CHAPTER I

A HISTORY OF LAND MAMMALS IN THE WESTERN HEMISPHERE

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A

HISTORY OF LAND MAMMALS

IN THE

WESTERN HEMISPHERE

BY

WILLIAM B. SCOTT

PH.D. (Heidelberg), HON.D.SC. (Harvard & Oxford), LL.D. (Univ.

of Pennsylvania)

BLAIR PROFESSOR OF GEOLOGY AND PALÆONTOLOGY

IN PRINCETON UNIVERSITY

_ILLUSTRATED WITH 32 PLATES AND MORE THAN 100 DRAWINGS

BY BRUCE HORSFALL_

New York

THE MACMILLAN COMPANY

1913

_All rights reserved_

COPYRIGHT, 1913,

BY THE MACMILLAN COMPANY.

Set up and electrotyped. Published November, 1913.

Norwood Press

J. S. Cushing Co.—Berwick & Smith Co.

Norwood, Mass., U.S.A.

Dedicated

TO

MY CLASSMATES

HENRY FAIRFIELD OSBORN AND FRANCIS SPEIR

IN MEMORY OF A NOTABLE SUMMER AFTERNOON IN 1876 AND IN TOKEN OF FORTY

YEARS’ UNCLOUDED FRIENDSHIP

Speak to the earth and it shall teach thee.

—JOB, xii, 8.

Can these bones live?

—EZEKIEL, xxxvii, 3.

PREFACE

One afternoon in June, 1876, three Princeton undergraduates were lying

under the trees on the canal bank, making a languid pretence of preparing

for an examination. Suddenly, one of the trio remarked: “I have been

reading an old magazine article which describes a fossil-collecting

expedition in the West; why can’t we get up something of the kind?” The

others replied, as with one voice, “We can; let’s do it.” This seemingly

idle talk was, for Osborn and myself, a momentous one, for it completely

changed the careers which, as we then believed, had been mapped out for

us. The random suggestion led directly to the first of the Princeton

palæontological expeditions, that of 1877, which took us to the “Bad

Lands” of the Bridger region in southwestern Wyoming. The fascination of

discovering and exhuming with our own hands the remains of the curious

creatures which once inhabited North America, but became extinct ages

ago, has proved an enduring delight. It was the wish to extend something

of this fascinating interest to a wider circle, that occasioned the

preparation of this book.

The western portion of North America has preserved a marvellous series

of records of the successive assemblages of animals which once dwelt

in this continent, and in southernmost South America an almost equally

complete record was made of the strange animals of that region. For

the last half-century, or more, many workers have coöperated to bring

this long-vanished world to light and to decipher and interpret the

wonderful story of mammalian development in the western hemisphere. The

task of making this history intelligible, not to say interesting, to the

layman, has been one of formidable difficulty, for it is recorded in

the successive modifications of the bones and teeth, and without some

knowledge of osteology, these records are in an unknown tongue. To meet

this need, Chapter III gives a sketch of the mammalian skeleton and

dentition, which the reader may use as the schoolboy uses a vocabulary

to translate his Latin exercise, referring to it from time to time, as

may be necessary to make clear the descriptions of the various mammalian

groups. Technical terms have been avoided as far as possible, but,

unfortunately, it is not practicable to dispense with them altogether.

The appended glossary will, it is hoped, minimize the inconvenience.

No one who has not examined it, can form any conception of the enormous

mass and variety of material, illustrating the history of American

mammals, which has already been gathered into the various museums. A full

account of this material would require many volumes, and one of the chief

problems in the preparation of this book has been that of making a proper

selection of the most instructive and illuminating portions of the long

and complicated story. Indeed, so rapid is the uninterrupted course of

discovery, that parts of the text became antiquated while in the press

and had to be rewritten. As first prepared, the work proved to be far

too long and it was necessary to excise several chapters, for it seemed

better to cover less ground than to make the entire history hurried and

superficial. The plan of treatment adopted involves a considerable amount

of repetition, but this is perhaps not a disadvantage, since the same

facts are considered from different points of view.

The facts which are here brought together have been ascertained by

many workers, and I have borrowed with the greatest freedom from my

fellow labourers in the field of palæontology. As every compiler of a

manual finds, it is not feasible to attribute the proper credit to each

discoverer. Huxley has so well explained the situation in the preface to

his “Anatomy of Vertebrated Animals,” that I may be permitted to borrow

his words: “I have intentionally refrained from burdening the text with

references; and, therefore, the reader, while he is justly entitled to

hold me responsible for any errors he may detect, will do well to give me

no credit for what may seem original, unless his knowledge is sufficient

to render him a competent judge on that head.”

A book of this character is obviously not the proper place for polemical

discussions of disputed questions. Whenever, therefore, the views

expressed differ widely from those maintained by other palæontologists,

I have attempted no more than to state, as fairly as I could, the

alternative interpretations and my own choice between them. Any other

course was forbidden by the limitations of space.

It is a pleasure to give expression to my sincere sense of gratitude

to the many friends who have helped me in an unusually laborious

undertaking. Professor Osborn and Dr. Matthew have placed at my disposal

the wonderful treasures of the American Museum of Natural History in New

York and in the most liberal manner have supplied me with photographs and

specimens for drawings, as well as with information regarding important

discoveries which have not yet been published. Dr. W. J. Holland,

Director of the Carnegie Museum in Pittsburgh, has likewise generously

provided many photographs from the noble collection under his charge,

kindly permitting the use of material still undescribed. To Professor

Charles Schuchert, of Yale University, I am also indebted for several

photographs.

The figures of existing animals are almost all from photographs taken

in the New York and London zoölogical gardens, and I desire to thank

Director Hornaday, of the Bronx Park, and Mr. Peacock, of the London

garden, for the very kind manner in which they have procured these

illustrations for my use. The photographs have been modified by painting

out the backgrounds of cages, houses, and the like, so as to give a less

artificial appearance to the surroundings.

