption of volcanoes, and the
subsidence of extensive areas—would be more powerful and would still
further aid the process of denudation. Yet again, the earth's rotation was
certainly more rapid in very remote times, and this would cause more
impetuous tides and still further add to the denuding power of the ocean. It
thus appears that, as we go back into the past, all the forces tending to the
continued destruction and renewal of the earth's surface would be in more
powerful action, and must therefore tend to reduce the time required for the
deposition and upheaval of the various geological formations. It may be
true, as many geologists assert, that the changes here indicated are so slow
that they would produce comparatively little effect within the time occupied
by the known sedimentary rocks, yet, whatever effect they did produce
would certainly be in the direction here indicated, and as several causes are
acting together, their combined effects may have been by no means
unimportant. It must also be remembered that such an increase of the
primary forces on which all geologic change depends would act with great
effect in still further intensifying those alternations of cold and warm
periods in each hemisphere, or, more frequently, of excessive and equable
seasons, which have been shown to be the result of astronomical, combined
with geographical, revolutions; and this would again increase the rapidity of
denudation and deposition, and thus still further reduce the time required
for the production of the known sedimentary rocks. It is evident therefore
that these various considerations all combine to prove that, in supposing
that the rate of denudation has been on the average only what it is now, we
are almost certainly over-estimating the time required to have produced the
whole series of formations from the Cambrian upwards.
Value of the Preceding Estimate of Geological Time.—It is not of course
supposed that the calculation here given makes any approach to accuracy,
but it is believed that it does indicate the order of magnitude of the time
required. We have a certain number of data, which are not guessed but the
result of actual measurement; such are, the amount of solid matter carried
down by rivers, the width of the belt within which this matter is mainly

deposited, and the maximum thickness of the known stratified rocks.[97] A
considerable but unknown amount of denudation is effected by the waves of
the ocean eating away coast lines. This was once thought to be of more
importance than sub-aërial denudation, but it is now believed to be
comparatively slow in its action.[98] Whatever it may be, however, it adds to
the rate of formation of new strata, and its omission from the calculation is
again on the side of making the lapse of time greater rather than less than
the true amount. Even if a considerable modification should be needed in
some of the assumptions it has been necessary to make, the result must still
show that, so far as the time required for the formation of the known
stratified rocks, the hundred million years allowed by physicists is not only
ample, but will permit of even more than an equal period anterior to the
lowest Cambrian rocks, as demanded by Mr. Darwin—a demand supported
and enforced by the arguments, taken from independent standpoints, of
Professor Huxley and Professor Ramsay.
Organic Modification Dependent on Change of Conditions.—Having thus
shown that the physical changes of the earth's surface may have gone on
much more rapidly and occupied much less time than has generally been
supposed, we have now to inquire whether there are any considerations
which lead to the conclusion that organic changes may have gone on with
corresponding rapidity.
There is no part of the theory of natural selection which is more clear and
satisfactory than that which connects changes of specific forms with
changes of external conditions or environment. If the external world
remains for a moderate period unchanged, the organic world soon reaches a
state of equilibrium through the struggle for existence; each species
occupies its place in nature, and there is then no inherent tendency to
change. But almost any change whatever in the external world disturbs this
equilibrium, and may set in motion a whole series of organic revolutions
before it is restored. A change of climate in any direction will be sure to
injure some and benefit other species. The one will consequently diminish,
the other increase in number; and the former may even become extinct. But
the extinction of a species will certainly affect other species which it either
preyed upon, or competed with, or served for food; while the increase of
any one animal may soon lead to the extinction of some other to which it
was inimical. These changes will in their turn bring other changes; and

before an equilibrium is again established, the proportions, ranges, and
numbers, of the species inhabiting the country may be materially altered.
The complex manner in which animals are related to each other is well
exhibited by the importance of insects, which in many parts of the world
limit the numbers or determine the very existence of some of the higher
animals. Mr. Darwin says:—"Perhaps Paraguay offers the most curious
instance of this; for here neither cattle, nor horses, nor dogs have ever run
wild, though they swarm southward and northward in a wild state; and
Azara and Rengger have shown that this is caused by the greater number in
Paraguay of a certain fly, which lays its eggs in the navels of these animals
when first born. The increase of these flies, numerous as they are, must be
habitually checked by some means, probably by other parasitic insects.
Hence, if certain insectivorous birds were to decrease in Paraguay, the
parasitic insects would probably increase; and this would lessen the number
of navel-frequenting flies—then cattle and horses would run wild; and this
would certainly alter (as indeed I have observed in parts of South America)
the vegetation: this again would largely affect the insects, and this, as we
have seen in Staffordshire, the insectivorous birds, and so onwards in ever
increasing circles of complexity."
Geographical changes would be still more important, and it is almost
impossible to exaggerate the modifications of the organic world that might
result from them. A subsidence of land separating a large island from a
continent would affect the animals and plants in a variety of ways. It would
at once modify the climate, and so produce a series of changes from this
cause alone; but more important would be its effect by isolating small
groups of individuals of many species and thus altering their relations to the
rest of the organic world. Many of these would at once be exterminated,
while others, being relieved from competition, might flourish and become
modified into new species. Even more striking would be the effects when
two continents, or any two land areas which had been long separated, were
united by an upheaval of the strait which divided them. Numbers of animals
would now be brought into competition for the first time. New enemies and
new competitors would appear in every part of the country; and a struggle
would commence which, after many fluctuations, would certainly result in
the extinction of some species, the modification of others, and a

