The chief palaeontological interests of the Pliocene deposits, as of the succeeding Post-Pliocene, centre round the Mammals of the period; and amongst the many forms of these we may restrict our attention to the orders of the Hoofed Quadrupeds (Ungulates), the Proboscideans, the Carnivora, and the Quadrumana. Almost all the other Mammalian orders are more or less fully represented in Pliocene times, but none of them attains any special interest till we enter upon the Post-Pliocene.

Amongst the Odd-toed Ungulates, in addition to the remains of true Tapirs (Tapirus Arvernensis), we meet with the bones of several species of Rhinoceros, of which the Rhinoceros Etruscus and R. Megarhinus (fig. 249) are the most important. The former of these (fig. 249, A) derives its specific name from its abundance in the Pliocene deposits of the Val d'Arno, near Florence, and though principally Pliocene in its distribution, it survived into the earlier portion of the Post-Pliocene period. Rhinoceros Etruscus agreed with the existing African forms in having two horns placed one behind the other, the front one being the longest; but it was comparatively slight and slender in its build, whilst the nostrils were separated by an incomplete bony partition. In the Rhinoceros megarhinus (fig. 249, B), on the other hand, no such partition exists between the nostrils, and the nasal bones are greatly developed in size. It was a two-horned form, and is found associated with Elephas meridionalis and E. Antiquus in the Pliocene deposits of the Val d'Arno, near Florence. Like the preceding, it survived, in diminished numbers, into the earlier portion of the Post-Pliocene period.



The Horses (Equidoe) are represented, both in Europe and America, by the three-toed Hipparions, which survive from the Miocene, but are now verging upon extinction. For the first time, also, we meet with genuine Horses (Equus), in which each foot is provided with a single complete toe only, encased in a single broad hoof. One of the American species of this period (the Equus excelsus) quite equalled the modern Horse in stature; and it is interesting to note the occurrence of indigenous horses in America at such a comparatively late geological epoch, seeing that this continent certainly possessed none of these animals when first discovered by the Spaniards.

Amongst the Even-toed Ungulates, we may note the occurrence of Swine (Suida), of forms allied to the Camels (Camelidoe), and of various kinds of Deer (Cervidoe); but the most interesting Pliocene Mammal belonging to this section is the great Hippopotamus major of Britain and Europe. This well-known species is very closely allied to the living Hippopotamus amphibius of Africa, from which it is separated only by its larger dimensions, and by certain points connected with the conformation of the skeleton. It is found very abundantly in the Pliocene deposits of Italy and France, associated with the remains of the Elephant, Mastodon, and Rhinoceros, and it survived into the earlier portion of the Post-Pliocene period. During this last-mentioned period, it extended its range northwards, and is found associated with the Reindeer, the Bison, and other northern animals. From this fact it has been inferred, with great probability, that the Hippotamus major was furnished with a long coat of hair and fur, thus differing from its nearly hairless modern representative, and resembling its associates, the Mammoth and the Woolly Rhinoceros.



Passing on to the Pliocene Proboscideans, we find that the great Deinotheria of the Miocene have now wholly disappeared, and the sole representatives of the order are Mastodons and Elephants. The most important member of the former group is the Mastodon Arvernensis (fig. 250), which ranged widely over Southern Europe and England, being generally associated with remains of the Elephas meridionalis, E. antiquus, Rhinoceros megarhinus, and Hippopotamus major. The lower jaw seems to have been destitute of incisor teeth; but the upper incisors are developed into great tusks, which sometimes reach a length of nine feet, and which have the simple curvature of the tusks of the existing Elephants. Amongst the Pliocene Elephants the two most important are the Elephas meridionalis and the Elephas antiquus. Of these, the Elephas meridionalis (fig. 251) is found abundantly in the Pliocene deposits of Southern Europe and England, and also survived into the earlier portion of the Post-Pliocene period. Its molar teeth are of the type of those of the existing African Elephant, the spaces enclosed by the transverse enamel-plates being more or less lozenge-shaped, whilst the curvature of the tusks is simple. The Elephas antiquus (fig. 252) is very generally associated with the preceding, and it survived to an even later stage of the Post-Pliocene period. The molar teeth are of the type of the existing Indian Elephant, with comparatively thin enamel-ridges, placed closer together than in the African type; whilst the tusks were nearly straight.



Amongst the Pliocene Carnivores, we meet with true Bears (Ursus Arvernensis), Hyaenas (such as Hyoena Hipparionum), and genuine Lions (such as the Felis angustus of North America); but the most remarkable of the beasts of prey of this period is the great "Sabre-toothed Tiger" (Machairodus), species of which existed in the earlier Miocene, and survived to the later Post-Pliocene. In this remarkable form we are presented with perhaps the most highly carnivorous type of all known beasts of prey. Not only are the jaws shorter in proportion even than those of the great Cats of the present day, but the canine teeth (fig. 253) are of enormous size, greatly flattened so as to assume the form of a poignard, and having their margins finely serrated. A part from the characters of the skull, the remainder of the skeleton, so far as known, exhibits proofs that the Sabre-toothed Tiger was extraordinarily muscular and powerful, and in the highest degree adapted for a life of rapine. Species of Machairodus must have been as large as the existing Lion; and the genus is not only European, but is represented both in South America and in India, so that the geographical range of these predaceous beasts must have been very extensive.



Lastly, we may note that the Pliocene deposits of Europe have yielded the remains of Monkeys (Quadrumana), allied to the existing Semnopitheci and Macaques.

LITERATURE.

The following list comprises a small selection of some of the more important and readily accessible works and memoirs relating to the Tertiary rocks and their fossils. With few exceptions, foreign works relating to the Tertiary strata of the continent of Europe or their organic remains have been omitted:—

(1) 'Elements of Geology.' Lyell. (2) 'Students' Elements of Geology.' Lyell. (3) 'Manual of Palaeontology.' Owen. (4) 'British Fossil Mammals and Birds.' Owen. (5) 'Traite de Paleontologie.' Pictet. (6) 'Cours Elementaire de Paleontologie.' D'Orbigny. (7) "Probable Age of the London Clay," &c.—'Quart. Journ. Geol. Soc.,' vol. iii. Prestwich. (8) 'Structure and Probable Age of the Bagshot Sands'—Ibid., vol. iii. Prestwich. (9) 'Tertiary Formations of the Isle of Wight'—Ibid., vol. ii. Prestwich. (10) 'Structure of the Strata between the London Clay and the Chalk,' &c.—Ibid., vols. vi., viii., and x. Prestwich. (11) 'Correlation of the Eocene Tertiaries of England, France, and Belgium'—Ibid., vol. xxvii. Prestwich. (12) 'On the Fluvio-marine Formations of the Isle of Wight'—Ibid., vol. ix. Edward Forbes. (13) 'Newer Tertiary Deposits of the Sussex Coast'—Ibid., vol. xiii. Godwin-Austen. (14) 'Kainozoic Formations of Belgium'—Ibid., vol. xxii. Godwin-Austen. (15) 'Tertiary Strata of Belgium and French Flanders'—Ibid., vol. viii. Lyell. (16) 'On Tertiary Leaf-beds in the Isle of Mull'—Ibid., vol. vii. The Duke of Argyll. (17) 'Newer Tertiaries of Suffolk and their Fauna'—Ibid., vol. xxvi. Ray Lankester. (18) 'Lower London Tertiaries of Kent'—Ibid., vol. xxii. Whitaker. (19) "Guide to the Geology of London"—'Mem. Geol. Survey.' Whitaker. (20) 'Memoirs of the Geological Survey of Great Britain.' (21) 'Introductory Outline of the Geology of the Crag District' (Supplement to Crag Mollusca, Palaeontographical Society). S. V. Wood, jun., and F. w. Harmer. (22) "Tertiary Fluvio-marine Deposits of the Isle of Wight." Edward Forbes. Edited by Godwin-Austen; with Descriptions of the Fossils by Morris, Salter, and Rupert Jones—'Memoirs of the Geological Survey.' (23) 'Geological Excursions round the Isle of Wight.' Mantell. (24) 'Catalogue of British Fossils.' Morris. (25) 'Catalogue of Fossils in the Museum of Practical Geology.' Etheridge. (26) 'Monograph of the Crag Polyzoa' (Palaeontographical Society). Busk. (27) 'Monograph of the Tertiary Brachiopoda' (Ibid.) Davidson. (28) 'Monograph of the Tertiary Malacostracous Crustacea' (Ibid.) Bell. (29) 'Monograph of the Tertiary Corals' (Ibid.) Milne-Edwards and Haime. (30) 'Supplement to the Tertiary Corals' (Ibid.) Martin Duncan. (31) 'Monograph of the Eocene Mollusca' (Ibid.) Fred. E. Edwards. (32) 'Monograph of the Eocene Mollusca' (Ibid.) Searles V. Wood. (33) 'Monograph of the Crag Mollusca' (Ibid.) Searles V. Wood. (34) 'Monograph of the Tertiary Entomostraca' (Ibid.) Rupert Jones. (35) 'Monograph of the Foraminifera of the Crag' (Ibid.) Rupert Jones, Parker, and H. B. Brady. (36) 'Monograph of the Radiaria of the London Clay' (Ibid.) Edward Forbes. (37) 'Monograph of the Cetacea of the Red Crag' (Ibid.) Owen. (38) 'Monograph of the Fossil Reptiles of the London Clay' (Ibid.) Owen and Bell. (39) "On the Skull of a Dentigerous Bird from the London Clay of Sheppey"—'Quart. Journ. Geol. Soc.,' vol. xxix. Owen. (40) 'Ossemens Fossiles.' Cuvier. (41) 'Fauna Antiqua Sivalensis.' Falconer and Sir Proby Cautley. (42) 'Palaeontological Memoirs.' Falconer. (43) 'Animaux Fossiles et Geologie de l'Attique.' Gaudry. (44) "Principal Characters of the Dinocerata"—'American Journ. of Science and Arts,' vol. xi. Marsh. (45) 'Principal Characters of the Brontotheridae' (Ibid.) Marsh. (46) 'Principal Characters of the Tillodontia' (Ibid.) Marsh. (47) "Extinct Vertebrata of the Eocene of Wyoming"—'Geological Survey of Montana,' &c., 1872. Cope. (48) "Ancient Fauna of Nebraska"—'Smithsonian Contributions to Knowledge,' vol. vi. Leidy. (49) 'Manual of Geology.' Dana. (50) "Palaeontology and Evolution" (Presidential Address to the Geological Society of London, 1870)—'Quart. Journ. Geol. Soc.,' vol. xxvi. Huxley.' (51) 'Mineral Conchology.' Sowerby. (52) 'Description des Coquilles Fossiles,' &c. Deshayes. (53) 'Description des Coquilles Tertiaires de Belgique.' Nyst. (54) 'Fossilen Polypen des Wiener Tertiaer-beckens.' Reuss. (55) 'Palaeontologische Studien ueber die aelteren Tertiaer-schichten der Alpen.' Reuss. (56) 'Land und Suess-wasser Conchylien der Vorwelt.' Sandberger. (57) 'Flora Tertiaria Helvetica.' Heer. (58) 'Flora Fossilis Arctica.' Heer. (59) 'Recherches sur le Climat et la Vegetation du Pays Tertiaire.' Heer. (60) 'Fossil Flora of Great Britain.' Lindley and Hutton. (61) 'Fossil Fruits and Seeds of the London Clay.' Bowerbank. (62) "Tertiary Leaf-beds of the Isle of Mull"—'Quart. Journ. Geol. Soc.,' vol. vii. Edward Forbes. (63) 'The Geology of England and Wales.' Horace B. Woodward.[25]