To my colleagues at Princeton I am under great obligations for much

valuable counsel and assistance. Professor Gilbert van Ingen has prepared

the maps and diagrams and Dr. W. J. Sinclair has devoted much labour

and care to the illustrations and has also read the proofs. Both of

these friends, as also Professors C. H. Smyth and E. G. Conklin and Drs.

Farr and McComas, have read various parts of the manuscript and made

many helpful suggestions in dealing with the problems of treatment and

presentation.

For thirteen years past I have been engaged in the study of the great

collections of fossil mammals, gathered in Patagonia by the lamented Mr.

Hatcher and his colleague, Mr. Peterson, now of the Carnegie Museum.

This work made it necessary for me to visit the museums of the Argentine

Republic, which I did in 1901, and was there received with the greatest

courtesy and kindness by Dr. F. Moreno, Director, and Dr. Santiago Roth,

of the La Plata Museum, and Dr. F. Ameghino, subsequently Director of the

National Museum at Buenos Aires. To all of these gentlemen the chapters

on the ancient life of South America are much indebted, especially to

Dr. Ameghino, whose untimely death was a great loss to science. It is

earnestly to be hoped that the heroic story of his scientific career may

soon be given to the world.

Finally, I desire to thank Mr. Horsfall for the infinite pains and care

which he has expended upon the illustrations for the work, to which so

very large a part of its value is due.

While the book is primarily intended for the lay reader, I cannot but

hope that it may also be of service to many zoölogists, who have been

unable to keep abreast of the flood of palæontological discovery and yet

wish to learn something of its more significant results. How far I have

succeeded in a most difficult task must be left to the judgment of such

readers.

PRINCETON, N.J., June 1, 1913.

CONTENTS

PAGE

CHAPTER I

METHODS OF INVESTIGATION—GEOLOGICAL 1

CHAPTER II

METHODS OF INVESTIGATION—PALÆONTOLOGICAL 29

CHAPTER III

THE CLASSIFICATION OF THE MAMMALIA 50

CHAPTER IV

THE SKELETON AND TEETH OF MAMMALS 61

CHAPTER V

THE GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS IN CENOZOIC TIMES 99

CHAPTER VI

THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 135

CHAPTER VII

THE SUCCESSIVE MAMMALIAN FAUNAS OF NORTH AND SOUTH AMERICA 192

CHAPTER VIII

HISTORY OF THE PERISSODACTYLA 288

CHAPTER IX

HISTORY OF THE ARTIODACTYLA 358

CHAPTER X

HISTORY OF THE PROBOSCIDEA 422

CHAPTER XI

HISTORY OF THE †AMBLYPODA AND †CONDYLARTHRA 443

CHAPTER XII

HISTORY OF THE †TOXODONTIA (OR †NOTOUNGULATA) 461

CHAPTER XIII

HISTORY OF THE †LITOPTERNA AND †ASTRAPOTHERIA 489

CHAPTER XIV

HISTORY OF THE CARNIVORA 516

CHAPTER XV

HISTORY OF THE PRIMATES 577

CHAPTER XVI

HISTORY OF THE EDENTATA 589

CHAPTER XVII

HISTORY OF THE MARSUPIALIA 624

CHAPTER XVIII

MODES OF MAMMALIAN EVOLUTION 645

GLOSSARY 665

INDEX 675

A HISTORY OF LAND MAMMALS IN THE WESTERN HEMISPHERE

CHAPTER I

METHODS OF INVESTIGATION—GEOLOGICAL

The term _Mammal_ has no exact equivalent in the true vernacular of

any modern language, the word itself, like its equivalents, the French

_Mammifère_ and the German _Säugethier_, being entirely artificial. As

a name for the class Linnæus adopted the term Mammalia, which he formed

from the Latin _mamma_ (_i.e._ teat) to designate those animals which

suckle their young; hence the abbreviated form Mammal, which has been

naturalized as an English word. “Beast,” as employed in the Bible, and

“Quadruped” are not quite the same as mammal, for they do not include the

marine forms, such as whales, dolphins, seals, walruses, or the flying

bats, and they are habitually used in contradistinction to Man, though

Man and all the forms mentioned are unquestionably mammals.

In attempting to frame a definition of the term _Mammal_, it is

impossible to avoid technicalities altogether, for it is the complete

unity of plan and structure which justifies the inclusion of all the

many forms that differ so widely in habits and appearance. _Mammals are

air-breathing vertebrates, which are warm-blooded and have a 4-chambered

heart; the body cavity is divided into pleural and abdominal chambers

by a diaphragm; except in the lowest division of the class, the young

are brought forth alive and are always suckled, the milk glands being

universal throughout the class. In the great majority of mammals the

body is clothed with hair; a character found in no other animals. In

a few mammals the skin is naked, and in still fewer there is a partial

covering of scales._ The list of characters common to all mammals, which

distinguish them from other animals, might be indefinitely extended,

for it includes all the organs and tissues of the body, the skeletal,

muscular, digestive, nervous, circulatory, and reproductive systems, but

the two or three more obvious or significant features above selected will

suffice for the purposes of definition.

While the structural plan is the same throughout the entire class,

there is among mammals a wonderful variety of form, size, appearance,

and adaptation to special habits. It is as though a musician had taken

a single theme and developed it into endless variations, preserving

an unmistakable unity through all the changes. Most mammals are

_terrestrial_, living, that is to say, not only on the land, but on the

ground, and are herbivorous in habit, subsisting chiefly or exclusively

upon vegetable substances, but there are many departures from this mode

of life. It should be explained, however, that the term _terrestrial_ is

frequently used in a more comprehensive sense for all land mammals, as

distinguished from those that are aquatic or marine. Monkeys, Squirrels,

Sloths and Opossums are examples of the numerous _arboreal_ mammals,

whose structure is modified to fit them for living and sleeping in the

trees, and in some, such as the Sloths, the modification is carried so

far that the creature is almost helpless on the ground. Another mode of

existence is the burrowing or _fossorial_, the animal living partly or

mostly, or even entirely underground, a typical instance of which is the

Mole. The Beaver, Muskrat and Otter, to mention only a few forms, are

_aquatic_ and spend most of their life in fresh waters, though perfectly

able to move about on the land. _Marine_ mammals, such as the Seals and

Whales, have a greatly modified structure which adapts them to life in

the sea.