considerable alteration in the proportionate numbers and the geographical
distribution of almost all.
Any other changes which led to the intermingling of species whose ranges
were usually separate would produce corresponding results. Thus, increased
severity of winter or summer temperature, causing southward migrations
and the crowding together of the productions of distinct regions, must
inevitably produce a struggle for existence, which would lead to many
changes both in the characters and the distribution of animals. Slow
elevations of the land would produce another set of changes, by affording
an extended area in which the more dominant species might increase their
numbers; and by a greater range and variety of alpine climates and
mountain stations, affording room for the development of new forms of life.
Geographical Mutations as a Motive Power in Bringing about Organic
Changes.—Now, if we consider the various geographical changes which, as
we have seen, there is good reason to believe have ever been going on in
the world, we shall find that the motive power to initiate and urge on
organic changes has never been wanting. In the first place, every continent,
though permanent in a general sense, has been ever subject to innumerable
physical and geographical modifications. At one time the total area has
increased, and at another has diminished; great plateaus have gradually
risen up, and have been eaten out by denudation into mountain and valley;
volcanoes have burst forth, and, after accumulating vast masses of eruptive
matter, have sunk down beneath the ocean, to be covered up with
sedimentary rocks, and at a subsequent period again raised above the
surface; and the loci of all these grand revolutions of the earth's surface
have changed their position age after age, so that each portion of every
continent has again and again been sunk under the ocean waves, formed the
bed of some inland sea, or risen high into plateaus and mountain ranges.
How great must have been the effects of such changes on every form of
organic life! And it is to such as these we may perhaps trace those great
changes of the animal world which have seemed to revolutionise it, and
have led us to class one geological period as the age of reptiles, another as
the age of fishes, and a third as the age of mammals.
But such changes as these must necessarily have led to repeated unions and
separations of the land masses of the globe, joining together continents

which were before divided, and breaking up others into great islands or
extensive archipelagoes. Such alterations of the means of transit would
probably affect the organic world even more profoundly than the changes of
area, of altitude, or of climate, since they afforded the means, at long
intervals, of bringing the most diverse forms into competition, and of
spreading all the great animal and vegetable types widely over the globe.
But the isolation of considerable masses of land for long periods also
afforded the means of preservation to many of the lower types, which thus
had time to become modified into a variety of distinct forms, some of which
became so well adapted to special modes of life that they have continued to
exist to the present day, thus affording us examples of the life of early ages
which would probably long since have become extinct had they been
always subject to the competition of the more highly organised animals. As
examples of such excessively archaic forms, we may mention the mud-
fishes and the ganoids, confined to limited fresh-water areas; the frogs and
toads, which still maintain themselves vigorously in competition with
higher forms; and among mammals the Ornithorhynchus and Echidna of
Australia; the whole order of Marsupials—which, out of Australia, where
they are quite free from competition, only exist abundantly in South
America, which was certainly long isolated from the northern continents;
the Insectivora, which, though widely scattered, are generally nocturnal or
subterranean in their habits; and the Lemurs, which are most abundant in
Madagascar, where they have long been isolated, and almost removed from
the competition of higher forms.
Climatal Revolutions as an Agent in Producing Organic Changes.—The
geographical and geological changes we have been considering are
probably those which have been most effective in bringing about the great
features of the distribution of animals, as well as the larger movements in
the development of organised beings; but it is to the alternations of warm
and cold, or of uniform and excessive climates—of almost perpetual spring
in arctic as well as in temperate lands, with occasional phases of cold
culminating at remote intervals in glacial epochs,—that we must impute
some of the more remarkable changes both in the specific characters and in
the distribution of organisms.[99] Although the geological evidence is
opposed to the belief in early glacial epochs except at very remote and
distant intervals, there is nothing which contradicts the occurrence of