[Footnote 25: This work—published whilst these sheets were going through the press—gives to the student a detailed view of all the strata of England and Wales, with their various sub-divisions, from the base of the Palaeozoic to the top of the Tertiary.]



CHAPTER XXI.

THE QUATERNARY PERIOD.

THE POST-PLIOCENE PERIOD.

Later than any of the Tertiary formations are various detached and more or less superficial accumulations, which are generally spoken of as the Post-Tertiary formations, in accordance with the nomenclature of Sir Charles Lyell—or as the Quaternary formations, in accordance with the general usage of Continental geologists. In all these formations we meet with no Mollusca except such as are now alive—with the partial and very limited exception of some of the oldest deposits of this period, in which a few of the shells occasionally belong to species not known to be in existence at the present day. Whilst the Shell-fish of the Quaternary deposits are, generally speaking, identical with existing forms, the Mammals are sometimes referable to living, sometimes to extinct species. In accordance with this, the Quaternary formations are divided into two groups: (1) The Post-Pliocene, in which the shells are almost invariably referable to existing species, but some of the Mammals are extinct; and (2) the Recent, in which the shells and the Mammals alike belong to existing species. The Post-Pliocene deposits are often spoken of as the Pleistocene formations (Gr. pleistos, most; kainos, new or recent), in allusion to the fact that the great majority of the living beings of this period belong to the species characteristic of the "new" or Recent period.

The Recent deposits, though of the highest possible interest, do not properly concern the palaeontologist strictly so-called, but the zoologist, since they contain the remains of none but existing animals. They are "Pre-historic," but they belong entirely to the existing terrestrial order. The Post-Pliocene deposits, on the other hand, contain the remains of various extinct Mammals; and though Man undoubtedly existed in, at any rate, the later portion of this period, if not throughout the whole of it, they properly form part of the domain of the palaeontologist.

The Post-Pliocene deposits are extremely varied, and very widely distributed; and owing to the mode of their occurrence, the ordinary geological tests of age are in their case but very partially available. The subject of the classification of these deposits is therefore an extremely complicated one; and as regards the age of even some of the most important of them, there still exists considerable difference of opinion. For our present purpose, it will be convenient to adopt a classification of the Post-Pliocene deposits founded on the relations which they bear in time to the great "Ice-age" or "Glacial period;" though it is not pretended that our present knowledge is sufficient to render such a classification more than a provisional one.

In the early Tertiary period, as we have seen, the climate of the northern hemisphere, as shown by the Eocene animals and plants, was very much hotter than it is at present—partaking, indeed, of a sub-tropical character. In the Middle Tertiary or Miocene period, the temperature, though not so high, was still much warmer than that now enjoyed by the northern hemisphere; and we know that the plants of temperate regions at this time flourished within the Arctic circle. In the later Tertiary or Pliocene period, again, there is evidence that the northern hemisphere underwent a further progressive diminution of temperature; though the climate of Europe generally seems at the close of the Tertiary period to have been if anything warmer, or at any rate not colder, than it is at the present day. With the commencement of the Quaternary period, however, this diminution of temperature became more decided; and beginning with a temperate climate, we find the greater portion of the northern hemisphere to become gradually subjected to all the rigours of intense Arctic cold. All the mountainous regions of Northern and Central Europe, of Britain, and of North America, became the nurseries of huge ice-streams, and large areas of the land appear to have been covered with a continuous ice-sheet. The Arctic conditions of this, the well-known "Glacial period," relaxed more than once, and were more than once re-established with lesser intensity. Finally, a gradual but steadily progressive amelioration of temperature took place; the ice slowly gave way, and ultimately disappeared altogether; and the climate once more became temperate, except in high northern latitudes.

The changes of temperature sketched out above took place slowly and gradually, and occupied the whole of the Post-Pliocene period. In each of the three periods marked out by these changes—in the early temperate, the central cold, and the later temperate period—certain deposits were laid down over the surface of the northern hemisphere; and these deposits collectively constitute the Post-Pliocene formations. Hence we may conveniently classify all the accumulations of this age under the heads of (1) Pre-Glacial deposits, (2) Glacial deposits, and (3) Post-Glacial deposits, according as they were formed before, during, or after the "Glacial period." It cannot by any means be asserted that we can definitely fix the precise relations in time of all the Post-Pliocene deposits to the Glacial period. On the contrary, there are some which hold a very disputed position as regards this point; and there are others which do not admit of definite allocation in this manner at all, in consequence of their occurrence in regions where no "Glacial Period" is known to have been established. For our present purpose, however, dealing as we shall have to do principally with the northern hemisphere, the above classification, with all its defects, has greater advantages than any other that has been yet proposed.

I. PRE-GLACIAL DEPOSITS.—The chief pre-glacial deposit of Britain is found on the Norfolk coast, reposing upon the Newer Pliocene (Norwich Crag), and consists of an ancient land-surface which is known as the "Cromer Forest-bed."

This consists of an ancient soil, having embedded in it the stumps of many trees, still in an erect position, with remains of living plants, and the bones of recent and extinct quadrupeds. It is overlaid by fresh-water and marine beds, all the shells of which belong to existing species, and it is finally surmounted by true "glacial drift." While all the shells and plants of the Cromer Forest-bed and its associated strata belong to existing species, the Mammals are partly living, partly extinct. Thus we find the existing Wolf (Canis lupus), Red Deer (Cervus elaphus), Roebuck (Cervus capreolus), Mole (Talpa Europtoea), and Beaver (Castor fiber), living in western England side by side with the Hippopotamus major, Elephas antiquus, Elephas meridionalis, Rhinoceros Etruscus, and R. Megarhinus of the Pliocene period, which are not only extinct, but imply an at any rate moderately warm climate. Besides the above, the Forest-bed has yielded the remains of several extinct species of Deer, of the great extinct Beaver (Trogontherium Cuvieri), of the Caledonian Bull or "Urus" (Bos primigenius), and of a Horse (Equus fossilis), little if at all distinguishable from the existing form.

The so-called "Bridlington Crag" of Yorkshire, and the "Chillesford Beds" of Suffolk, are probably to be regarded as also belonging to this period; though many of the shells which they contain are of an Arctic character, and would indicate that they were deposited in the commencement of the Glacial period itself. Owing, however, to the fact that a few of the shells of these deposits are not known to occur in a living condition, these, and some other similar accumulations, are sometimes considered as referable to the Pliocene period.

II. GLACIAL DEPOSITS.—Under this head is included a great series of deposits which are widely spread over both Europe and America, and which were formed at a time when the climate of these countries was very much colder than it is at present, and approached more or less closely to what we see at the present day in the Arctic regions. These deposits are known by the general name of the Glacial deposits, or by the more specialised names of the Drift, the Northern Drift, the Boulder-clay, the Till, &c.

These glacial deposits are found in Britain as far south as the Thames, over the whole of Northern Europe, in all the more elevated portions of Southern and Central Europe, and over the whole of North America, as far south as the 39th parallel. They generally occur as sands, clays, and gravels, spread in widely-extended sheets over all the geological formations alike, except the most recent, and are commonly spoken of under the general term of "Glacial drift." They vary much in their exact nature in different districts, but they universally consist of one, or all, of the following members:—

1. Unstratified clays, or loams, containing numerous angular or sub-angular blocks of stone, which have often been transported for a greater or less distance from their parent rock, and which often exhibit polished, grooved, or striated surfaces. These beds are what is called Boulder-clay, or Till.

2. Sands, gravels, and clays, often more or less regularly stratified, but containing erratic blocks, often of large size, and with their edges unworn, derived from considerable distances from the place where they are now found. In these beds it is not at all uncommon to find fossil shells; and these, though of existing species, are generally of an Arctic character, comprising a greater or less number of forms which are now exclusively found in the icy waters of the Arctic seas. These beds are often spoken of as "Stratified Drift."

3. Stratified sands and gravels, in which the pebbles are worn and rounded, and which have been produced by a rearrangement of ordinary glacial beds by the sea. These beds are commonly known as "Drift-gravels," or "Regenerated Drift".

Some of the last-mentioned of these are doubtless post-glacial; but, in the absence of fossils, it is often impossible to arrive at a positive opinion as to the precise age of superficial accumulations of this nature. It is also the opinion of high authorities that a considerable number of the so-called "cave-deposits," with the bones of extinct Mammals, truly belong to the Glacial period, being formed during warm intervals when the severity of the Arctic cold had become relaxed. It is further believed that some, at any rate, of the so-called "high-level" river-gravels and "brick-earths" have likewise been deposited during mild or warm intervals in the great age of ice; and in two or three instances this has apparently been demonstrated—deposits of this nature, with the bones of extinct animals and the implements of man, having been shown to be overlaid by true Boulder-clay.