Within the limits of each of these categories we may note that there

are many degrees of specialization or adaptation to particular modes of

life. Thus, for example, among the marine mammals, the Whales and their

allies, Porpoises, etc., are so completely adapted to a life in the

seas that they cannot come upon the land, and even stranding is fatal

to them, while the Seals frequently land and move about upon the shore.

It should further be observed that mammals of the most diverse groups

are adapted to similar modes of existence. Thus in one natural group or

_order_ of related forms, occur terrestrial, burrowing, arboreal and

aquatic members, and the converse statement is of course equally true,

that animals of similar life-habits are not necessarily related to one

another, and very frequently, in fact, are not so related. Among the

typically marine mammals, for example, there are at least three and

probably four distinct series, which have independently become adapted to

life in the sea.

* * * * *

Before attempting to set forth an outline of what has been learned

regarding the history of mammalian life in the western hemisphere, it

is essential to give the reader some conception of the manner in which

that knowledge has been obtained. Without such an understanding of the

methods employed in the investigation the reader can only blindly accept

or as blindly reject what purports to be the logical inference from

well-established evidence. How is that evidence to be discovered? and how

may trustworthy conclusions be derived from it?

The first and most obvious step is to gather all possible information

concerning the mammals of the present day, their structure (comparative

anatomy), functions (physiology), and their geographical arrangement.

This latter domain, of the geographical distribution of mammals, is one

of peculiar significance. Not only do the animals of North America differ

radically from those of Central and South America, but within the limits

of each continent are more or less well-defined areas, the animals of

which differ in a subordinate degree from those of other areas. The study

of the modern world, however, would not of itself carry us very far

toward the goal of our inquiries, which is an _explanation_, not merely

a statement, of the facts. The present order of things is the outcome

of an illimitably long sequence of events and can be understood only in

proportion to our knowledge of the past. In other words, it is necessary

to treat the problems involved in our inquiry _historically_; to trace

the evolution of the different mammalian groups from their simpler

beginnings to the more complex and highly specialized modern forms; to

determine, so far as that may be done, the place of origin of each group

and to follow out their migrations from continent to continent.

While we shall deal chiefly, almost exclusively, with the mammals of the

New World, something must be said regarding those of other continents,

for, as will be shown in the sequel, both North and South America have,

at one time or another, been connected with various land-masses of the

eastern hemisphere. By means of those land-connections, there has been

an interchange of mammals between the different continents, and each

great land-area of the recent world contains a more or less heterogeneous

assemblage of forms of very diverse places of origin. Indeed, migration

from one region to another has played a most important part in bringing

about the present distribution of living things. From what has already

been learned as to the past life of the various continents and their

shifting connections with one another, it is now feasible to analyze the

mammalian faunas of most of them and to separate the indigenous from the

immigrant elements. Among the latter may be distinguished those forms

which are the much modified descendants of ancient migrants from those

which arrived at a much later date and have undergone but little change.

To take a few examples from North America, it may be said that the Bears,

Moose, Caribou and Bison are late migrants from the Old World; that the

Virginia and Black-tailed Deer and the Prong-horned Antelope are of

Old World origin, but their ancestors came in at a far earlier period

and the modern species are greatly changed from the ancestral migrants.

The Armadillo of Texas and the Canada Porcupine are almost the only

survivors, north of Mexico, of the great migration of South American

mammals which once invaded the northern continent. On the other hand,

the raccoons and several families of rodents are instances of indigenous

types which may be traced through a long American ancestry.

Fully to comprehend the march of mammalian development, it thus becomes

necessary to reconstruct, at least in outline, the geography of the

successive epochs through which the developmental changes have taken

place, the connections and separations of land-masses, the rise of

mountain ranges, river and lake systems and the like. Equally significant

factors in the problem are climatic changes, which have had a profound

and far-reaching effect upon the evolution and geographical spread of

animals and plants, and the changes in the vegetable world must not be

ignored, for, directly or indirectly, animals are dependent upon plants.

To one who has paid no attention to questions of this kind, it might well

seem an utterly hopeless task to reconstruct the long vanished past, and

he would naturally conclude that, at best, only fanciful speculations,

with no foundation of real knowledge, could be within our reach. Happily,

such is by no means the case. Geology offers the means of a successful

attack upon these problems and, although very much remains to be done,

much has already been accomplished in elucidating the history, especially

in its later periods, with which the story of the mammals is more

particularly concerned.

It is manifestly impossible to present here a treatise upon the science

of Geology, even in outline sketch. Considerations of space are

sufficient to forbid any such attempt. Certain things must be taken for

granted, the evidence for which may be found in any modern text-book of

Geology. For example, it is entirely feasible to establish the mode of

formation of almost any rock (aside from certain problematical rocks,

which do not enter into our discussion) and to determine whether it

was laid down in the sea, or on the land, or in some body of water not

directly connected with the sea, such as a lake or river. With the aid

of the microscope, it is easy to discriminate volcanic material from

the ordinary water-borne and wind-borne sediments and, in the case of

the rocks which have solidified from the molten state, to distinguish

those masses which have cooled upon the surface from those which have

solidified deep within the earth.

While the nature and mode of formation of the rocks may thus be

postulated, it will be needful to explain at some length the character of

the evidence from which the history of the earth may be deciphered. First

of all, must be made clear the method by which the events of the earth’s

history may be arranged in chronological order, for a history without

chronology is unintelligible. The events which are most significant for

our purpose are recorded in the rocks which are called _stratified,

bedded or sedimentary_, synonymous terms. Such rocks were made mostly

from the débris of older rocks, in the form of gravel, sand or mud, and

were laid down under water, or, less extensively, spread by the action of

the wind upon a land-surface. Important members of this group of rocks

are those formed, more or less completely, from the finer fragments given

out in volcanic eruptions, carried and sorted by the wind and finally

deposited, it may be at great distances from their point of origin,

upon a land-surface, or on the bottom of some body of water. Stratified

or bedded rocks, as their name implies, are divided into more or less

parallel layers or beds, which may be many feet or only a minute fraction

of an inch in thickness. Such a division means a pause in the process of

deposition or a change in the character of the material deposited over

a given area. Owing to the operation of gravity, the layers of sediment

are spread out in a horizontal attitude, which disregard the minor

irregularities of the bottom, just as a deep snow buries the objects

which lie upon the surface.