repeated changes of climate, which, though too small in amount to produce
any well-marked physical or organic change, would yet be amply sufficient
to keep the organic world in a constant state of movement, and which, by
subjecting the whole flora and fauna of a country at comparatively short
intervals to decided changes of physical conditions, would supply that
stimulus and motive power which, as we have seen, is all that is necessary
to keep the processes of "natural selection" in constant operation.
The frequent recurrence of periods of high and of low excentricity must
certainly have produced changes of climate of considerable importance to
the life of animals and plants. During periods of high excentricity with
summer in perihelion, that season would be certainly very much hotter,
while the winters would be longer and colder than at present; and although
geographical conditions might prevent any permanent increase of snow and
ice even in the extreme north, yet we cannot doubt that the whole northern
hemisphere would then have a very different climate than when the
changing phase of precession brought a very cool summer and a very mild
winter—a perpetual spring, in fact. Now, such a change of climate would
certainly be calculated to bring about a considerable change of species, both
by modification and migration, without any such decided change of type
either in the vegetation or the animals that we could say from their fossil
remains that any change of climate had taken place. Let us suppose, for
instance, that the climate of England and that of Canada were to be
mutually exchanged, and that the change took five or six thousand years to
bring about, it cannot be doubted that considerable modifications in the
fauna and flora of both countries would be the result, although it is
impossible to predict what the precise changes would be. We can safely say,
however, that some species would stand the change better than others, while
it is highly probable that some would be actually benefited by it, and that
others would be injured. But the benefited would certainly increase, and the
injured decrease, in consequence, and thus a series of changes would be
initiated that might lead to most important results. Again, we are sure that
some species would become modified in adaptation to the change of climate
more readily than others, and these modified species would therefore
increase at the expense of others not so readily modified; and hence would
arise on the one hand extinction of species, and on the other the production
of new forms.

But this is the very least amount of change of climate that would certainly
occur every 10,500 years when there was a high excentricity, for it is
impossible to doubt that a varying distance of the sun in summer from 86 to
99 millions of miles (which is what occurred during—as supposed—the
Miocene period, 850,000 years ago) would produce an important difference
in the summer temperature and in the actinic influence of sunshine on
vegetation. For the intensity of the sun's rays would vary as the square of
the distance, or nearly as 74 to 98, so that the earth would be actually
receiving one-fourth less sun-heat during summer at one time than at the
other. An equally high excentricity occurred 2,500,000 years back, and no
doubt was often reached during still earlier epochs, while a lower but still
very high excentricity has frequently prevailed, and is probably near its
average value. Changes of climate, therefore, every 10,500 years, of the
character above indicated and of varying intensity, have been the rule rather
than the exception in past time; and these changes must have been variously
modified by changing geographical conditions so as to produce climatic
alterations in different directions, giving to the ancient lands either dry or
wet seasons, storms or calms, equable or excessive temperatures, in a
variety of combinations of which the earth perhaps affords no example
under the present low phase of excentricity and consequent slight inequality
of sun-heat.
Present Condition of the Earth One of Exceptional Stability as Regards
Climate.—It will be seen, by a reference to the diagram at page 171, that
during the last three million years the excentricity has been less than it is
now on eight occasions, for short periods only, making up a total of about
280,000 years; while it has been more than it is now for many long periods,
of from 300,000 to 700,000 years each, making a total of 2,720,000 years;
or nearly as 10 to 1. For nearly half the entire period, or 1,400,000 years,
the excentricity has been nearly double what it is now, and this is not far
from its mean condition. We have no reason for supposing that this long
period of three million years, for which we have tables, was in any way
exceptional as regards the degree or variation of excentricity; but, on the
contrary, we may pretty safely assume that its variations during this time
fairly represent its average state of increase and decrease during all known
geological time. But when the glacial epoch ended, 72,000 years ago, the
excentricity was about double its present amount; it then rapidly decreased

till, at 60,000 years back, it was very little greater than it is now, and since
then it has been uniformly small. It follows that, for about 60,000 years
before our time, the mutations of climate every 10,500 years have been
comparatively unimportant, and that the temperate zones have enjoyed an
exceptional stability of climate. During this time those powerful causes of
organic change which depend on considerable changes of climate and the
consequent modifications, migrations, and extinctions of species, will not
have been at work; the slight changes that did occur would probably be so
slow and so little marked that the various species would be able to adapt
themselves to them without much disturbance; and the result would be an
epoch of exceptional stability of species.
But it is from this very period of exceptional stability that we obtain our
only scale for measuring the rate of organic change. It includes not only the
historical period, but that of the Swiss Lake dwellings, the Danish shell-
mounds, our peat-bogs, our sunken forests, and many of our superficial
alluvial deposits—the whole in fact, of the iron, bronze, and neolithic ages.
Even some portion of the palæolithic age, and of the more recent gravels
and cave-earths may come into the same general period if they were formed
when the glacial epoch was passing away. Now throughout all these ages
we find no indication of change of species, and but little, comparatively, of
migration. We thus get an erroneous idea of the permanence and stability of
specific forms, due to the period immediately antecedent to our own being a
period of exceptional permanence and stability as regards climatic and
geographical conditions.[100]
Date of Last Glacial Epoch and its Bearing on the Measurement of
Geological Time.—Directly we go back from this stable period we come
upon changes both in the forms and in the distribution of species; and when
we pass beyond the last glacial epoch into the Pliocene period we find
ourselves in a comparatively new world, surrounded by a considerable
number of species altogether different from any which now exist, together
with many others which, though still living, now inhabit distant regions. It
seems not improbable that what is termed the Pliocene period, was really
the coming on of the glacial epoch, and this is the opinion of Professor Jules
Marcou.[101] According to our views, a considerable amount of
geographical change must have occurred at the change from the Miocene to
the Pliocene, favouring the refrigeration of the northern hemisphere, and

