Fig. 45.
Fig.45.—Two stages in the development of the Graafian follicle.A, With the follicular fluid beginning to appear;B, after the space has largely increased.caps, Capsule;disc, cumulus proligerus;memb, membrana granulosa;ov, ovum;sp, space containing fluid. (After Hertwig.)
Fig. 46.
Fig.46.—Ovarian egg ofEchidna.b, Basilar membrane;fe, follicular epithelium;o, oil globules;vm, vitelline membrane;y1,y2, yolk. (Partly after Caldwell.)
The only ova which depart at all in structure from that above described are those of the Monotremata. The credit of thisdiscovery rests with Owen and with Professor Poulton, who pointed out in 1884,[35]that the ovum ofOrnithorhynchusis very large as compared with those of other Mammalia (6 mm. as against .2 mm.), that it is filled with yolk, and that it completely fills the follicle, being surrounded by two layers of follicular cells only. This latter fact was proved by Caldwell. Subsequently Gyldberg[36]and I[37]described the ovarian ovum ofEchidna, showing it to be identical with that ofOrnithorhynchus. Later still a more elaborate and beautifully illustrated paper was published by Caldwell[38]upon the early stages of development in the Monotremata and Marsupials, in which the ovum of the former was accurately described (see Fig. 46). In the particulars mentioned above, the ovum of the Monotremata is practically identical with that of the large-yolked ova of the Sauropsida.
Fig. 47.
Fig.47.—Lepus cuniculus.The anterior end of the vagina, with the right uterus, Fallopian tube, and ovary. (Nat. size.) Part of the ventral wall of the vagina is removed, and the proximal end of the left uterus is shown in longitudinal section,fl.t, Fallopian tube;fl.t′, its peritoneal aperture;l.ut, left uterus;l.ut′, left os uteri;ov, ovary;r.ut, right uterus;r.ut′, right os uteri;s, vaginal septum;va, vagina. (From Parker'sZootomy.)
It is the general rule among vertebrate animals that the ovaries are completely independent of the ducts which convey their products to the exterior. In certain fishes, however, there is an absolute continuity between the two structures, which is believed to be due to a simple concrescence between the originally distinct ovary and oviduct. The latter has grown round the former, an obvious advantage in preventing the eggs from wandering into the abdominal cavity and becoming lost. In the Mammalia we find discontinuity as a general rule. But in quite a number of forms folds of the lining membrane of the abdominal cavity are developed, which practically ensure the passage of the ova into the oviduct when they are extruded from the ovaries. The oviduct, moreover, has a large and fimbriated mouth, called in human anatomy—which is provided with a number of fanciful names—the morsus diaboli. This almost wraps round the ovary, and thus prevents the ova from straying in the wrong direction. Moreover, the ovary itself is often so arranged that it can easily be withdrawn into a pocket of the peritoneum, from which the obvious exit is by the gaping mouth of the oviduct. This disposition of the generative parts is still further modified in a few animals, such as the Rat[39]and the Kinkajou.[40]In these animals the mouth of the oviduct actually opens into the interior of a closed chamber which contains the ovary. In this case there is but one route for theextruded ova to follow. This series of steps in the perfecting of the mode of safe extrusion of the ova is highly interesting, and is a piece of evidence in favour of the high position of the mammals.
Fig. 48.
Fig.48.—Female urino-genital apparatus of various Marsupials.A,Didelphys dorsigera(young);B,Trichosurus;C,Phascolomys wombat.B, Urinary bladder;Cl, "cloaca";Fim, fimbriae;g, clitoris;N, kidney;Od, Fallopian tube;Ot, aperture of Fallopian tube;Ov, ovary;r, rectum;Sp, septum dividing vagina;Sug, urino-genital sinus;Ur, ureter;Ut, uterus;Ut′, opening of the uterus into the median vagina (VgB);Vg, lateral vagina;Vg′, its opening into the urino-genital sinus; † (inB), point of approximation of uteri; † (inC) and *, rectal glands. (From Wiedersheim'sComparative Anatomy.)