A moment’s consideration will show that, in any series of stratified

rocks which have not been greatly disturbed from their original

horizontal position, _the order of succession or superposition of the

beds must necessarily be the chronological order of their formation_.

(Fig. 1.) Obviously, the lowest beds must have been deposited first and

therefore are the oldest of the series, while those at the top must

be the newest or youngest and the beds intermediate in position are

intermediate in age. This inference depends upon the simple principle

that each bed must have been laid down before the next succeeding

one can have been deposited upon it. While this is so clear as to be

almost self-evident, it is plain that such a mode of determining the

chronological order of the rocks of the earth’s crust can be of only

local applicability and so far as the beds may be traced in unbroken

continuity. It is of no direct assistance in correlating the events in

the history of one continent with those of another and it fails even in

comparing the distinctly separated parts of the same continent. Some

method of universal applicability must be devised before the histories of

scattered regions can be combined to form a history of the earth. Such a

universal method is to be found in the _succession of the forms of life_,

so far as that is recorded in the shape of _fossils_, or the recognizable

remains of animal and vegetable organisms preserved in the rocks.

[Illustration: FIG. 1.—Diagram section of a series of beds, illustrating

superposition. A is the oldest, B, C, D, etc., succeeding in ascending

order.]

This principle was first enunciated by William Smith, an English

engineer, near the close of the eighteenth century, who thus laid the

foundations of Historical Geology. In the diagram, Fig. 2, is reproduced

Smith’s section across England from Wales to near London, which shows

the successive strata or beds, very much tilted from their original

horizontal position by the upheaval of the sea-bed upon which they

were laid down. The section pictures the side of an imaginary gigantic

trench cut across the island and was constructed by a simple geometrical

method from the surface exposures of the beds, such as mining engineers

continually employ to map the underground extension of economically

important rocks, and shows how an enormous thickness of strata may be

studied from the surface. The older beds are exposed at the western end

of the section in Wales and, passing eastward, successively later and

later beds are encountered, the newest appearing at the eastern end. Very

many of the strata are richly fossiliferous, and thus a long succession

of fossils was obtained in the _order of their appearance_, and this

order has been found to hold good, not only in England, but throughout

the world. The order of succession of the fossils was thus in the first

instance actually ascertained from the succession of the strata in which

they are found and has been verified in innumerable sections in many

lands and is thus a matter of observed and verifiable fact, not merely

a postulate or working hypothesis. Once ascertained, however, the order

of succession of living things upon the earth may be then employed as an

independent and indispensable means of geologically dating the rocks in

which they occur.

[Illustration: FIG. 2.—William Smith’s section across the south of

England. The vertical scale is exaggerated, which makes the inclination

of the beds appear too steep.

N. B. The original drawing is in colors, which are not indicated by the

dotted strata.]

This is the _palæontological method_, which finds analogies in many

other branches of learned inquiry. The student of manuscripts discovers

that there is a development, or regular series of successive changes,

in handwriting, and from the handwriting alone can make a very close

approximation to the date of a manuscript. The order in which those

changes came about was ascertained from the comparative study of

manuscripts, the date of which could be ascertained from other evidence,

but, when once established, the changes in handwriting are used to fix

the period of undated manuscripts. Just so, the succession of fossils,

when learned from a series of superposed beds, may then be employed to

fix the geological date of strata in another region. Similarly, the

archæologist has observed that there is an evolution or development in

every sort of the work of men’s hands and therefore makes use of coins,

inscriptions, objects of art, building materials and methods, etc., to

date ancient structures. In the German town of Trier (or Trèves) on the

Moselle, the cathedral has as a nucleus a Roman structure, the date and

purpose of which had long been matters of dispute, though the general

belief was that the building had been erected under Constantine the

Great. In the course of some repairs made not very long ago, it became

necessary to cut deep into the Roman brickwork, and there, embedded

in the undisturbed mortar, was a coin of the emperor Valentinian II,

evidently dropped from the pocket of some Roman bricklayer. That coin

fixed a date older than which the building cannot be, though it may be

slightly later, and it well illustrates the service rendered by fossils

in determining geological chronology.

Other methods of making out the chronology of the earth’s history have

been proposed from time to time and all of them have their value, though

none of them renders us independent of the use of fossils, which have the

pre-eminent advantage of not recurring or repeating themselves at widely

separated intervals of time, as all physical processes and changes do. An

organism, animal or plant, that has become extinct never returns and is

not reproduced in the evolutionary process.

Great and well founded as is our confidence in fossils as fixing the

geological date of the rocks in which they occur, it must not be

forgotten that the succession of the different kinds of fossils in

time was first determined from the superposition of the containing

strata. Hence, it is always a welcome confirmation of the chronological

inferences drawn from the study of fossils, when those inferences can

be unequivocally established by the succession of the beds themselves.

For example, in the Tertiary deposits of the West are two formations or

groups of strata, called respectively the Uinta and the White River,

which had never been known to occur in the same region and whose relative

age therefore could not be determined by the method of superposition.

Each of the formations, however, has yielded a large number of

well-preserved fossil mammals, and the comparative study of these mammals

made it clear that the Uinta must be older than the White River and that

no very great lapse of time, geologically speaking, occurred between the

end of the former and the beginning of the latter. Only two or three

years ago an expedition from the American Museum of Natural History

discovered a place in Wyoming where the White River beds lie directly

upon those of the Uinta, thus fully confirming the inference as to the

relative age of these two formations which had long ago been drawn from

the comparative study of their fossil mammals.