The fossils of the undoubted Glacial deposits are principally shells, which are found in great numbers in certain localities, sometimes with Foraminifera, the bivalved cases of Ostracode Crustaceans, &c. Whilst some of the shells of the "Drift" are such as now live in the seas of temperate regions, others, as previously remarked, are such as are now only known to live in the seas of high latitudes; and these therefore afford unquestionable evidence of cold conditions. Amongst these Arctic forms of shells which characterise the Glacial beds may be mentioned Pecten Islandicus (fig. 254), Pecten Groenlandicus, Scalaria Groenlandica, Leda truncata, Astarte borealis, Tellina proxima, Nattra clausa, &c.



III. POST-GLACIAL DEPOSITS.—As the intense cold of the Glacial period became gradually mitigated, and temperate conditions of climate were once more re-established, various deposits were formed in the northern hemisphere, which are found to contain the remains of extinct Mammals, and which, therefore, are clearly of Post-Pliocene age. To these deposits the general name of Post-Glacial formations is given; but it is obvious that, from the nature of the case, and with our present limited knowledge, we cannot draw a rigid line of demarcation between the deposits formed towards the close of the Glacial period, or during warm "interglacial" periods, and those laid down after the ice had fairly disappeared. Indeed it is extremely improbable that any such rigid line of demarcation should ever have existed; and it is far more likely that the Glacial and Post-Glacial periods, and their corresponding deposits, shade into one another by an imperceptible gradation. Accepting this reservation, we may group together, under the general head of "Post-Glacial Deposits," most of the so-called "Valley-gravels," "Brick-earths," and "Cave-deposits," together with some "raised beaches" and various deposits of peat. Though not strictly within the compass of this work, a few words may be said here as to the origin and mode of formation of the Brick-earths, Valley-gravels, and Cave-deposits, as the subject will thus be rendered more clearly intelligible.

Every river produces at the present day beds of fine mud and loam, and accumulations of gravel, which it deposits at various parts of its course—the gravel generally occupying the lowest position, and the finer sands and mud coming above. Numerous deposits of a similar nature are found in most countries in various localities, and at various heights above the present channels of our rivers. Many of these fluviatile (Lat. fluvius, a river) deposits consist of fine loam, worked for brick-making, and known as "Brick-earths;" and they have yielded the remains of numerous extinct Mammals, of which the Mammoth (Elephas primigenius) is the most abundant. In the valley of the Rhine these fluviatile loams (known as "Loess") attain a thickness of several hundred feet, and contain land and fresh-water shells of existing species. With these occur the remains of Mammals, such as the Mammoth and Woolly Rhinoceros. Many of these Brick-earths are undoubtedly Post-Glacial, but others seem to be clearly "inter-glacial;" and instances have recently been brought forward in which deposits of Brick-earth containing bones and shells of fresh-water Molluscs have been found to be overlaid by regular unstratified boulder-clay.

The so-called "Valley-gravels," like the Brick-earths, are fluviatile deposits, but are of a coarser nature, consisting of sands and gravels. Every river gives origin to deposits of this kind at different points along the course of its valley; and it is not uncommon to find that there exist in the valley of a single river two or more sets of these gravel-beds, formed by the river itself, but formed at times when the river ran at different levels, and therefore formed at different periods. These different accumulations are known as the "high-level" and "low-level" gravels; and a reference to the accompanying diagram will explain the origin and nature of these deposits (fig. 255). When a river begins to occupy a particular line of drainage, and to form its own channel, it will deposit fluviatile sands and gravels along its sides. As it goes on deepening the bed or valley through which it flows, it will deposit other fluviatile strata at a lower level beside its new bed. In this way have arisen the terms "high-level" and "low-level" gravels. We find, for instance, a modern river flowing through a valley which it has to a great extent or entirely formed itself; by the side of its immediate channel we may find gravels, sand, and loam (fig. 255, 2 2') deposited by the river flowing in its present bed. These are recent fluviatile or alluvial deposits. At some distance from the present bed of the river, and at a higher level, we may find other sands and gravels, quite like the recent ones in character and origin, but formed at a time when the stream flowed at a higher level, and before it had excavated its valley to its present depth. These (fig. 255, 3 3') are the so-called "low-level gravels" of a river. At a still higher level, and still farther removed from the present bed of the river, we may find another terrace, composed of just the same materials as the lower one, but formed at a still earlier period, when the excavation of the valley had proceeded to a much less extent. These (fig. 255, 4 4') are the so-called "high-level gravels" of a river, and there may be one or more terraces of these.



The important fact to remember about these fluviatile deposits is this—that here the ordinary geological rule is reversed. The high-level gravels are, of course, the highest, so far as their actual elevation above the sea is concerned; but geologically the lowest, since they are obviously much older than the low-level gravels, as these are than the recent gravels. How much older the high-level gravels may be than the low-level ones, it is impossible to say. They occur at heights varying from 10 to 100 feet above the present river-channels, and they are therefore older than the recent gravels by the time required by the river to dig out its own bed to this depth. How long this period may be, our data do not enable us to determine accurately; but if we are to calculate from the observed rate of erosion of the actually existing rivers, the period between the different valley-gravels must be a very long one.

The lowest or recent fluviatile deposits which occur beside the bed of the present river, are referable to the Recent period, as they contain the remains of none but living Mammals. The two other sets of gravels are Post-Pliocene, as they contain the bones of extinct Mammals, mixed with land and fresh-water shells of existing species. Among the more important extinct Mammals of the low-level and high-level valley-gravels may be mentioned the Elephas antiquus, the Mammoth (Elephas primigenius), the Woolly Rhinoceros (R. Tichorhinus), the Hippopotamus, the Cave-lion, and the Cave-bear. Along with these are found unquestionable traces of the existence of Man, in the form of rude flint implements of undoubted human workmanship.

The so-called "Cave-deposits," again, though exhibiting peculiarities due to the fact of their occurrence in caverns or fissures in the rocks, are in many respects essentially similar to the older valley-gravels. Caves, in the great majority of instances, occur in limestone. When this is not the case, it will generally be found that they occur along lines of sea-coast, or along lines which can be shown to have anciently formed the coast-line. There are many caves, however, in the making of which it can be shown that the sea has had no hand; and these are most of the caves of limestone districts. These owe their origin to the solvent action upon lime of water holding carbonic acid in solution. The rain which falls upon a limestone district absorbs a certain amount of carbonic acid from the air, or from the soil. It then percolates through the rock, generally along the lines of jointing so characteristic of limestones, and in its progress it dissolves and carries off a certain quantity of carbonate of lime. In this way, the natural joints and fissures in the rock are widened, as can be seen at the present day in any or all limestone districts. By a continuance of this action for a sufficient length of time, caves may ultimately be produced. Nothing, also, is commoner in a limestone district than for the natural drainage to take the line of some fissure, dissolving the rock in its course. In this way we constantly meet in limestone districts with springs issuing from the limestone rock—sometimes as large rivers—the waters of which are charged with carbonate of lime, obtained by the solution of the sides of the fissure through which the waters have flowed. By these and similar actions, every district in which limestones are extensively developed will be found to exhibit a number of natural caves, rents, or fissures. The first element, therefore, in the production of cave-deposits, is the existence of a period in which limestone rocks were largely dissolved, and caves were formed in consequence of the then existing drainage taking the line of some fissure.

Secondly, there must have been a period in which various deposits were accumulated in the caves thus formed. These cavern-deposits are of very various nature, consisting of mud, loam, gravel, or breccias of different kinds. In all cases, these materials have been introduced into the cave at some period subsequent to, or contemporaneous with, the formation of the cave. Sometimes the cave communicates with the surface by a fissure through which sand, gravel, &c., may be washed by rains or by floods from some neighbouring river. Sometimes the cave has been the bed of an ancient stream, and the deposits have been formed as are fluviatile deposits at the surface. Or, again, the river has formerly flowed at a greater elevation than it does at present, and the cave has been filled with fluviatile deposits by the river at a time prior to the excavation of its bed to the present depth (fig. 256). In this last case, the cave-deposits obviously bear exactly the same relation in point of antiquity to recent deposits, as do the low-level and high-level valley-gravels to recent river-gravels. In any case, it is necessary for the physical geography of the district to change to some extent, in order that the cave-deposits should be preserved. If the materials have been introduced by a fissure, the cave will probably become ultimately filled to the roof, and the aperture of admission thus blocked up. If a river has flowed through the cave, the surface configuration of the district must be altered so far as to divert the river into a new channel. And if the cave is placed in the side of a river-valley, as in fig. 256, the river must have excavated its channel to such a depth that it can no longer wash out the contents of the cave even in high floods.



If the cave be entirely filled, the included deposits generally get more or less completely cemented together by the percolation through them of water holding carbonate of lime in solution. If the cave is only partially filled, the dropping of water from the roof holding lime in solution, and its subsequent evaporation, would lead to the formation over the deposits below of a layer of stalagmite, perhaps several inches, or even feet, in thickness. In this way cave-deposits, with their contained remains, may be hermetically sealed up and preserved without injury for an altogether indefinite period of time.

In all caves in limestone in which deposits containing bones are found, we have then evidence of three principal sets of changes. (1.) A period during which the cave was slowly hollowed out by the percolation of acidulated water; (2.) A period in which the cave became the channel of an engulfed river, or otherwise came to form part of the general drainage-system of the district; (3.) A period in which the cave was inhabited by various animals.

As a typical example of a cave with fossiliferous Post-Pliocene deposits, we may take Kent's Cavern, near Torquay, in which a systematic and careful examination has revealed the following sequence of accumulations in descending order:—

(a) Large blocks of limestone, which lie on the floor of the cave, having fallen from the roof, and which are sometimes cemented together by stalagmite.

(b) A layer of black mould, from three to twelve inches thick, with human bones, fragments of pottery, stone and bronze implements, and the bones of animals now living in Britain. This, therefore, is a recent deposit.

(c) A layer of stalagmite, from sixteen to twenty inches thick, but sometimes as much as five feet, containing the bones of Man, together with those of extinct Post-Pliocene Mammals.

(d) A bed of red cave-earth, sometimes four feet in thickness, with numerous bones of extinct Mammals (Mammoth, Cave-bear, &c.), together with human implements of flint and horn.