The oviducal apparatus of the mammal is more specialised than that of lower vertebrates. It is most simple, as might be imagined, in the egg-laying Monotremes, where, indeed, it is on the same level as that of reptiles. But in the Eutheria the fimbriated mouth of the oviduct passes into a narrow and winding tube, the Fallopian tube; this widens into a uterus, and the two uteri combine into a single tube in the higher forms. They are called the Monodelphia on this account. In the Marsupials the uteri are distinct though they often join above, and from this junction depends a median "uterus." After the uterus or the uteri follows in every case a single vagina.
The testes of the Mammalia, like those of other vertebrates, occupy primitively a position within the body cavity precisely corresponding to that of the ovaries. And in the lowly-organised Monotremata, and some other forms, such as the Whales, they retain that primitive position within the body. It is, however, distinctive of the Mammalia as opposed to lower vertebrates that the testes descend later into a scrotum, which is simply a protrusion of the skin of the body surrounded by muscles, and, of course, containing a section of the body cavity in which lie the testes. The penis of the Mammalia, represented by the clitoris and associated structures in the female, is of a structure entirely peculiar to this group.
Fig. 49.
Fig.49.—Brain of Dog.A, ventral;B, dorsal;C, lateral aspect.B.ol, Olfactory lobe;Cr.ce, crura cerebri;Fi.p, great longitudinal fissure;HH,HH1, lateral lobes of cerebellum;Hyp, hypophysis;Med, spinal cord;NH, medulla oblongata;Po, pons Varolii;VH, cerebral hemispheres;Wu, middle lobe (vermis) of cerebellum;I-XII, cerebral nerves. (From Wiedersheim'sComparative Anatomy.)
The Brain.—Inasmuch as Professor Wiedersheim has said with perfect truth that "the brain of the extinct UngulateDinocerasshows so striking a likeness to that of a lizard that one would be compelled to explain it as that of a lizard without a knowledge of the skeleton," it is clear that to define the mammalian brain is a difficult matter. The existing Mammalia, however, all possess brains which can be readily distinguished from those of vertebrates lying lower in the scale. They are of relatively large size, brought about mainly by the dimensions of the cerebral hemispheres, which have an importance in this class of vertebrates that they have not elsewhere. Coupled with this large size of the hemispheres is a more elaborate system of transverse commissures uniting the two; and this culminates in the higher Mammalia, where the corpus callosum attains a large size and great physiological importance. Avery marked feature, moreover, of the mammal's brain is the development of regular fissures upon its surface, which fissures are only absent fromOrnithorhynchus, various small Rodents, Bats, and Insectivores, among living mammals. It is sometimes, but erroneously, said that the more complicated the fissures of the brain are, the higher in intelligence and "zoological position" is the possessor of that brain. Instances can undoubtedly be quoted to support such a view; but they aremerely selected cases, which do not indicate a wide applicability of such a generalisation. Thus it is true that the brain of a Man is more elaborate in its furrows and convolutions than is that of a Cat. The real fact of the matter is, that the complexity of the brain from this point of view increases with the size of the animal within the group.
Fig. 50.
Fig.50.—Lepus cuniculus.Longitudinal vertical section of the brain. (Nat. size.)a.co, Anterior commissure;b.fo, body of the fornix;cb, cerebellum, showing arbor vitae;c.c, crus cerebri;c.h1, parencephalon or cerebral hemisphere;c.h2, temporal lobe;c.ma, corpus mammillare;cp.cl, corpus callosum;f.m, foramen of Monro;inf, infundibulum;l.t, lamina terminalis;ly, lyra;m.co, middle commissure;m.o, medulla oblongata;o.ch, optic chiasma;o.l1,o.l2, corpora quadrigemina or optic lobes;olf, olfactory lobe;p.co, posterior commissure;pd.pn, peduncle of the pineal "gland,"pn;p.fo, anterior pillar of the fornix;pty, pituitary body;pv.a, pons Varolii;sp.lu, septum lucidum;v4, fourth ventricle;vl.ip, velum interpositum;v.vn, valve of Vicussens;II, optic nerve. (From Parker'sZootomy.)