The palæontological method of determining the geological date of the

stratified rocks is thus an indispensable means of correlating the

scattered exposures of the strata in widely separated regions and in

different continents, it may be with thousands of miles of intervening

ocean. The general principle employed is that _close similarity of

fossils in the rocks of the regions compared points to an approximately

contemporaneous date of formation of those rocks_. This principle must

not, however, be applied in an off-hand or uncritical manner, or it will

lead to serious error. In the first place, the evolutionary process is a

very slow one and geological time is inconceivably long, so that deposits

which differ by some thousands of years may yet have the same or nearly

the same fossils. The method is not one of sufficient refinement to

detect such relatively small differences. To recur to the illustration

of the development in handwriting, the palæographer can hardly do more

than determine the decade in which a manuscript was written; no one would

expect him to fix upon the exact year, still less the month, from the

study of handwriting alone. As is the month in recorded human history, so

is the millennium in the long course of the earth’s development.

[Illustration: FIG. 3.—Bluff on Beaver Creek, Fremont Co., Wyoming. The

White River beds were deposited on the worn and weathered surface of

the Uinta, the heavy, broken line marking the separation between them.

The valley was carved out long after the deposition of the White River

strata.]

In the second place, there are great differences in the contemporary life

of separate regions and such geographical differences there have always

been, so far as we can trace back the history of animals and plants. A

new organism does not originate simultaneously all over the world but,

normally at least, in a single area and spreads from that centre until

it encounters insuperable obstacles. Such spreading is a slow process

and hence it is that new forms often appear in one region much earlier

than in others and in the very process of extending their range, the

advancing species may themselves be considerably modified and reach their

new and distant homes as different species from those which originated

the movement. Extinction, likewise, seldom occurs simultaneously over

the range of a group, but now here and now there in a way that to our

ignorance appears to be arbitrary and capricious. The process may go on

until extinction is total, or may merely result in a great restriction of

the range of a given group, or may break up that range into two or more

distinct areas.

Of such incomplete extinctions many instances might be given, but one

must suffice. The camel-tribe, strange as it may appear, originated in

North America and was long confined to that continent, while at the

present day it is represented only by the llamas of South America and

the true camels of Asia, having completely vanished from its early home.

These facts and a host of similar ones make plain how necessary it is to

take geographical considerations into account in all problems that deal

with the synchronizing of the rocks of separate areas and continents.

Properly to estimate the significance of a difference in the fossils

of two regions and to determine how far it is geographical, due to a

separation in space, or geological and caused by separation in time, is

often a very difficult matter and requires a vast amount of minute and

detailed study. Once more, the principle involved is illustrated by the

study of manuscripts. Down to the time when the printing press superseded

the copyist, each of the nations of Europe had its own traditions and

its more or less independent course of handwriting development. A great

monastery, in which the work of copying manuscripts went on century after

century, became an independent geographical centre with its particular

styles. Thus the palæographer, like the geologist, is confronted by

geographical problems as well as by those of change and development in

general.

In addition to the method of geologically dating the rocks by means of

the fossils which they contain, there are other ways which may give a

greater precision to the result. Climatic changes, when demonstrable,

are of this character, for they may speedily and simultaneously affect

vast areas of the earth’s surface or even the entire world. From time to

time in the past, glacial conditions have prevailed over immense regions,

several continents at once, it may be, as in one instance in which India,

South Africa, Australia, South America were involved. The characteristic

accumulations made by the glaciers in these widely separated regions

must be contemporaneous in a sense that can rarely be predicated of the

ordinary stratified rocks. Such climatic changes as the formation and

disappearance of the ice-fields give a sharper and more definite standard

of time comparisons than do the fossils alone, and yet the fossils are

in turn needed to show which of several possible glacial periods are

actually being compared.

Again, great movements of the earth’s crust, which involve vast and

widely separated regions and bring the sea in over great areas of land,

or raise great areas into land, which had been submerged, may also yield

more precise time-measurements, because occurring within shorter periods

than do notable changes in the system of living things. Such changes in

animals and plants may be compared to the almost imperceptible movement

of the hour-hand of a clock, while the recorded climatic revolutions

and crustal movements often supply the place of the minute-hand. It is

obvious, however, that if the hour-hand be wanting, the minute-hand

alone can be of very limited use. There have been a great many vast

submergences and emergences of land in the history of the earth, and only

the fossils can give us the assurance that we are comparing the same

movement in distant continents, and not two similar movements separated

by an enormous interval of time.

It may thus fairly be admitted that it is possible to arrange the rocks

which compose the accessible parts of the earth’s crust in chronological

order and to correlate in one system the rocks of the various continents.

The terms used for the more important divisions of geological time are,

in descending order of magnitude, era, period, epoch, age or stage, and

the general scheme of the eras and periods, which is in almost uniform

use throughout the world, is given in the table, which is arranged so as

to give the succession graphically, with the most ancient rocks at the

bottom and the latest at the top.

Cenozoic era { Quaternary period

{ Tertiary period

{ Cretaceous period

Mesozoic era { Jurassic period

{ Triassic period

{ Permian period

{ Carboniferous period

Palæozoic era { Devonian period

{ Silurian period

{ Ordovician period

{ Cambrian period

Pre-Cambrian eras { Algonkian period

{ Archæan period

It must not be supposed that all the divisions of similar rank, such as

the eras, for example, were of equal length, as measured by the thickness

of the rocks assigned to those divisions. On the contrary, they must

have been of very unequal length and are of very different divisibility.

The Pre-Cambrian eras, with only two periods, were probably far longer

than all subsequent time, and all that the major divisions imply is that

they represent changes in the system of life of approximately equivalent

importance. It is impossible to give any trustworthy estimate of the

actual lengths of these divisions in years, though many attempts to do so

have been made. All that can be confidently affirmed is that geological

time, like astronomical distances, is of inconceivable vastness and its

years can be counted only in hundreds of millions.

To discuss in any intelligible manner the history of mammals, it will be

necessary to go much farther than the above table in the subdivision of

that part of geological time in which mammalian evolution ran its course.