(e) A second bed of stalagmite, in places twelve feet in thickness, with bones of the Cave-bear.

(f) A red-loam and cave-breccia, with remains of the Cave-bear and human implements.

The most important Mammals which are found in cave-deposits in Europe generally, are the Cave-bear, the Cave-lion, the Cave-hyaena, the Reindeer, the Musk-ox, the Glutton, and the Lemming—of which the first three are probably identical with existing forms, and the remainder are certainly so—together with the Mammoth and the Woolly Rhinoceros, which are undoubtedly extinct. Along with these are found the implements, and in some cases the bones, of Man himself, in such a manner as to render it absolutely certain that an early race of men was truly contemporaneous in Western Europe with the animals above mentioned.

IV. UNCLASSIFIED POST-PLIOCENE DEPOSITS.—Apart from any of the afore mentioned deposits, there occur other accumulations—sometimes superficial, sometimes in caves—which are found in regions where a "Glacial period" has not been fully demonstrated, or where such did not take place; and which, therefore, are not amenable to the above classification. The most important of these are known to occur in South America and Australia; and though their numerous extinct Mammalia place their reference to the Post-Pliocene period beyond doubt, their relations to the glacial period and its deposits in the northern hemisphere have not been precisely determined.



CHAPTER XXII.

THE POST-PLIOCENE PERIOD—Continued.

As regards the life of the Post-Pliocene period, we have, in the first place, to notice the effect produced throughout the northern hemisphere by the gradual supervention of the Glacial period. Previous to this the climate must have been temperate or warm-temperate; but as the cold gradually came on, two results were produced as regards the living beings of the area thus affected. In the first place, all those Mammals which, like the Mammoth, the Woolly Rhinoceros, the Lion, the Hyaena, and the Hippopotamus, require, at any rate, moderately warm conditions, would be forced to migrate southwards to regions not affected by the new state of things. In the second place, Mammals previously inhabiting higher latitudes, such as the Reindeer, the Musk-ox, and the Lemming, would be enabled by the increasing cold to migrate southwards, and to invade provinces previously occupied by the Elephant and the Rhinoceros. A precisely similar, but more slowly-executed process, must have taken place in the sea, the northern Mollusca moving southwards as the arctic conditions of the Glacial period became established, whilst the forms proper to temperate seas receded. As regards the readily locomotive Mammals, also, it is probable that this process was carried on repeatedly in a partial manner, the southern and northern forms alternately fluctuating backwards and forwards over the same area, in accordance with the fluctuations of temperature which have been shown by Mr James Geikie to have characterised the Glacial period as a whole. We can thus readily account for the intermixture which is sometimes found of northern and southern types of Mammalia in the same deposits, or in deposits apparently synchronous, and within a single district. Lastly, at the final close of the arctic cold of the Glacial period, and the re-establishment of temperate conditions over the northern hemisphere, a reversal of the original process took place—the northern Mammals retiring within their ancient limits, and the southern forms pressing northwards and reoccupying their original domains.

The Invertebrate animals of the Post-Pliocene deposits require no further mention—all the known forms, except a few of the shells in the lowest beds of the formation, being identical with species now in existence upon the globe. The only point of importance in this connection has been previously noticed—namely, that in the true Glacial deposits themselves a considerable number of the shells belong to northern or Arctic types.

As regards the Vertebrate animals of the period, no extinct forms of Fishes, Amphibians, or Reptiles are known to occur, but we meet with both extinct Birds and extinct Mammals. The remains of the former are of great interest, as indicating the existence during Post-Pliocene times, at widely remote points of the southern hemisphere, of various wingless, and for the most part gigantic, Birds. All the great wingless Birds of the order Cursores which are known as existing at the present day upon the globe, are restricted to regions which are either wholly or in great part south of the equator. Thus the true Ostriches are African; the Rheas are South American; the Emeus are Australian; the Cassowaries are confined to Northern Australia, Papua, and the Indian Archipelago; the species of Apteryx are natives of New Zealand; and the Dodo and Solitaire (wingless, though probably not true Cursores), both of which have been exterminated within historical times, were inhabitants of the islands of Mauritius and Rodriguez, in the Indian Ocean. In view of these facts, it is noteworthy that, so far as known, all the Cursorial Birds of the Post-Pliocene period should have been confined to the same hemisphere as that inhabited by the living representatives of the order. It is still further interesting to notice that the extinct forms in question are only found in geographical provinces which are now, or have been within historical times, inhabited by similar types. The greater number of the remains of these have been discovered in New Zealand, where there now live several species of the curious wingless genus Apteryx; and they have been referred by Professor Owen to several generic groups, of which Dinornis is the most important (fig. 257). Fourteen species of Dinornis have been described by the distinguished palaeontologist just mentioned, all of them being large wingless birds of the type of the existing Ostrich, having enormously powerful hind-limbs adapted for running, but with the wings wholly rudimentary, and the breast-bone devoid of the keel or ridge which characterises this bone in all birds which fly. The largest species is the Dinornis giganteus, one of the most gigantic of living or fossil birds, the shank (tibia) measuring a yard in length, and the total height being at least ten feet. Another species, the Dinornis Elephantopus (fig. 257), though not standing more than about six feet in height, was of an even more ponderous construction—"the framework of the skeleton being the most massive of any in the whole class of Birds," whilst "the toe-bones almost rival those of the Elephant" (Owen). The feet in Dinornis were furnished with three toes, and are of interest as presenting us with an undoubted Bird big enough to produce the largest of the foot-prints of the Triassic Sandstones of Connecticut. New Zealand has now been so far explored, that it seems questionable if it can retain in its recesses any living example of Dinornis; but it is certain that species of this genus were alive during the human period, and survived up to quite a recent date. Not only are the bones very numerous in certain localities, but they are found in the most recent and superficial deposits, and they still contain a considerable proportion of animal matter; whilst in some instances bones have been found with the feathers attached, or with the horny skin of the legs still adhering to them. Charred bones have been found in connection with native "ovens;" and the traditions of the Maories contain circumstantial accounts of gigantic wingless Birds, the "Moas," which were hunted both for their flesh and their plumage. Upon the whole, therefore, there can be no doubt but that the Moas of New Zealand have been exterminated at quite a recent period—perhaps within the last century—by the unrelenting pursuit of Man,—a pursuit which their wingless condition rendered them unable to evade.



In Madagascar, bones have been discovered of another huge wingless Bird, which must have been as large as, or larger than, the Dinornis giganteus, and which has been described under the name of AEpiornis maximus. With the bones have been found eggs measuring from thirteen to fourteen inches in diameter, and computed to have the capacity of three Ostrich eggs. At least two other smaller species of AEpiornis have been described by Grandidier and Milne-Edwards as occurring in Madagascar; and they consider the genus to be so closely allied to the Dinornis of New Zealand, as to prove that these regions, now so remote, were at one time united by land. Unlike New Zealand, where there is the Apteryx, Madagascar is not known to possess any living wingless Birds; but in the neighbouring island of Mauritius the wingless Dodo (Didus ineptus) has been exterminated less than three hundred years ago; and the little island of Rodriguez, in the same geographical province, has in a similar period lost the equally wingless Solitaire (Pezophaps), both of these, however, being generally referred to the Rasores.

The Mammals of the Post-Pliocene period are so numerous, that in spite of the many points of interest which they present, only a few of the more important forms can be noticed here, and that but briefly. The first order that claims our attention is that of the Marsupials, the headquarters of which at the present day is the Australian province. In Oolitic times Europe possessed its small Marsupials, and similar forms existed in the same area in the Eocene and Miocene periods; but if size be any criterion, the culminating point in the history of the order was attained during the Post-Pliocene period in Australia. From deposits of this age there has been disentombed a whole series of remains of extinct, and for the most part gigantic, examples of this group of Quadrupeds. Not to speak of Wombats and Phalangers, two forms stand out prominently as representatives of the Post-Pliocene animals of Australia. One of these is Diprotodon (fig. 258), representing, with many differences, the well-known modern group of the Kangaroos. In its teeth, Diprotodon shows itself to be closely allied to the living, grass-eating Kangaroos; but the hind-limbs were not so disproportionately long. In size, also, Diprotodon must have many times exceeded the dimensions of the largest of its living successors, since the skull measures no less than three feet in length. The other form in question is Thylacoleo (fig. 259), which is believed by Professor Owen to belong to the same group as the existing "Native Devil" (Dasyurus) of Van Diemen's Land, and therefore to have been flesh-eating and rapacious in its habits, though this view is not accepted by others. The principal feature in the skull of Thylacoleo is the presence, on each side of each jaw, of a single huge tooth, which is greatly compressed, and has a cutting edge. This tooth is regarded by Owen as corresponding to the great cutting tooth of the jaw of the typical Carnivores, but Professor Flower considers that Thylacoleo is rather related to the Kangaroo-rats. The size of the crown of the tooth in question is not less than two inches and a quarter; and whether carnivorous or not, it indicates an animal of a size exceeding that of the largest of existing Lions.