The Gorilla and the Chimpanzee have a more furrowed brain than has the little Marmoset; the Bear a more complicated brain than the Weasel, etc. The most highly-convoluted brains of all mammals are those of the Elephants, and there does not seem in the Ungulates to be so marked a relation between size and abundance of fissures as there is among other mammals. A regular plan of the fissures can be detected with certainty for each group considered by itself; but it is not so easy to homologise the details of arrangement from group to group. This is so far in accord with the view that the existing groups of mammals have diverged from each otherab initio.
Another marked characteristic of the mammalian as opposed to other brains is the relatively small importance in size and yet the fourfold nature of the optic lobes. What was the case with the optic lobes of the early Ungulates is difficult to understand, on account of the fact that the casts are necessarily imperfect.Altogether the enormous progress in the complexity of the brain from the early Tertiary mammals down to the present, is one of the most remarkable revelations of palaeontology. It goes perhaps some way in explaining the remarkable diversity in mode of life exhibited by the mammals as compared, for example, with the birds, whose brains have not diverged so much or in so many directions from the primitive form.
The present Distribution of the Mammalia.—In the following pages some of the principal facts in the geographical range of the orders, families, and many of the genera of Mammalia will be given. It has been justly observed by Mr. Sclater that the habitat of an animal is as much a part of its definition as is its structure or external form. No systematic account of the Mammalia would therefore be complete without such geographical facts. But that branch of zoology which is concerned with the past and present distribution of animals is wider in scope than this. Zoogeography deals not only with the actual facts in the range of animals, but with the inferences as to past changes in the relations of land and sea which the facts seem to indicate, and with speculations as to the place of origin of the different groups, of which more than hints are sometimes given by their past and present distribution. In addition to this, the earth can be mapped out into provinces and regions which are definable by their animal inhabitants. In the present volume, dealing only with the Mammalia, it will be obviously impossible to enter fully into the entire subject of zoogeography. All that will be attempted is a brief general survey of the science so far as it can be illustrated by the Mammalia. For fuller knowledge the reader is referred to the treatises mentioned below.[41]
There are certain facts in the distribution of animals which are commonplaces of knowledge, but which may be set forth with definiteness. Everybody knows that an animal has a given range: Elephants, for example, are found in India and certain adjacent parts of Asia, and again in Africa; the Rhinoceroses have roughly the same range; the Tiger is limited to Asia; theJaguar to America, and so forth. The entire expanse of country which is inhabited by an animal is called its area of distribution. Such areas are larger or smaller. The Lion ranges over the whole of Africa, a small part of India, and some neighbouring countries; on the other hand, the InsectivoreSolenodonis limited to Cuba and Hayti, a separate species to each. Among other groups of animals are instances of an even more restricted range. There are humming-birds confined to the slopes of a single mountain, and fishes limited in their range to a single small lake.
A species may be found everywhere within the area of its distribution, or it may be confined to a number of limited tracts within that area. In this case it is usual to speak of "stations." In such cases the species in question is generally suited to some particular kind of environment. Thus the Otter and other aquatic mammals will only be found where there is water; and intervening tracts of waterless country will contain no Otters. Goats and Chamois live only upon mountains; the intervening plains are destitute of them. This discontinuity of distribution within the area is very general. But a discontinuity of area is also seen—not so commonly however; and, indeed, when it does occur, it is a matter of a genus and not of a species. Thus the Tapir is found in the East Indies on the one hand and in South and Central America on the other, being absent in the intermediate tracts.