As mammals represent the highest stage of development yet attained in the

animal world, it is only the latter part of the earth’s history which

is concerned with them; the earlier and incomparably longer portion of

that history may be passed over. Mammals are first recorded in the later

Triassic, the first of the three periods which make up the Mesozoic era.

They have also been found, though very scantily, in the other Mesozoic

periods, the Jurassic and Cretaceous, but it was the Cenozoic era that

witnessed most of the amazing course of mammalian development and

diversification, and hence the relatively minute subdivisions necessary

for the understanding of this history deal only with the Cenozoic, the

latest of the great eras.

In the subjoined table the periods and epochs are those which are in

general use throughout the world, the ages and stages are those which

apply to the western interior of North America, each region, even of the

same continent, requiring a different classification. The South American

formations are given in a separate table, as it is desirable to avoid the

appearance of an exactitude in correlation which cannot yet be attained.

CENOZOIC ERA

Quaternary period { Recent epoch

{ Pleistocene epoch = Glacial and Interglacial stages.

{ Pliocene epoch

{ Miocene epoch

Tertiary period { Oligocene epoch

{ Eocene epoch

{ Paleocene epoch

Continuing the subdivision of the Tertiary period still farther, we have

the following arrangement:

TERTIARY PERIOD (North America)

{ Upper Wanting

{ Middle Blanco age

_Pliocene_ { { Thousand Creek age

{ Lower { Snake Creek age

{ { Republican River age

{ Upper Loup Fork age

_Miocene_ { Middle Deep River age

{ Lower Arikaree age

_Oligocene_ { Upper John Day age

{ Lower White River age

{ Upper Uinta age

_Eocene_ { Middle Bridger age

{ Lower { Wind River age

{ Wasatch age

_Paleocene_ { Upper Torrejon age } Fort Union

{ Lower Puerco age }

This is a representative series of the widespread and manifold non-marine

Tertiary deposits of the Great Plains, but a much more extensive and

subdivided scheme would be needed to show with any degree of fullness

the wonderfully complete record of that portion of the continent during

the Tertiary period. A much more elaborate table will be found in

Professor Osborn’s “Age of Mammals,” p. 41. There are some differences

of practice among geologists as to this scheme of classification, though

the differences are not those of principle. No question arises concerning

the reality of the divisions, or their order of succession in time, but

merely as to the rank or relative importance which should be attributed

to some of them, and that is a very minor consideration.

Much greater difficulty and, consequently, much more radical differences

of interpretation arise when the attempt is made to correlate or

synchronize the smaller subdivisions, as found in the various continents,

with one another, because of the geographical differences in contemporary

life. Between Europe and North America there has always been a certain

proportion of mammalian forms in common, a proportion that was at

one time greater, at another less, and this community renders the

correlation of the larger divisions of the Tertiary in the two continents

comparatively easy, and even in the minor subdivisions very satisfactory

progress has been made, so that it is possible to trace in some detail

the migrations of mammals from the eastern to the western hemisphere and

_vice versa_. Such intermigrations were made possible by the land-bridges

connecting America with Europe across the Atlantic, perhaps on the line

of Greenland and Iceland, and with Asia where now is Bering Strait.

These connections were repeatedly made and repeatedly broken during the

Mesozoic and Cenozoic eras down to the latest epoch, the Pleistocene.

By comparing the fossil mammals of Europe with those of North America

for any particular division of geological time, it is practicable to

determine whether the way of intermigration was open or closed, because

separation always led to greater differences between the faunas of the

two continents through divergent evolution.

Correlation with South America is exceedingly difficult and it is in

dealing with this problem that the widest differences of opinion have

arisen among geologists. Through nearly all the earlier half of the

Tertiary period the two Americas were separated and, because of this

separation, their land mammals were utterly different. Hence, the lack

of elements common to both continents puts great obstacles in the way of

establishing definite time-relations between their geological divisions.

Only the marine mammals, whales and dolphins, were so far alike as to

offer some satisfactory basis of comparison. When, in the later Tertiary,

a land-connection was established between the two continents, migrations

of mammals from each to the other began, and thenceforward there were

always certain elements common to both, as there are to-day. In spite

of the continuous land between them, the present faunas of North and

South America are very strikingly different, South America being, with

the exception of Australia, zoölogically the most peculiar region of the

earth.

In the following table of the South American Cenozoic, the assignment of

the ages to their epochs is largely tentative, especially as regards the

more ancient divisions, and represents the views generally held by the

geologists of Europe and the United States; those of South America, on

the contrary, give an earlier date to the ages and stages and refer the

older ones to the Cretaceous instead of the Tertiary.

CENOZOIC ERA (South America)

Quaternary period { Recent epoch

{ Pleistocene epoch—Pampean Beds, Brazilian caverns

{ { Monte Hermoso age

{ Pliocene epoch { Catamarca age

{ { Paraná age

{

Tertiary period { Miocene epoch { Santa Cruz age

{ { Patagonian age

{

{ Oligocene epoch { Deseado age (_Pyrotherium_ Beds)

{ { Astraponotus Beds

{

{ Eocene epoch { Casa Mayor age (_Notostylops_ Beds)

The Pleistocene and Pliocene deposits are most widely distributed over

the Pampas of Argentina, but the former occur also in Ecuador, Brazil,

Chili, and Bolivia. The other formations cover extensive areas in

Patagonia, and some extend into Tierra del Fuego.

We have next to consider the methods by which past geographical

conditions may be ascertained, a task which, though beset with

difficulties, is very far from being a hopeless undertaking. As has

already been pointed out, it is perfectly possible for the geologist to

determine the circumstances of formation of the various kinds of rocks,

to distinguish terrestrial from aquatic accumulations and, among the

latter, to identify those which were laid down in the sea and those which

were formed in some other body of water. By platting on a map all the

marine rocks of a given geological date, an approximate estimate may be

formed as to the extension of the sea over the present land for that

particular epoch. It is obvious, however, that for those areas which

then were land and now are covered by the sea, no such direct evidence

can be obtained, and only indirect means of ascertaining the former

land-connections can be employed. It is in the treatment of this indirect

evidence that the greatest differences of opinion arise and, if two

maps of the same continent for the same epoch, by separate authors, be

compared, it will be seen that the greatest discrepancies between them

are concerning former land-connections and extensions.