The order of the Edentates, comprising the existing Sloths, Ant-eaters, and Armadillos, and entirely restricted at the present day to South America, Southern Asia, and Africa, is one alike singular for the limited geographical range of its members, their curious habits of life, and the well-marked peculiarities of their anatomical structure. South America is the metropolis of the existing forms; and it is an interesting fact that there flourished within Post-Pliocene times in this continent, and to some extent in North America also, a marvellous group of extinct Edentates, representing the living Sloths and Armadillos, but of gigantic size. The most celebrated of these is the huge Megatherium Cuvieri (fig. 260) of the South American Pampas. The Megathere was a colossal Sloth-like animal which attained a length of from twelve to eighteen feet, with bones more massive than those of the Elephant. Thus the thigh-bone is nearly thrice the thickness of the same bone in the largest of existing Elephants, its circumference at its narrowest point nearly equalling its total length; the massive bones of the shank (tibia and fibula) are amalgamated at their extremities; the heel-bone (calcaneum) is nearly half a yard in length; the haunch-bones (ilia) are from four to five feet across at their crests; and the bodies of the vertebrae at the root of the tail are from five to seven inches in diameter, from which it has been computed that the circumference of the tail at this part might have been from five to six feet. The length of the fore-foot is about a yard, and the toes are armed with powerful curved claws. It is known now that the Megathere, in spite of its enormous weight and ponderous construction, walked, like the existing Ant-eaters and Sloths, upon the outside edge of the fore-feet, with the claws more or less bent inwards towards the palm of the hand. As in the great majority of the Edentate order, incisor and canine teeth are entirely wanting, the front of the jaws being toothless. The jaws, however, are furnished with five upper and four lower molar teeth on each side. These grinding teeth are from seven to eight inches in length, in the form of four-sided prisms, the crowns of which are provided with well-marked transverse ridges; and they continue to grow during the whole life of the animal. There are indications that the snout was prolonged, and more or less flexible; and the tongue was probably prehensile. From the characters of the molar teeth it is certain that the Megathere was purely herbivorous in its habits; and from the enormous size and weight of the body, it is equally certain that it could not have imitated its modern allies, the Sloths, in the feat of climbing, back downwards, amongst the trees. It is clear, therefore, that the Megathere sought its sustenance upon the ground; and it was originally supposed to have lived upon roots. By a masterly piece of deductive reasoning, however, Professor Owen showed that this great "Ground-Sloth" must have truly lived upon the foliage of trees, like the existing Sloths—but with this difference, that instead of climbing amongst the branches, it actually uprooted the tree bodily. In this tour de force, the animal sat upon its huge haunches and mighty tail, as on a tripod, and then grasping the trunk with its powerful arms, either wrenched it up by the roots or broke it short off above the ground. Marvellous as this may seem, it can be shown that every detail in the skeleton of the Megathere accords with the supposition that it obtained its food in this way. Similar habits were followed by the allied Mylodon (fig. 261), another of the great "Ground-Sloths," which inhabited South America during the Post-Pliocene period. In most respects, the Mylodon is very like the Megathere; but the crowns of the molar teeth are flat instead of being ridged. The nearly-related genus Megalonyx, unlike the Megathere, but like the Mylodon, extended its range northwards as far as the United States.



Just as the Sloths of the present day were formerly represented in the same geographical area by the gigantic Megatheroids, so the little banded and cuirassed Armadillos of South America were formerly represented by gigantic species, constituting the genus Glyptodon. The Glyptodons (fig. 262) differed from the living Armadillos in having no bands in their armour, so that they must have been unable to roll themselves up. It is rare at the present day to meet with any Armadillo over two or three feet in length; but the length of the Glyptodon clavipes, from the tip of the snout to the end of the tail, was more than nine feet.



There are no canine or incisor teeth in the Glyptodon, but there are eight molars on each side of each jaw, and the crowns of these are fluted and almost trilobed. The head is covered by a helmet of bony plates, and the trunk was defended by an armour of almost hexagonal bony pieces united by sutures, and exhibiting special patterns of sculpturing in each species. The tail was also defended by a similar armour, and the vertebrae were mostly fused together so as to form a cylindrical bony rod. In addition to the above-mentioned forms, a number of other Edentate animals have been discovered by the researches of M. Lund in the Post-Pliocene deposits of the Brazilian bone-caves. Amongst these are true Ant-eaters, Armadillos, and Sloths, many of them of gigantic size, and all specifically or generically distinct from existing forms.

Passing over the aquatic orders of the Sirenians and Cetaceans, we come next to the great group of the Hoofed Quadrupeds, the remains of which are very abundant in Post-Pliocene deposits both in Europe and North America. Amongst the Odd-toed Ungulates the most important are the Rhinoceroses, of which three species are known to have existed in Europe during the Post-Pliocene period. Two of these are the well-known Pliocene forms, the Rhinoceros Etruscus and the R. Megarhinus still surviving in diminished numbers; but the most famous is the Rhinoceros tichorhinus (fig. 263), or so-called "Woolly Rhinoceros." This species is known not only by innumerable bones, but also by a carcass, at the time of its discovery complete, which was found embedded in the frozen soil of Siberia towards the close of last century, and which was partly saved from destruction by the exertions of the naturalist Pallas. From this, we know that the Tichorhine Rhinoceros, like its associate the Mammoth, was provided with a coating of hair, and therefore was enabled to endure a more severe climate than any existing species. The skin was not thrown into the folds which characterise most of the existing forms; and the technical name of the species refers to the fact that the nostrils were completely separated by a bony partition. The head carried two horns, placed one behind the other, the front one being unusually large. As regards its geographical range, the Woolly Rhinoceros is found in Europe in vast numbers north of the Alps and Pyrenees, and it also abounded in Siberia; so that it would appear to be a distinctly northern form, and to have been adapted for a temperate climate. It is not known to occur in Pliocene deposits, but it makes its first appearance in the Pre-Glacial deposits, surviving the Glacial period, and being found in abundance in Post-Glacial accumulations. It was undoubtedly a contemporary of the earlier races of men in Western Europe; and it may perhaps be regarded as being the actual substantial kernel of some of the "Dragons" of fable.



The only other Odd-toed Ungulate which needs notice is the so-called Equus fossilis of the Post-Pliocene of Europe. This made its appearance before the Glacial period, and appears to be in reality identical with the existing Horse (Equus caballus). True Horses also occur in the Post-Pliocene of North America; but, from some cause or another, they must have been exterminated before historic times.



Amongst the Even-toed Ungulates, the great Hippopotamus major of the Pliocene still continued to exist in Post-Pliocene times in Western Europe; and the existing Wild Boar (Sus scrofa), the parent of our domestic breeds of Pigs, appeared for the first time. The Old World possessed extinct representatives of its existing Camels, and lost types of the living Llamas inhabited South America. Amongst the Deer, the Post-Pliocene accumulations have yielded the remains of various living species, such as the Red Deer (Cervus elaphus), the Reindeer (Cervus tarandus), the Moose or Elk (Alces malchis), and the Roebuck (Cervus capreolus), together with a number of extinct forms. Among the latter, the great "Irish Elk" (Cervus megaceros) is justly celebrated both for its size and for the number and excellent preservation of its discovered remains. This extinct species (fig. 264) has been found principally in peat-mosses and Post-Pliocene lake-deposits, and is remarkable for the enormous size of the spreading antlers, which are widened out towards their extremities, and attain an expanse of over ten feet from tip to tip. It is not a genuine Elk, but is intermediate between the Reindeer and the Fallow-deer. Among the existing Deer of the Post-Pliocene, the most noticeable is the Reindeer, an essentially northern type, existing at the present day in Northern Europe, and also (under the name of the "Caribou") in North America. When the cold of the Glacial period became established, this boreal species was enabled to invade Central and Western Europe in great herds, and its remains are found abundantly in cave-earths and other Post-Pliocene deposits as far south as the Pyrenees.



In addition to the above, the Post-Pliocene deposits of Europe and North America have yielded the remains of various Sheep and Oxen. One of the most interesting of the latter is the "Urus" or Wild Bull (Bos primigenius, fig. 265), which, though much larger than any of the existing fossils, is believed to be specifically undistinguishable from the domestic Ox (Bos taurus), and to be possibly the ancestor of some of the larger European varieties of oxen. In the earlier part of its existence the Urus ranged over Europe and Britain in company with the Woolly Rhinoceros and the Mammoth; but it long survived these, and does not appear to have been finally exterminated till about the twelfth century. Another remarkable member of the Post-Pliocene Cattle, also to begin with an associate of the Mammoth and Rhinoceros, is the European Bison or "Aurochs" (Bison priscus). This "maned" ox formerly abounded in Europe in Post-Glacial times, and was not rare even in the later periods of the Roman empire, though much diminished in numbers, and driven back into the wilder and more inaccessible parts of the country. At present this fine species has been so nearly exterminated that it no longer exists in Europe save in Lithuania, where its preservation has been secured by rigid protective laws. Lastly, the Post-Pliocene deposits have yielded the remains of the singular living animal which is known as the Musk-ox or Musk-sheep (Ovibos moschatus). At the present day, the Musk-ox is an inhabitant of the "barren grounds" of Arctic America, and it is remarkable for the great length of its hair. It is, like the Reindeer, a distinctively northern animal; but it enjoyed during the Glacial period a much wider range than it has at the present day, the conditions suitable for its existence being then extended over a considerable portion of the northern hemisphere. Thus remains of the Musk-Ox are found in greater or less abundance in Post-Pliocene deposits over a great part of Europe, extending even to the south of France; and closely-related forms are found in similar deposits in the United States.



Coming to the _Proboscideans_, we find that the _Mastodons_ seem to have disappeared in Europe at the close of the Pliocene period, or at the very commencement of the Post-Pliocene. In the New World, on the other hand, a species of Mastodon (_M. Americanus_ or _M. Ohioticus_) is found abundantly in deposits of Post-Pliocene age, from Canada to Texas. Very perfect skeletons of this species have been exhumed from morasses and swamps, and large individuals attained a length (exclusive of the tusks) of seventeen feet and a height of eleven feet, the tusks being twelve feet in length. Remains of _Elephants_ are also abundant in the Post-Pliocene deposits of both the Old and the New World. Amongst these, we find in Europe the two familiar Pliocene species _E. Meridionales_ and _E. Antiquus_ still surviving, but in diminished numbers. With these are found in vast abundance the remains of the characteristic Elephant of the Post-Pliocene, the well-known "Mammoth" (Elephas primigenius_), which is accompanied in North America by the nearly-allied, but more southern species, the _Elephas Americanus_. The Mammoth (fig. 266) is considerably larger than the largest of the living Elephants, the skeleton being over sixteen feet in length, exclusive of the tusks, and over nine feet in height. The tusks are bent almost into a circle, and are sometimes twelve feet in length, measured along their curvature. In the frozen soil of Siberia several carcasses of the Mammoth have been discovered with the flesh and skin still attached to the bones, the most celebrated of these being a Mammoth which was discovered at the beginning of this century at the mouth of the Lena, on the borders of the Frozen Sea, and the skeleton of which is now preserved at St Petersburg (fig. 266). From the occurrence of the remains of the Mammoth in vast numbers in Siberia, it might have been safely inferred that this ancient Elephant was able to endure a far more rigorous climate than its existing congeners. This inference has, however, been rendered a certainty by the specimens just referred to, which show that the Mammoth was protected against the cold by a thick coat of reddish-brown wool, some nine or ten inches long, interspersed with strong, coarse black hair more than a foot in length. The teeth of the Mammoth (fig.267) are of the type of those of the existing Indian Elephant, and are found in immense numbers in certain localities. The Mammoth was essentially northern in its distribution, never passing south of a line drawn through the Pyrenees, the Alps, the northern shores of the Caspian, Lake Baikal, Kamschatka, and the Stanovi Mountains (Dawkins). It occurs in the Pre-Glacial forest-bed of Cromer in Norfolk, survived the Glacial period, and is found abundantly in Post-Glacial deposits in France, Germany, Britain, Russia in Europe, Asia, and North America, being often associated with the Reindeer, Lemming, and Musk-ox. That it survived into the earlier portion of the human period is unquestionable, its remains having been found in a great number of instances associated with implements of human manufacture; whilst in one instance a recognisable portrait of it has been discovered, carved on bone.



