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.
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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.