leading, in the way already pointed out, to the glacial epoch whenever a
high degree of excentricity prevailed. As many reasons combine to make us
fix the height of the glacial epoch at the period of high excentricity which
occurred 200,000 years back, and as the Pliocene period was probably not
of long duration, we must suppose the next great phase of very high
excentricity (850,000 years ago) to fall within the Miocene epoch. Dr. Croll
believes that this must have produced a glacial period, but we have shown
strong reasons for believing that, in concurrence with favourable
geographical conditions, it led to uninterrupted warm climates in the
temperate and northern zones. This, however, did not prevent the
occurrence of local glaciation wherever other conditions led to its initiation,
and the most powerful of such conditions is a great extent of high land.
Now we know that the Alps acquired a considerable part of their elevation
during the latter part of the Miocene period, since Miocene rocks occur at
an elevation of over 6,000 feet, while Eocene beds occur at nearly 10,000
feet. But since that time there has been a vast amount of denudation, so that
these rocks may have been at first raised much higher than we now find
them, and thus a considerable portion of the Alps may have been more
elevated than they are now. This would certainly lead to an enormous
accumulation of snow, which would be increased when the excentricity
reached a maximum, as already fully explained, and may then have caused
glaciers to descend into the adjacent sea, carrying those enormous masses
of rock which are buried in the Upper Miocene of the Superga in Northern
Italy. An earlier epoch of great altitude in the Alps coinciding with the very
high excentricity 2,500,000 years ago, may have caused the local glaciation
of the Middle Eocene period when the enormous erratics of the Flysch
conglomerate were deposited in the inland seas of Northern Switzerland,
the Carpathians, and the Apennines. This is quite in harmony with the
indications of an uninterrupted warm climate and rich vegetation during the
very same period in the adjacent low countries, just as we find at the present
day in New Zealand a delightful climate and a rich vegetation of
Metrosideros, fuchsias and tree-ferns on the very borders of huge glaciers,
descending to within 700 feet of the sea-level. It is not pretended that these
estimates of geological time have any more value than probable guesses;
but it is certainly a curious coincidence that two remarkable periods of high
excentricity should have occurred, at such periods and at such intervals
apart, as very well accord with the comparative remoteness of the two

deposits in which undoubted signs of ice-action have been found, and that
both these are localised in the vicinity of mountains which are known to
have acquired a considerable elevation at about the same period of time.
In the tenth edition of the Principles of Geology, Sir Charles Lyell, taking
the amount of change in the species of mollusca as a guide, estimated the
time elapsed since the commencement of the Miocene as one-third that of
the whole Tertiary epoch, and the latter at one-fourth that of geological time
since the Cambrian period. Professor Dana, on the other hand, estimates the
Tertiary as only one-fifteenth of the Mesozoic and Palæozoic combined. On
the estimate above given, founded on the dates of phases of high
excentricity, we shall arrive at about four million years for the Tertiary
epoch, and sixteen million years for the time elapsed since the Cambrian,
according to Lyell, or sixty millions according to Dana. The estimate
arrived at from the rate of denudation and deposition (twenty-eight million
years) is nearly midway between these, and it is, at all events, satisfactory
that the various measures result in figures of the same order of magnitude,
which is all one can expect when discussing so difficult and exceedingly
speculative a subject.
The only value of such estimates is to define our notions of geological time,
and to show that the enormous periods, of hundreds of millions of years,
which have sometimes been indicated by geologists, are neither necessary
nor warranted by the facts at our command; while the present result places
us more in harmony with the calculations of physicists, by leaving a very
wide margin between geological time as defined by the fossiliferous rocks,
and that far more extensive period which includes all possibility of life
upon the earth.
Concluding Remarks.—In the present chapter I have endeavoured to show
that, combining the measured rate of denudation with the estimated
thickness and probable extent of the known series of sedimentary rocks, we
may arrive at a rude estimate of the time occupied in the formation of those
rocks. From another point of departure—that of the probable date of the
Miocene period, as determined by the epoch of high excentricity supposed
to have aided in the production of the Alpine glaciation during that period,
and taking the estimate of geologists as to the proportionate amount of
change in the animal world since that epoch—we obtain another estimate of