Amongst other Elephants which occur in Post-Pliocene deposits may be mentioned, as of special interest, the pigmy Elephants of Malta. One of these—the Elephas Melitensis, or so-called "Donkey-Elephant"—was not more than four and a half feet in height. The other—the Elephas Falconeri, of Busk—was still smaller, its average height at the withers not exceeding two and a half to three feet.



Whilst herbivorous animals abounded during the Post-Pliocene, we have ample evidence of the coexistence with them of a number of Carnivorous forms, both in the New and the Old World. The Bears are represented in Europe by at least three species, two of which—namely, the great Grizzly Bear (Ursus ferox) and the smaller Brown Bear (Ursus arctos)—are in existence at the present day. The third species is the celebrated Cave-bear (Ursus speloeus, fig. 268), which is now extinct. The Cave-bear exceeded in its dimensions the largest of modern Bears; and its remains, as its name implies; have been found mainly in cavern-deposits. Enormous numbers of this large and ferocious species must have lived in Europe in Post-Glacial times; and that they survived into the human period, is clearly shown by the common association of their bones with the implements of man. They are occasionally accompanied by the remains of a Glutton (the Gulo speloeus), which does not appear to be really separable from the existing Wolverine or Glutton of northern regions (the Gulo luscus). In addition, we meet with the bones of the Wolf, Fox, Weasel, Otter, Badger, Wild Cat, Panther, Hyaena, and Lion, &c., together with the extinct Machairodus or "Sabre-toothed Tiger." The only two of these that deserve further mention are the Hyaena and the Lion. The Cave-hyaena (Hyoena speloea, fig. 269) is regarded by high authorities as nothing more than a variety of the living Spotted Hyaena (H. Crocuta) of South Africa. This well-known species inhabited Britain and a considerable portion of Europe during a large part of the Post-Pliocene period; and its remains often occur in great abundance. Indeed, some caves, such as the Kirkdale Cavern in Yorkshire, were dens inhabited during long periods by these animals, and thus contain the remains of numerous individuals and of successive generations of Hyaenas, together with innumerable gnawed and bitten bones of their prey. That the Cave-hyaena was a contemporary with Man in Western Europe during Post-Glacial times is shown beyond a doubt by the common association of its bones with human implements.



Lastly, the so-called Cave-lion (Felis speloea), long supposed to be a distinct species, has been shown to be nothing more than a large variety of the existing Lion (Felis leo). This animal inhabited Britain and Western Europe in times posterior to the Glacial period, and was a contemporary of the Cave-hyaena, Cave-bear, Woolly Rhinoceros, and Mammoth. The Cave-lion also unquestionably survived into the earlier portion of the human period in Europe.

The Post-Pliocene deposits of Europe have further yielded the remains of numerous Rodents—such as the Beaver, the Northern Lemming, Marmots, Mice, Voles, Rabbits, &c.—together with the gigantic extinct Beaver known as the Trogontherium Cuvieri (fig. 270). The great Castoroides Ohioensis of the Post-Pliocene of North America is also a great extinct Beaver, which reached a length of about five feet. Lastly, the Brazilian bone-caves have yielded the remains of numerous Rodents of types now characteristic of South America, such as Guinea-pigs, Capybaras, tree-inhabiting Porcupines, and Coypus.



The deposits just alluded to have further yielded the remains of various Monkeys, such as Howling Monkeys, Squirrel Monkeys, and Marmosets, all of which belong to the group of Quadrumana which is now exclusively confined to the South American continent—namely, the "Platyrhine" Monkeys.

We still have very briefly to consider the occurrence of Man in Post-Pliocene deposits; but before doing so, it will be well to draw attention to the evidence afforded by the Post-Pliocene Mammals as to the climate of Western Europe at this period. The chief point which we have to notice is, that a considerable revolution of opinion has taken place on this point. It was originally believed that the presence of such animals as Elephants, Lions, the Rhinoceros, and the Hippopotamus afforded an irrefragable proof that the climate of Europe must have been a warm one, at any rate during Post-Glacial times. The existence, also, of numbers of Mammoths in Siberia, was further supposed to indicate that this high temperature extended itself very far north. Upon the whole, however, the evidence is against this view. Not only is there great difficulty in supposing that the Arctic conditions of the Glacial period were immediately followed by anything warmer than a cold-temperate climate; but there is nothing in the nature of the Mammals themselves which would absolutely forbid their living in a temperate climate. The Hippopotamus major, though probably clad in hair, offers some difficulty—since, as pointed out by Professor Busk, it must have required a climate sufficiently warm to insure that the rivers were not frozen over in the winter; but it was probably a migratory animal, and its occurrence may be accounted for by this. The Woolly Rhinoceros and the Mammoth are known with certainty to have been protected with a thick covering of wool and hair; and their extension northwards need not necessarily have been limited by anything except the absence of a sufficiently luxuriant vegetation to afford them food. The great American Mastodon, though not certainly known to have possessed a hairy covering, has been shown to have lived upon the shoots of Spruce and Firs, trees characteristic of temperate regions—as shown by the undigested food which has been found with its skeleton, occupying the place of the stomach. The Lions and Hyaenas, again, as shown by Professor Boyd Dawkins, do not indicate necessarily a warm climate. Wherever a sufficiency of herbivorous animals to supply them with food can live, there they can live also; and they have therefore no special bearing upon the question of climate. After a review of the whole evidence, Professor Dawkins concludes that the nearest approach at the present day to the Post-Pliocene climate of Western Europe is to be found in the climate of the great Siberian plains which stretch from the Altai Mountains to the Frozen Sea. "Covered by impenetrable forests, for the most part of Birch, Poplar, Larch, and Pines, and low creeping dwarf Cedars, they present every gradation in climate from the temperate to that in which the cold is too severe to admit of the growth of trees, which decrease in size as the traveller advances northwards, and are replaced by the grey mosses and lichens that cover the low marshy 'tundras.' The maximum winter cold, registered by Admiral Von Wrangel at Nishne Kolymsk, on the banks of the Kolyma, is—65 deg. in January. 'Then breathing becomes difficult; the Reindeer, that citizen of the Polar region, withdraws to the deepest thicket of the forest, and stands there motionless as if deprived of life;' and trees burst asunder with the cold. Throughout this area roam Elks, Black Bears, Foxes, Sables, and Wolves, that afford subsistence to the Jakutian and Tungusian fur-hunters. In the northern part countless herds of Reindeer, Elks, Foxes, and Wolverines make up for the poverty of vegetation by the rich abundance of animal life. 'Enormous flights of Swans, Geese, and Ducks arrive in the spring, and seek deserts where they may moult and build their nests in safety. Ptarmigans run in troops amongst the bushes; little Snipes are busy along the brooks and in the morasses; the social Crows seek the neighbourhood of new habitations; and when the sun shines in spring, one may even sometimes hear the cheerful note of the Finch, and in autumn that of the Thrush.' Throughout this region of woods, a hardy, middle-sized breed of horses lives under the mastership and care of man, and is eminently adapted to bear the severity of the climate.... The only limit to their northern range is the difficulty of obtaining food. The severity of the winter through the southern portion of this vast wooded area is almost compensated for by the summer heat and its marvellous effect on vegetation."—(Dawkins, 'Monograph of Pleistocene Mammalia.')

Finally, a few words must be said as to the occurrence of the remains of Man in Post-Pliocene deposits. That Man existed in Western Europe and in Britain during the Post-Pliocene period, is placed beyond a doubt by the occurrence of his bones in deposits of this age, along with the much more frequent occurrence of implements of human manufacture. At what precise point of time during the Post-Pliocene period he first made his appearance is still a matter of conjecture. Recent researches would render it probable that the early inhabitants of Britain and Western Europe were witnesses of the stupendous phenomena of the Glacial period; but this cannot be said to have been demonstrated. That Man existed in these regions during the Post-Glacial division of Post-Pliocene time cannot be doubted for a moment. As to the physical peculiarities of the ancient races that lived with the Mammoth and the Woolly Rhinoceros, little is known compared with what we may some day hope to know. Such information as we have, however, based principally on the skulls of the Engis, Neanderthal, Cro-Magnon, and Bruniquel caverns, would lead to the conclusion that Post-Pliocene Man was in no respect inferior in his organisation to, or less highly developed than, many existing races. All the known skulls of this period, with the single exception of the Neanderthal cranium, are in all respects average and normal in their characters; and even the Neanderthal skull possessed a cubic capacity at least equal to that of some existing races. The implements of Post-Pliocene Man are exclusively of stone or bone; and the former are invariably of rude shape and undressed. These "palaeolithic" tools (Gr. palaios; ancient; lithos, stone) point to a very early condition of the arts; since the men of the earlier portion of the Recent period, though likewise unacquainted with the metals, were in the habit of polishing or dressing the stone implements which they fabricated.

It is impossible here to enter further into this subject; and it would be useless to do so without entering as well into a consideration of the human remains of the Recent period—a period which lies outside the province of the present work. So far as Post-Pliocene Man is concerned, the chief points which the palaeontological student has to remember have been elsewhere summarised by the author as follows:—

1. Man unquestionably existed during the later portion of what Sir Charles Lyell has termed the "Post-Pliocene" period. In other words, Man's existence dates back to a time when several remarkable Mammals, previously mentioned, had not yet become extinct; but he does not date back to a time anterior to the present Molluscan fauna.