The only kind of indirect evidence bearing upon ancient land-connections,

now broken by the sea, that need be considered here is that derived from

the study of animals and plants, both recent and fossil. All-important in

this connection is the principle that the same or closely similar species

do not arise independently in areas between which there is no connection.

It is not impossible that such an independent origin of organisms which

the naturalist would class as belonging to the same species may have

occasionally taken place, but, if so, it must be the rare exception to

the normal process. This principle leads necessarily to the conclusion

that the more recently and broadly two land-areas, now separated by the

sea, have been connected, the more nearly alike will be their animals

and plants. Such islands as Great Britain, Sumatra and Java must have

been connected with the adjacent mainland within a geologically recent

period, while the extreme zoölogical peculiarity of Australia can be

explained only on the assumption that its present isolation is of very

long standing. The principle applies to the case of fossils as well as to

that of modern animals, and has already been made use of, in a preceding

section, in dealing with the ancient land-connections of North America.

It was there shown that the connection of this continent with the Old

World and the interruptions of that connection are reflected and recorded

in the greater or less degree of likeness in the fossil mammals at any

particular epoch. Conversely, the very radical differences between the

fossil mammals of the two Americas imply a long-continued separation

of those two continents, and their junction in the latter half of the

Tertiary period is proved by the appearance of southern groups of mammals

in the northern continent, and of northern groups in South America.

Inasmuch as the connection between North and South America still

persists, the geology of the Isthmus of Panama should afford testimony

in confirmation of the inferences drawn from a study of the mammals. Of

course, the separating sea did not necessarily cross the site of the

present isthmus; it might have cut through some part of Central America,

but a glance at the map immediately suggests the isthmus as the place

of separation and subsequent connection. As a matter of fact, isthmian

geology is in complete accord with the evidence derived from the mammals.

Even near the summit of the hills which form the watershed between the

Atlantic and the Pacific and through which the great Culebra Cut passes,

are beds of marine Tertiary shells, showing that at that time the land

was completely submerged. This does not at all preclude the possibility

of other transverse seas at the same period; indeed, much of Central

America was probably under the sea also, but the geology of that region

is still too imperfectly known to permit positive statements.

When several different kinds of testimony, each independent of the other,

can be secured and all are found to be in harmony, the strength of the

conclusion is thereby greatly increased. Many distinct lines of evidence

support the inference that North and South America were completely

severed for a great part of the Tertiary period. This is indicated in the

clearest manner, not only by the geological structure of the Isthmus and

by the mammals, living and extinct, as already described, but also by the

fresh-water fishes, the land-shells, the reptiles and many other groups

of animals and plants.

The distribution of marine fossils may render the same sort of service

in elucidating the history of the sea as land-mammals do for the

continents, demonstrating the opening and closing of connections between

land-areas and between oceans. The sea, it is true, is one and undivided,

the continental masses being great islands in it, but, nevertheless,

the sea is divisible into zoölogical provinces, just as is the land.

Temperature, depth of water, character of the bottom, etc., are factors

that limit the range of marine organisms, as climate and physical

barriers circumscribe the spread of terrestrial animals. Professor Perrin

Smith has shown that in the Mesozoic era Bering Strait was repeatedly

opened and closed, and that each opening and closing was indicated by

the geographical relationships of the successive assemblages of marine

animals that are found in the Mesozoic rocks of California and Nevada.

When the Strait was open, the coast-line between North America and Asia

was interrupted and the North Pacific was cooled by the influx of water

from the Arctic Sea. At such times, sea-animals from the Russian and

Siberian coasts extended their range along the American side as far south

as Mexico, and no forms from the eastern and southern shores of Asia

accompanied them. On the other hand, when the Strait was closed, the

Arctic forms were shut out and the continuous coast-line and warmer water

enabled the Japanese, Indian, and even Mediterranean animals to extend

their range to the Pacific coast of North America. A comparison of the

marine fishes of the two sides of the Isthmus of Panama shows an amount

and degree of difference between the two series as might be expected from

the length of time that they have been separated by the upheaval of the

land.

In working out the geographical conditions for any particular epoch

of the earth’s history, it is possible to go much farther than merely

gaining an approximate estimate of the distribution of land and sea; many

other important facts may be gathered from a minute examination of the

rocks in combination with a genetic study of topographical forms. By this

physiographical method, as it is called, the history of several of the

great mountain-ranges has been elaborated in great detail. It is quite

practicable to give a geological date for the initial upheaval and to

determine whether one or many such series of movements have been involved

in bringing about the present state of things. Similarly, the history of

plains and plateaus, hills and valleys, lake and river systems, may be

ascertained, and for the earth’s later ages, at least, a great deal may

be learned regarding the successive forms of the land-surfaces in the

various continents. It would be very desirable to explain the methods by

which these results are reached, but this could hardly be done without

writing a treatise on physiography, for which there is no room in this

chapter. We must be permitted to make use of the results of that science

without being called upon to prove their accuracy.

No factor has a more profound effect in determining the character and

distribution of living things than climate, of which the most important

elements, for our purpose, are temperature and moisture. One of the most

surprising results of geological study is the clear proof that almost all

parts of the earth have been subjected to great vicissitudes of climate,

and a brief account of the evidence which has led to this unlooked for

result will not be out of place here.