the duration of geological time, which, though founded on far less secure
data, agrees pretty nearly with the former estimate. The time thus arrived at
is immensely less than the usual estimates of geologists, and is so far within
the limits of the duration of the earth as calculated by Sir William Thomson,
as to allow for the development of the lower organisms an amount of time
anterior to the Cambrian period several times greater than has elapsed
between that period and the present day. I have further shown that, in the
continued mutations of climate produced by high excentricity and opposite
phases of precession, even though these did not lead to glacial epochs, we
have a motive power well calculated to produce far more rapid organic
changes than have hitherto been thought possible; while in the enormous
amount of specific variation (as demonstrated in an earlier chapter), we
have ample material for that power to act upon, so as to keep the organic
world in a state of rapid change and development proportioned to the
comparatively rapid changes in the earth's surface.

We have now finished the series of preliminary studies of the biological
conditions and physical changes which have affected the modification and
dispersal of organisms, and have thus brought about their actual distribution
on the surface of the earth. These studies will, it is believed, place us in a
condition to solve most of the problems presented by the distribution of
animals and plants, whenever the necessary facts, both as to their
distribution and their affinities, are sufficiently well known; and we now
proceed to apply the principles we have established to the interpretation of
the phenomena presented by some of the more important and best known of
the islands of our globe, limiting ourselves to these for reasons which have
been already sufficiently explained in our preface.

PART II
INSULAR FAUNAS AND FLORAS

CHAPTER XI

THE CLASSIFICATION OF ISLANDS

Importance of Islands in the Study of the Distribution of Organisms—
Classification of Islands with Reference to Distribution—Continental
Islands—Oceanic Islands.

In the preceding chapters, forming the first part of our work, we have
discussed, more or less fully, the general features presented by animal
distribution, as well as the various physical and biological changes which
have been the most important agents in bringing about the present condition
of the organic world.
We now proceed to apply these principles to the solution of the numerous
problems presented by the distribution of animals; and in order to limit the
field of our inquiry, and at the same time to deal only with such facts as
may be rendered intelligible and interesting to those readers who have not
much acquaintance with the details of natural history, we propose to
consider only such phenomena as are presented by the islands of the globe.
Importance of Islands in the Study of the Distribution of Organisms.—
Islands possess many advantages for the study of the laws and phenomena
of distribution. As compared with continents they have a restricted area and
definite boundaries, and in most cases their geographical and biological
limits coincide. The number of species and of genera they contain is always
much smaller than in the case of continents, and their peculiar species and
groups are usually well defined and strictly limited in range. Again, their
relations with other lands are often direct and simple, and even when more
complex are far easier to comprehend than those of continents; and they
exhibit besides certain influences on the forms of life and certain

peculiarities in their distribution which continents do not present, and
whose study offers many points of interest.
In islands we have the facts of distribution presented to us, sometimes in
their simplest forms, in other cases becoming gradually more and more
complex; and we are therefore able to proceed step by step in the solution
of the problems they present. But as in studying these problems we have
necessarily to take into account the relations of the insular and continental
faunas, we also get some knowledge of the latter, and acquire besides so
much command over the general principles which underlie all problems of
distribution, that it is not too much to say that when we have mastered the
difficulties presented by the peculiarities of island life we shall find it
comparatively easy to deal with the more complex and less clearly defined
problems of continental distribution.
Classification of Islands with Reference to Distribution.—Islands have had
two distinct modes of origin—they have either been separated from
continents of which they are but detached fragments, or they have
originated in the ocean and have never formed part of a continent or any
large mass of land. This difference of origin is fundamental, and leads to a
most important difference in their animal inhabitants; and we may therefore
first distinguish the two classes—oceanic and continental islands.
Mr. Darwin appears to have been the first writer who called attention to the
number and importance, both from a geological and biological point of
view, of oceanic islands. He showed that with very few exceptions all the
remoter islands of the great oceans were of volcanic or coralline formation,
and that none of them contained indigenous mammalia or amphibia. He
also showed the connection of these two phenomena, and maintained that
none of the islands so characterised had ever formed part of a continent.
This was quite opposed to the opinions of the scientific men of the day, who
almost all held the idea of continental extensions, and of oceanic islands
being their fragments, and it was long before Mr. Darwin's views obtained
general acceptance. Even now the belief still lingers; and we continually
hear of old Atlantic or Pacific continents, of "Atlantis" or "Lemuria," of
which hypothetical lands many existing islands, although wholly volcanic,
are thought to be the remnants. We have already seen that Darwin
connected the peculiar geological structure of oceanic islands with the