2. The antiquity of the so-called Post-Pliocene period is a matter which must be mainly settled by the evidence of Geology proper, and need not be discussed here.

3. The extinct Mammals with which man coexisted in Western Europe are mostly of large size, the most important being the Mammoth (_Elephas primogenius_), the Woolly Rhinoceros (_Rhinoceros tichorhinus_), the Cave-lion (_Felis speloea_), the Cave-hyaena(_Hyoena speloea), and the Cave-bear (_Ursus speloeus_). We do not know the causes which led to the extinction of these Mammals; but we know that hardly any Mammalian species has become extinct during the historical period.

4. The extinct Mammals with which man coexisted are referable in many cases to species which presumably required a very different climate to that now prevailing in Western Europe. How long a period, however, has been consumed in the bringing about of the climatic changes thus indicated, we have no means of calculating with any approach to accuracy.

5. Some of the deposits in which the remains of man have been found associated with the bones of extinct Mammals, are such as to show incontestably that great changes in the physical geography and surface-configuration of Western Europe have taken place since the period of their accumulation. We have, however, no means at present of judging of the lapse of time thus indicated except by analogies and comparisons which may be disputed.

6. The human implements which are associated with the remains of extinct Mammals, themselves bear evidence of an exceedingly barbarous condition of the human species. Post-Pliocene or "Palaeolithic" Man was clearly unacquainted with the use of any of the metals. Not only so, but the workmanship of these ancient races was much inferior to that of the later tribes, who were also ignorant of the metals, and who also used nothing but weapons and tools of stone, bone, &c.

7. Lastly, it is only with the human remains of the Post-Pliocene period that the palaeontologist proper has to deal. When we enter the "Recent" period, in which the remains of Man are associated with those of existing species of Mammals, we pass out of the region of pure palaeontology into the domain of the Archaeologist and the Ethnologist.

LITERATURE.

The following are some of the principal works and memoirs to which the student may refer for information as to the Post-Pliocene deposits and the remains which they contain, as well as to the primitive races of mankind:—

(1) 'Elements of Geology.' Lyell. (2) 'Antiquity of Man.' Lyell. (3) 'Palaeontological Memoirs.' Falconer. (4) 'The Great Ice-age.' James Geikie. (5) 'Manual of Palaeontology.' Owen. (6) 'British Fossil Mammals and Birds.' Owen. (7) 'Cave-Hunting.' Boyd Dawkins. (8) 'Prehistoric Times.' Lubbock. (9) 'Ancient Stone Implements.' Evans. (10) 'Prehistoric Man.' Daniel Wilson. (11) 'Prehistoric Races of the United States.' Foster. (12) 'Manual of Geology.' Dana. (13) 'Monograph of Pleistocene Mammalia' (Palaeontographical Society). Boyd Dawkins and Sanford. (14) 'Monograph of the Post-Tertiary Entomostraca of Scotland, &c., with an Introduction on the Post-Tertiary Deposits of Scotland' (Ibid.) G. S. Brady, H. W. Crosskey, and D. Robertson. (15) "Reports on Kent's Cavern"—'British Association Reports.' Pengelly. (16) "Reports on the Victoria Cavern, Settle"—'British Association Reports.' Tiddeman. (17) 'Ossemens Fossiles.' Cuvier. (18) 'Reliquiae Diluvianae.' Buckland. (19) "Fossil Mammalia"—'Zoology of the Voyage of the Beagle.' Owen. (20) 'Description of the Tooth and Part of the Skeleton of the Glyptodon.' Owen. (21) "Memoir on the Extinct Sloth Tribe of North America"—'Smithsonian Contributions to Knowledge.' Leidy. (22) "Report on Extinct Mammals of Australia"—'British Association,' 1844. Owen. (23) 'Description of the Skeleton of an Extinct Gigantic Sloth (Mylodon robtutus).' Owen. (24) "Affinities and Probable Habits of Thylacoleo"—'Quart. Journ. Geol. Soc.,' vol. xxiv. Flower. (25) 'Prodromus of the Palaeontology of Victoria.' M'Coy. (26) 'Les Ossemens Fossiles des Cavernes de Liege.' Schmerling. (27) 'Die Fauna der Pfahlbauten in der Schweiz.' Ruetimeyer. (28) "Extinct and Existing Bovine Animals of Scandinavia"—'Annals of Natural History,' ser. 2, vol. iv., 1849. Nilsson. (29) 'Man's Place in Nature.' Huxley. (30) 'Les Temps Antehistoriques en Belgique.' Dupont. (31) "Classification of the Pleistocene Strata of Britain and the Continent"—'Quart. Journ. Geol. Soc.,' vol. xxviii. Boyd Dawkins. (32) 'Distribution of the Post-Glacial Mammalia' (Ibid.), vol. xxv. Boyd Dawkins. (33) 'On British Fossil Oxen' (Ibid.), vols. xxii. and xxiii. Boyd Dawkins. (34) 'British Prehistoric Mammals' (Congress of Prehistoric Archaeology, 1868). Boyd Dawkins. (35) 'Reliquiae Aquitanicae.' Lartet and Christy. (36) 'Zoologie et Paleontologie Francaises.' Gervais. (37) 'Notes on the Post-Pliocene Geology of Canada.' Dawson. (38) "On the Connection between the existing Fauna and Flora of Great Britain and certain Geological Changes"—'Mem. Geol. Survey.' Edward Forbes. (39) 'Cavern-Researches.' M'Enery. Edited by Vivian. (40) "Quaternary Gravels"—'Quart. Journ. Geol. Soc.,' vol. xxv. Tylor.



CHAPTER XXIII.

THE SUCCESSION OF LIFE UPON THE GLOBE.

In conclusion, it may not be out of place if we attempt to summarise, in the briefest possible manner, some of the principal results which may be deduced as to the succession of life upon the earth from the facts which have in the preceding portion of this work been passed in review. That there was a time when the earth was void of life is universally admitted, though it may be that the geological record gives us no direct evidence of this. That the globe of to-day is peopled with innumerable forms of life whose term of existence has been, for the most part, but as it were of yesterday, is likewise an assertion beyond dispute. Can we in any way connect the present with the remote past, and can we indicate even imperfectly the conditions and laws under which the existing order was brought about? The long series of fossiliferous deposits, with their almost countless organic remains, is the link between what has been and what is; and if any answer to the above question can be arrived at, it will be by the careful and conscientious study of the facts of Palaeontology. In the present state of our knowledge, it may be safely said that anything like a dogmatic or positive opinion as to the precise sequence of living forms upon the globe, and still more as to the manner in which this sequence may have been brought about, is incapable of scientific proof. There are, however, certain general deductions from the known facts which may be regarded as certainly established.

In the first place, it is certain that there has been a succession of life upon the earth, different specific and generic types succeeding one another in successive periods. It follows from this, that the animals and plants with which we are familiar as living, were not always upon the earth, but that they have been preceded by numerous races more or less differing from them. What is true of the species of animals and plants, is true also of the higher zoological divisions; and it is, in the second place, quite certain that there has been a similar succession in the order of appearance of the primary groups ("sub-kingdoms," "classes," &c.) of animals and vegetables. These great groups did not all come into existence at once, but they made their appearance successively. It is true that we cannot be said to be certainly acquainted with the first absolute appearance of any great group of animals. No one dare assert positively that the apparent first appearance of Fishes in the Upper Silurian is really their first introduction upon the earth: indeed, there is a strong probability against any such supposition. To whatever extent, however, future discoveries may push back the first advent of any or of all of the great groups of life, there is no likelihood that anything will be found out which will materially alter the relative succession of these groups as at present known to us. It is not likely, for example, that the future has in store for us any discovery by which it would be shown that Fishes were in existence before Molluscs, or that Mammals made their appearance before Fishes. The sub-kingdoms of Invertebrate animals were all represented in Cambrian times—and it might therefore be inferred that these had all come simultaneously into existence; but it is clear that this inference, though incapable of actual disproof, is in the last degree improbable. Anterior to the Cambrian is the great series of the Laurentian, which, owing to the metamorphism to which it has been subjected, has so far yielded but the singular Eozooen. We may be certain, however, that others of the Invertebrate sub-kingdoms besides the Protozoa were in existence in the Laurentian period; and we may infer from known analogies that they appeared successively, and not simultaneously.

When we come to smaller divisions than the sub-kingdoms—such as classes, orders, and families—a similar succession of groups is observable. The different classes of any given sub-kingdom, or the different orders of any given class, do not make their appearance together and all at once, but they are introduced upon the earth in succession. More than this, the different classes of a sub-kingdom, or the different orders of a class, in the main succeed one another in the relative order of their zoological rank—the lower groups appearing first and the higher groups last. It is true that in the Cambrian formation—the earliest series of sediments in which fossils are abundant—we find numerous groups, some very low, others very high, in the zoological scale, which appear to have simultaneously flashed into existence. For reasons stated above, however, we cannot accept this appearance as real; and we must believe that many of the Cambrian groups of animals really came into being long before the commencement of the Cambrian period. At any rate, in the long series of fossiliferous deposits of later date than the Cambrian the above-stated rule holds good as a broad generalisation—that the lower groups, namely, precede the higher in point of time; and though there are apparent exceptions to the rule, there are none of such a nature as not to admit of explanation. Some of the leading facts upon which this generalisarion is founded will be enumerated immediately; but it will be well, in the first place, to consider briefly what we precisely mean when we speak of "higher" and "lower" groups.