The evidence of climatic changes is of two principal kinds, (1) that

derived from a study of the rocks themselves, and (2) that given by the

fossils of the various epochs. So far as the rocks laid down in the sea

are concerned, little has yet been ascertained regarding the climatic

conditions of their formation, but the strata which were deposited on

the land, or in some body of water other than the sea, often give the

most positive and significant information concerning the circumstances of

climate which prevailed at the time of their formation. Certain deposits,

such as gypsum and rock-salt, are accumulated only in salt lakes, which,

in turn, are demonstrative proof of an arid climate. A salt lake could

not exist in a region of normal rainfall and, from the geographical

distribution of such salt-lake deposits, it may be shown that arid

conditions have prevailed in each of the continents and, not only once,

but many times. As a rule, such aridity of climate was relatively local

in extent, but sometimes it covered vast areas. For example, in the

Permian, the last of the Palæozoic, and the Triassic, the first of the

Mesozoic periods (see Table, p. 15) nearly all the land-areas of the

northern hemisphere were affected, either simultaneously or in rapid

succession.

Until a comparatively short time ago, it was very generally believed that

the Glacial or Pleistocene epoch, which was so remarkable and conspicuous

a feature of the Quaternary period, was an isolated phenomenon, unique in

the entire history of the earth. Now, however, it has been conclusively

shown that such epochs of cold have been recurrent and that no less than

five of these have left unmistakable records in as many widely separated

periods of time.

When the hypothesis of a great “Ice Age” in the Pleistocene was first

propounded by the elder Agassiz, it was naturally received with general

incredulity, but the gradual accumulation of proofs has resulted in

such an overwhelming weight of testimony, that the glacial hypothesis

is now accepted as one of the commonplaces of Geology. The proofs

consist chiefly in the characteristic glacial accumulations, moraines

and drift-sheets, which cover such enormous areas in Europe and North

America and, on a much smaller scale, in Patagonia, and in the equally

characteristic marks of glacial wear left upon the rocks over which

the ice-sheets moved. Many years later it was proved that the Permian

period had been a time of gigantic glaciation, chiefly in the southern

hemisphere, when vast ice-caps moved slowly over parts of South America,

South Africa, Australia and even of India. The evidence is of precisely

the same nature as in the case of the Pleistocene glaciation. In not less

than three more ancient periods, the Devonian, Cambrian, and Algonkian,

proofs of glacial action have been obtained.

While the rocks themselves thus afford valuable testimony as to the

climatic conditions which prevailed at the time and place of their

formation, this testimony is fragmentary, missing for very long periods,

and must be supplemented from the information presented by the fossils.

As in all matters where fossils are involved, the evidence must be

cautiously used, for hasty inferences have often led to contradictory

and absurd conclusions. When properly employed, the fossils give a more

continuous and complete history of climatic changes than can, in the

present state of knowledge, be drawn from a study of the rocks alone.

For this purpose plants are particularly useful, because the great

groups of the vegetable kingdom are more definitely restricted in their

range by the conditions of temperature and moisture than are most of the

correspondingly large groups of animals. Not that fossil animals are

of no service in this connection; quite the contrary is true, but the

evidence from them must be treated more carefully and critically. To

illustrate the use of fossils as recording climatic changes in the past,

one or two examples may be given.

In the Cretaceous period a mild and genial climate prevailed over all

that portion of the earth whose history we know, and was, no doubt,

equally the case in the areas whose geology remains to be determined.

The same conditions extended far into the Arctic regions, and abundant

remains of a warm-temperate vegetation have been found in Greenland,

Alaska and other Arctic lands. Where now only scanty and minute dwarf

willows and birches can exist, was then a luxuriant forest growth

comprising almost all of the familiar trees of our own latitudes, a most

decisive proof that in the Cretaceous the climate of the Arctic regions

must have been much warmer than at present and that there can have been

no great accumulation of ice in the Polar seas. Conditions of similar

mildness obtained through the earlier part of the Tertiary. In the Eocene

epoch large palm-trees were growing in Wyoming and Idaho, while great

crocodiles and other warm-country reptiles abounded in the waters of the

same region.

It is of particular interest to inquire how far the fossils of Glacial

times confirm the inferences as to a great climatic change which are

derived from a study of the rocks, for this may be taken as a test-case.

Any marked discrepancy between the two would necessarily cast grave doubt

upon the value of the testimony of fossils as to climatic conditions.

The problem is one of great complexity, for the Pleistocene was not one

long epoch of unbroken cold, but was made up of Glacial and Interglacial

ages, alternations of colder and milder conditions, and some, at least,

of the Interglacial ages had a climate warmer than that of modern times.

Such great changes of temperature led to repeated migrations of the

mammals, which were driven southward before the advancing ice-sheets

and returned again when the glaciers withdrew under the influence

of ameliorating climates. Any adequate discussion of these complex

conditions is quite out of the question in this place and the facts must

be stated in simplified form, as dealing only with the times of lowered

temperature and encroaching glaciers.

The plants largely fail us here, for little is known of Glacial

vegetation, but, on the other hand, a great abundance of the fossil

remains of animal life of that date has been collected, and its testimony

is quite in harmony with that afforded by the ice-markings and the

ice-made deposits. Arctic shells in the marine deposits of England, the

valley of the Ottawa River and of Lake Champlain, Walruses on the coast

of New Jersey, Reindeer in the south of France, and Caribou in southern

New England, Musk-oxen in Kentucky and Arkansas, are only a few examples

of the copious evidence that the climate of the regions named in Glacial

times was far colder than it is to-day.

I have thus endeavored to sketch, necessarily in very meagre outlines,

the nature of the methods employed to reconstruct the past history of

the various continents and the character of the evidence upon which we

must depend. Should the reader be unconvinced and remain sceptical as to

the possibility of any such reconstruction, he must be referred to the

numerous manuals of Geology, in which these methods are set forth with a

fulness which cannot be imitated within the limits of a single chapter.

The methods are sound, consisting as they do merely in the application of

“systematized common sense” (in Huxley’s phrase) to observed facts, but

by no means all applications of them are to be trusted. Not to mention

ill-considered and uncritical work, or inverted pyramids of hypothesis

balanced upon a tiny point of fact, it should be borne in mind that

such a complicated and difficult problem as the reconstruction of past

conditions can be solved only by successive approximations to the truth,

each one partial and incomplete, but less so than the one which preceded

it.