permanence of the great oceans which contain them, and we have shown
that several distinct lines of evidence all point to the same conclusion. We
may therefore define oceanic islands, as follows:—Islands of volcanic or
coralline formation, usually far from continents and always separated from
them by very deep sea, entirely without indigenous land mammalia or
amphibia, but with a fair number of birds and insects, and usually with
some reptiles. This definition will exclude only two islands which have
been sometimes classed as oceanic—New Zealand and the Seychelles.
Rodriguez, which was once thought to be another exception, has been
shown by the explorations during the Transit of Venus Expedition to be
essentially volcanic, with some upraised coralline limestone.
Continental Islands.—Continental islands are always more varied in their
geological formation, containing both ancient and recent stratified rocks.
They are rarely very remote from a continent, and they always contain some
land mammals and amphibia, as well as representatives of the other classes
and orders in considerable variety. They may, however, be divided into two
well-marked groups—ancient and recent continental islands—the
characters of which may be easily defined.
Recent continental islands are always situated on submerged banks
connecting them with a continent, and the depth of the intervening sea
rarely exceeds 100 fathoms. They resemble the continent in their geological
structure, while their animal and vegetable productions are either almost
identical with those of the continent, or if otherwise, the difference consists
in the presence of closely allied species of the same types, with occasionally
a very few peculiar genera. They possess in fact all the characteristics of a
portion of the continent, separated from it at a recent geological period.
Ancient continental islands differ greatly from the preceding in many
respects. They are not united to the adjacent continent by a shallow bank,
but are usually separated from it by a depth of sea of several hundreds to
more than a thousand fathoms. In geological structure they agree generally
with the more recent islands; like them they possess mammalia and
amphibia, usually in considerable abundance, as well as all other classes of
animals; but these are highly peculiar, almost all being distinct species, and
many forming distinct and peculiar genera or families. They are also well
characterised by the fragmentary nature of their fauna, many of the most

characteristic continental orders or families being quite unrepresented,
while some of their animals are allied, not to such forms as inhabit the
adjacent continent, but to others found only in remote parts of the world.
This very remarkable set of characters marks off the islands which exhibit
them as a distinct class, which often present the greatest anomalies and
most difficult problems to the student of distribution.
Oceanic Islands.—The total absence of warm-blooded terrestrial animals in
an island otherwise well suited to maintain them, is held to prove that such
island is no mere fragment of any existing or submerged continent, but one
that has been actually produced in mid-ocean. It is true that if a continental
island were to be completely submerged for a single day and then again
elevated, its higher terrestrial animals would be all destroyed, and if it were
situated at a considerable distance from land it would be reduced to the
same zoological condition as an oceanic island. But such a complete
submergence and re-elevation appears never to have taken place, for there is
no single island on the globe which has the physical and geological features
of a continental, combined with the zoological features of an oceanic island.
It is true that some of the coral-islands may be formed upon submerged
lands of a continental character, but we have no proof of this; and even if it
were so, the existing islands are to all intents and purposes oceanic.
We will now pass on to a consideration of some of the more interesting
examples of these three classes, beginning with oceanic islands.
All the animals which now inhabit such oceanic islands must either
themselves have reached them by crossing the ocean, or be the descendants
of ancestors who did so. Let us then see what are, in fact, the animal and
vegetable inhabitants of these islands, and how far their presence can be
accounted for. We will begin with the Azores, or Western Islands, because
they have been thoroughly well explored by naturalists, and in their
peculiarities afford us an important clue to some of the most efficient means
of distribution among several classes of animals.

CHAPTER XII

OCEANIC ISLANDS:—THE AZORES AND BERMUDA

The Azores, or Western Islands

Position and Physical Features—Chief Zoological Features of the
Azores—Birds—Origin of the Azorean Bird Fauna—Insects of the
Azores—Land-Shells of the Azores—The Flora of the Azores—The
Dispersal of Seeds—Birds as Seed-Carriers—Facilities for Dispersal
of Azorean Plants—Important Deduction from the Peculiarities of the
Azorean Fauna and Flora.

Bermuda

Position and Physical Features—The Red Clay of Bermuda—Zoology
of Bermuda—Birds of Bermuda—Comparison of the Bird Faunas of
Bermuda and the Azores—Insects of Bermuda—Land Mollusca—
Flora of Bermuda—Concluding Remarks on the Azores and Bermuda.