It is well known that naturalists are in the habit of "classifying" the innumerable animals which now exist upon the globe; or, in other words, of systematically arranging them into groups. The precise arrangement adopted by one naturalist may differ in minor details from that adopted by another; but all are agreed as to the fundamental points of classification, and all, therefore, agree in placing certain groups in a certain sequence. What, then, is the principle upon which this sequence is based? Why, for example, are the Sponges placed below the Corals; these below the Sea-urchins; and these, again, below the Shell-fish? Without entering into a discussion of the principles of zoological classification, which would here be out of place, it must be sufficient to say that the sequence in question is based upon the relative type of organisation of the groups of animals classified. The Corals are placed above the Sponges upon the ground that, regarded as a whole, the plan or type of structure of a Coral is more complex than that of a Sponge. It is not in the slightest degree that the Sponge is in any respect less highly organised or less perfect, as a Sponge, than is the Coral as a Coral. Each is equally perfect in its own way; but the structural pattern of the Coral is the highest, and therefore it occupies a higher place in the zoological scale. It is upon this principle, then, that the primary subdivisions of the animal kingdom (the so-called "sub-kingdoms") are arranged in a certain order. Coming, again, to the minor subdivisions (classes, orders, &c.) of each sub-kingdom, we find a different but entirely analogous principle employed as a means of classification. The numerous animals belonging to any given sub-kingdom are formed upon the same fundamental plan of structure; but they nevertheless admit of being arranged in a regular series of groups. All the Shell-fish, for example, are built upon a common plan, this plan representing the ideal Mollusc; but there are at the same time various groups of the Mollusca, and these groups admit of an arrangement in a given sequence. The principle adopted in this case is simply of the relative elaboration of the common type. The Oyster is built upon the same ground-plan as the Cuttle-fish; but this plan is carried out with much greater elaboration, and with many more complexities, in the latter than in the former: and in accordance with this, the Cephalopoda constitute a higher group than the Bivalve Shell-fish. As in the case of superiority of structural type, so in this case also, it is not in the least that the Oyster is an imperfect animal. On the contrary, it is just as perfectly adapted by its organisation to fill its own sphere and to meet the exigencies of its own existence as is the Cuttle-fish; but the latter lives a life which is, physiologically, higher than the former, and its organisation is correspondingly increased in complexity.

This being understood, it may be repeated that, in the main, the succession of life upon the globe in point of time has corresponded with the relative order of succession of the great groups of animals in zoological rank; and some of the more striking examples of this may be here alluded to. Amongst the Echinoderms, for instance, the two orders generally admitted to be the "lowest" in the zoological scale—namely, the Crinoids and the Cystoids—are likewise the oldest, both, appearing in the Cambrian, the former slowly dying out as we approach the Recent period, and the latter disappearing wholly before the close of the Palaeozoic period. Amongst the Crustaceans, the ancient groups of the Trilobites, Ostracodes, Phyllopods, Eurypterids, and Limuloids, some of which exist at the present day, are all "low" types; whereas the highly-organised Decapods do not make their appearance till near the close of the Palaeozoic epoch, and they do not become abundant till we reach Mesozoic times. Amongst the Mollusca, those Bivalves which possess breathing-tubes (the "siphonate" Bivalves) are generally admitted to be higher than those which are destitute of these organs (the "asiphonate" Bivalves); and the latter are especially characteristic of the Palaeozoic period, whilst the former abound in Mesozoic and Kainozoic formations. Similarly, the Univalves with breathing-tubes and a corresponding notch in the mouth of the shell ("siphonostomatous" Univalves) are regarded as higher in the scale than the round-mouthed vegetable-eating Sea-snails, in which no respiratory siphons exist ("holostomatous" Univalves); but the latter abound in the Palaeozoic rocks—whereas the former do not make their appearance till the Jurassic period, and their higher groups do not seem to have existed till the close of the Cretaceous. The Cephalopods, again—the highest of all the groups of Mollusca—are represented in the Palaeozoic rocks exclusively by Tetrabranchiate forms, which constitute the lowest of the two orders of this class; whereas the more highly specialised Dibranchiates do not make their appearance till the commencement of the Mesozoic. The Palaeozoic Tetrabranchiates, also, are of a much simpler type than the highly complex Ammonitidoe of the Mesozoic.

Similar facts are observable amongst the Vertebrate animals. The Fishes are the lowest class of Vertebrates, and they are the first to appear, their first certain occurrence being in the Upper Silurian; whilst, even if the Lower Silurian and Upper Cambrian "Conodonts" were shown to be the teeth of Fishes, there would still remain the enormously long periods of the Laurentian and Lower Cambrian, during which there were Invertebrates, but no Vertebrates. The Amphibians, the next class in zoological order, appears later than the Fishes, and is not represented till the Carboniferous; whilst its highest group (that of the Frogs and Toads) does not make its entrance upon the scene till Tertiary times are reached. The class of the Reptiles, again, the next in order, does not appear till the Permian, and therefore not till after Amphibians of very varied forms had been in existence for a protracted period. The Birds seem to be undoubtedly later than the Reptiles; but, owing to the uncertainty as to the exact point of their first appearance, it cannot be positively asserted that they preceded Mammals, as they should have done. Finally, the Mesozoic types of Mammals are mainly, if not exclusively, referable to the Marsupials, one of the lowest orders of the class; whilst the higher orders of the "Placental" Quadrupeds are not with certainty known to have existed prior to the commencement of the Tertiary period.

Facts of a very similar nature are offered by the succession of Plants upon the globe. Thus the vegetation of the Palaeozoic period consisted principally of the lowly-organised groups of the Cryptogamous or Flowerless plants. The Mesozoic formations, up to the Chalk, are especially characterised by the naked-seeded Flowering plants—the Conifers and the Cycads; whilst the higher groups of the Angiospermous Exogens and Monocotyledons characterise the Upper Cretaceous and Tertiary rocks.

Facts of the above nature—and they could be greatly multiplied—seem to point clearly to the existence of some law of progression, though we certainly are not yet in a position to formulate this law, or to indicate the precise manner in which it has operated. Two considerations, also, must not be overlooked. In the first place, there are various groups, some of them highly organised, which make their appearance at an extremely ancient date, but which continue throughout geological time almost unchanged, and certainly unprogressive. Many of these "persistent types" are known—such as various of the Foraminifera, the Linguloe, the Nautili, &c.; and they indicate that under given conditions, at present unknown to us, it is possible for a life-form to subsist for an almost indefinite period without any important modification of its structure. In the second place, whilst the facts above mentioned point to some general law of progression of the great zoological groups, it cannot be asserted that the primeval types of any given group are necessarily "lower," zoologically speaking, than their modern representatives. Nor does this seem to be at all necessary for the establishment of the law in question. It cannot be asserted, for example, that the Ganoid and Placoid Fishes of the Upper Silurian are in themselves less highly organised than their existing representatives; nor can it even be asserted that the Ganoid and Placoid orders are low groups of the class Pisces. On the contrary, they are high groups; but then it must be remembered that these are probably not really the first Fishes, and that if we meet with Fishes at some future time in the Lower Silurian or Cambrian, these may easily prove to be representatives of the lower orders of the class. This question cannot be further entered into here, as its discussion could be carried out to an almost unlimited length; but whilst there are facts pointing both ways, it appears that at present we are not justified in asserting that the earlier types of each group—so far as these are known to us, or really are without predecessors—are necessarily or invariably more "degraded" or "embryonic" in their structure than their more modern representatives.

It remains to consider very briefly how far Palaeontology supports the doctrine of "Evolution," as it is called; and this, too, is a question of almost infinite dimensions, which can but be glanced at here. Does Palaeontology teach us that the almost innumerable kinds of animals and plants which we know to have successively flourished upon the earth in past times were produced separately and wholly independently of each other, at successive periods? or does it point to the theory that a large number of these supposed distinct forms, have been in reality produced by the slow modification of a comparatively small number of primitive types? Upon the whole, it must be unhesitatingly replied that the evidence of Palaeontology is in favour of the view that the succession of life-forms upon the globe has been to a large extent regulated by some orderly and constantly-acting law of modification and evolution. Upon no other theory can we comprehend how the fauna of any given formation is more closely related to that of the formation next below in the series, and to that of the formation next above, than to that of any other series of deposits. Upon no other view can we comprehend why the Post-Tertiary Mammals of South America should consist principally of Edentates, Llamas, Tapirs, Peccaries, Platyrhine Monkeys, and other forms now characterising this continent; whilst those of Australia should be wholly referable to the order of Marsupials. On no other view can we explain the common occurrence of "intermediate" or "transitional" forms of life, filling in the gaps between groups now widely distinct.

On the other hand, there are facts which point clearly to the existence of some law other than that of evolution, and probably of a deeper and more far-reaching character. Upon no theory of evolution can we find a satisfactory explanation for the constant introduction throughout geological time of new forms of life, which do not appear to have been preceded by pre-existent allied types; The Graptolites and Trilobites have no known predecessors, and leave no known successors. The Insects appear suddenly in the Devonian, and the Arachnides and Myriapods in the Carboniferous, under well-differentiated and highly-specialised types. The Dibranchiate Cephalopods appear with equal apparent suddenness in the older Mesozoic deposits, and no known type of the Palaeozoic period can be pointed to as a possible ancestor. The Hippuritidoe of the Cretaceous burst into a varied life to all appearance almost immediately after their first introduction into existence. The wonderful Dicotyledonous flora of the Upper Cretaceous period similarly surprises us without any prophetic annunciation from the older Jurassic.

Many other instances could be given; but enough has been said to show that there is a good deal to be said on both sides, and that the problem is one environed with profound difficulties. One point only seems now to be universally conceded, and that is, that the record of life in past time is not interrupted by gaps other than those due to the necessary imperfections of the fossiliferous series, to the fact that many animals are incapable of preservation in a fossil condition, or to other causes of a like nature. All those who are entitled to speak on this head are agreed that the introduction of new and the destruction of old species have been slow and gradual processes, in no sense of the term "catastrophistic." Most are also willing to admit that "Evolution" has taken place in the past, to a greater or less extent, and that a greater or less number of so-called species of fossil animals are really the modified descendants of pre-existent forms. How this process of evolution has been effected, to what extent it has taken place, under what conditions and laws it has been carried out, and how far it may be regarded as merely auxiliary and supplemental to some deeper law of change and progress, are questions to which, in spite of the brilliant generalisations of Darwin, no satisfactory answer can as yet be given. In the successful solution of this problem—if soluble with the materials available to our hands—will lie the greatest triumph that Palaeontology can hope to attain; and there is reason to think that, thanks to the guiding-clue afforded by the genius of the author of the 'Origin of Species,' we are at least on the road to a sure, though it may be a far-distant, victory.