We will commence our investigation into the phenomena presented by
oceanic islands, with two groups of the North Atlantic, in which the facts
are of a comparatively simple nature and such as to afford us a valuable
clue to a solution of the more difficult problems we shall have to deal with
further on. The Azores and Bermuda offer great contrasts in physical
features, but striking similarities in geographical position. The one is
volcanic, the other coralline; but both are surrounded by a wide expanse of
ocean of enormous depth, the one being about as far from Europe as the
other is from America. Both are situated in the temperate zone, and they
differ less than six degrees in latitude, yet the vegetation of the one is
wholly temperate, while that of the other is almost tropical. The productions
of the one are related to Europe, as those of the other are to America, but
they present instructive differences; and both afford evidence of the highest
value as to the means of dispersal of various groups of organisms across a
wide expanse of ocean.

THE AZORES, OR WESTERN ISLANDS.
These islands, nine in number, form a widely scattered group, situated
between 37° and 39° 40′ N. Lat. and stretching in a south-east and north-

west direction over a distance of nearly 400 miles. The largest of the
islands, San Miguel, is about forty miles long, and is one of the nearest to
Europe, being rather under 900 miles from the coast of Portugal, from
which it is separated by an ocean 2,500 fathoms deep. The depth between
the islands does not seem to be known, but the 1,000 fathom line encloses
the whole group pretty closely, while a depth of about 1,800 fathoms is
reached within 300 miles in all directions. These great depths render it in
the highest degree improbable that the Azores have ever been united with
the European continent; while their being wholly volcanic is equally
opposed to the view of their having formed part of an extensive Atlantis
including Madeira and the Canaries. The only exception to their volcanic
structure is the occurrence in one small island only (Santa Maria) of some
marine deposits of Upper Miocene age—a fact which proves some
alterations of level, and perhaps a greater extension of this island at some
former period, but in no way indicates a former union of the islands, or any
greater extension of the whole group. It proves, however, that the group is
of considerable antiquity, since it must date back to Miocene times; and this
fact may be of importance in considering the origin and peculiar features of
the fauna and flora. It thus appears that in all physical features the Azores
correspond strictly with our physical definition of "oceanic islands," while
their great distance from any other land, and the depth of the ocean around
them, make them typical examples of the class. We should therefore expect
them to be equally typical in their fauna and flora; and this is the case as
regards the most important characteristics, although in some points of detail
they present exceptional phenomena.

OUTLINE MAP OF THE AZORES.

Note. The light the sea less than 1,000 fathoms
shows where
— tint is deep.
The dark more than 1,000 fathoms
" " "
tint deep.
The figures show depths in fathoms.

Chief Zoological Features of the Azores.[102]—The great feature of oceanic
islands—the absence of all indigenous land-mammalia and amphibia—is
well shown in this group; and it is even carried further, so as to include all
terrestrial vertebrata, there being no snake, lizard, frog, or fresh-water fish,
although the islands are sufficiently extensive, possess a mild and equable
climate, and are in every way adapted to support all these groups. On the
other hand, flying creatures, as birds and insects, are abundant; and there is
also one flying mammal—a small European bat. It is true that rabbits,
weasels, rats and mice, and a small lizard peculiar to Madeira and Teneriffe,
are now found wild in the Azores, but there is good reason to believe that
these have all been introduced by human agency. The same may be said of
the gold-fish and eels now found in some of the lakes, there being not a
single fresh-water fish which is truly indigenous to the islands. When we
consider that the nearest part of the group is about 900 miles from Portugal,
and more than 550 miles from Madeira, it is not surprising that none of
these terrestrial animals can have passed over such a wide expanse of ocean
unassisted by man.

Let us now see what animals are believed to have reached the group by
natural means, and thus constitute its indigenous fauna. These consist of
birds, insects, and land-shells, each of which must be considered separately.
Birds.—Fifty-three species of birds have been observed at the Azores, but
the larger proportion (thirty-one) are either aquatic or waders—birds of
great powers of flight, whose presence in the remotest islands is by no
means remarkable. Of these two groups twenty are residents, breeding in
the islands, while eleven are stragglers only visiting the islands
occasionally, and all are common European species. The land-birds, twenty-
two in number, are more interesting, four only being stragglers, while
eighteen are permanent residents. The following is a list of these resident
land-birds:—
1. Common Buzzard (Buteo vulgaris)
2. Long-eared Owl (Asio otus)
3. Barn Owl (Strix flammea)
4. Blackbird (Turdus merula)
5. Robin (Erythacus rubecula)
6. Blackcap (Sylvia atricapilla)
7. Gold-crest (Regulus cristatus)
8. Wheatear (Saxicola œnanthe)
9. Grey Wagtail (Motacilla sulphurea)
10. Atlantic Chaffinch (Fringilla tintillon)
11. Azorean Bullfinch (Pyrrhula murina)
12. Canary (Serinus canarius)
13. Common Starling (Sturnus vulgaris)
14. Lesser Spotted Woodpecker (Dryobates minor)
15. Wood-pigeon (Columba palumbus)
16. Rock Dove (Columba livia)
17. Red-legged Partridge (Caccabis rufa)
18. Common Quail (Coturnix communis)
All the above-named birds are common in Europe and North Africa except
three—the Atlantic chaffinch and the canary which inhabit Madeira and the
Canary Islands, and the Azorean bullfinch, which is peculiar to the islands
we are considering.

Origin of the Azorean Bird-fauna.—The questions we have now before us
are—how did these eighteen species of birds first reach the Azores, and
how are we to explain the presence of a single peculiar species while all the
rest are identical with European birds? In order to answer them, let us first
see what stragglers now actually visit the Azores from the nearest
continents. The four species given in Mr. Godman's list are the kestrel, the
oriole, the snow-bunting, and the hoopoe; but he also tells us that there are
certainly others, and adds: "Scarcely a storm occurs in spring or autumn
without bringing one or more species foreign to the islands; and I have
frequently been told that swallows, larks, grebes, and other species not
referred to here, are not uncommonly seen at those seasons of the year."
We have, therefore, every reason to believe that the birds which are now
residents originated as stragglers, which occasionally found a haven in these
remote islands when driven out to sea by storms. Some of them, no doubt,
still often arrive from the continent, but these cannot easily be distinguished
as new arrivals among those which are permanent inhabitants. Many facts
mentioned by Mr. Godman show that this is the case. A barn-owl, much
exhausted, flew on board a whaling-ship when 500 miles S.W. of the
Azores; and even if it had come from Madeira it must have travelled quite
as far as from Portugal to the islands. Mr. Godman also shot a single
specimen of the wheatear in Flores after a strong gale of wind, and as no
one on the island knew the bird, it was almost certainly a recent arrival.
Subsequently a few were found breeding in the old crater of Corvo, a small
adjacent island; and as the species is not found in any other island of the
group, we may infer that this bird is a recent immigrant in process of
establishing itself.
Another fact which is almost conclusive in favour of the bird-population
having arrived as stragglers is, that they are most abundant in the islands
nearest to Europe and Africa. The Azores consist of three divisions—an
eastern, consisting of two islands, St. Michael's and St. Mary's; a central of
five, Terceira, Graciosa, St. George's, Pico, and Fayal; and a western of two,
Flores and Corvo. Now had the whole group once been united to the
continent, or even formed parts of one extensive Atlantic island, we should
certainly expect the central group, which is more compact and has a much
larger area than all the rest, to have the greatest number and variety of birds.
But the fact that birds are most numerous in the eastern group, and diminish

as we go westward, is entirely opposed to this theory, while it is strictly in
accordance with the view that they are all stragglers from Europe, Africa, or
the other Atlantic islands. Omitting oceanic wanderers, and including all
birds which have probably arrived involuntarily, the numbers are found to
be forty species in the eastern group, thirty-six in the central, and twenty-
nine in the western.
To account for the presence of one peculiar species—the bullfinch (which,
however, does not differ from the common European bullfinch more than
do some of the varieties of North American birds from their type-species) is
not difficult; the wonder rather being that there are not more peculiar forms.
In our third chapter we have seen how great is the amount of individual
variation in birds, and how readily local varieties become established
wherever the physical conditions are sufficiently distinct. Now we can
hardly have a greater difference of conditions than between the continent of
Europe or North Africa, and a group of rocky islands in mid-Atlantic,
situated in the full course of the Gulf Stream and with an excessively mild
though stormy climate. We have every reason to believe that special
modifications would soon become established in any animals completely
isolated under such conditions. But they are not, as a rule, thus completely
isolated, because, as we have seen, stragglers arrive at short intervals; and
these, mixing with the residents, keep up the purity of the breed. It follows,
that only those species which reach the Azores at very remote intervals will
be likely to acquire well-marked distinctive characters; and this appears to
have happened with the bullfinch alone, a bird which does not migrate, and
is therefore less likely to be blown out to sea, more especially as it inhabits
woody districts. A few other Azorean birds, however, exhibit slight
differences from their European allies.
There is another reason for the very slight amount of peculiarity presented
by the fauna of the Azores as compared with many other oceanic islands,
dependent on its comparatively recent origin. The islands themselves may
be of considerable antiquity, since a few small deposits, believed to be of
Miocene age, have been found on them, but there can be little doubt that
their present fauna, at all events as concerns the birds, had its origin since
the date of the last glacial epoch. Even now icebergs reach the latitude of
the Azores but a little to the west of them; and when we consider the proofs
of extensive ice-action in North America and Europe, we can hardly doubt