It is clear that tracts of country eminently suitable for the housing of a particular mammal do not always possess that kind, or even an allied form. Africa, for example, possesses no arboreal Anteaters; there are no Anteaters at all (of the order Edentata) in Australia, though there are plenty of ants for them to feed upon, and tropical conditions of climate prevail. But as in these cases the inference may be denied on the grounds that no experiments exist to prove or to disprove the assertion, the matter may be better emphasised by such cases as the introduction of the Rabbit into Australia, and various mammals, such as Goats, into oceanic islands. The plague caused by the former is a matter of notoriety. But although climate and conditions and animal inhabitants do not march accurately together, there is certainly some connexion between temperature and the range of animals. Mr. Lydekker writes on this point as follows: "The llama-like animals, respectively known as vicunas and guanacos, are met with incompany on the highlands of the Cordillera in Peru and Ecuador, but as we go farther south the latter are found on the plains of southern Argentina and Patagonia, as well as on the island of Tierra del Fuego at the sea level. Here then is a clear proof of the intimate connexion existing between temperature and station; the guanaco being an animal which can only live in cold or temperate climates, finds suitable conditions for its existence in tropical latitudes solely at a height of so many thousands of feet, although farther south it is able to thrive at the sea level." This, however, cannot be pushed too far—the world cannot be mapped out into areas bounded by parallels of temperature as was once attempted—since there are plenty of cases like that of the Tiger, which is as much at home in a tropical jungle as on the icy plains of Northern Asia.
Seeing that there are in many cases no climatic barriers to the spreading of a given race of animals over a larger area of distribution than it actually occupies, it becomes important to inquire why there are so many cases of restriction in range.
It is possible to see, at any rate, three causes which are responsible for a large number of such cases. In the first place, a given species of animal must have originated at a certain spot; its multiplication in individuals must always be a slow matter, since enemies, and untoward events generally, would conspire to check the natural multiplication by geometrical progression. A long time might therefore elapse before the species greatly extended its range. A restricted distribution may therefore, in some cases, mean a modern race. In the second place, there are definite physical barriers which check the migration of species. The terrestrial Mammalia cannot cross wide arms of the sea; that they can and do swim for considerable distances has been proved in several instances; but, as has been pointed out, it is unlikely that a purely terrestrial mammal would voluntarily swim out into an unknown sea. And then if it did, and successfully reached the opposite side, nothing would happen unless it were a pregnant female; or, if not pregnant, till a male swam very soon afterwards in exactly the same direction. Many travellers have told of floating islands, formed of torn-up trees and brushwood, which have been seen at the mouths of large rivers, with animal passengers upon them. These are, however, so much at the mercy of currents and storms,that but little reliance can be placed on them as a means of transit; besides, here again, two individuals, or a pregnant female, would be required to effect a settlement on a foreign shore. The existence of oceanic islands is often urged as a proof of this inability to cross tracts of sea; even those which are comparatively near an extensive continent, such as, for example, Fernando Noronha in the Atlantic, are destitute of mammals (except, indeed, the ubiquitous Mouse, which is believed to have been carried there, often in company with the equally widely-spread Rat, in ships). This argument, however, is not so conclusive as might appear; it doubtless is in the case of far-distant islands. But the size of the islands has to be taken into account. For there are islands, such as the Galapagos, or, to take a less contested instance, some of the islands of the Malagasy Archipelago, undoubtedly continental, which have an exceedingly reduced number of mammals. An area of a certain size seems to be a necessity.
The converse of this is in many cases easy to show, that is, the wide range of animals when there are no marine barriers to stop their spreading. John Hunter, the celebrated anatomist and surgeon (not often quoted, however, as an authority upon geographical distribution), observes: "It is a curious circumstance in the natural history of animals to find most of the northern animals the same both on the continent of America and what is called the Old World, while those of the warmer parts of both continents are not so. Thus we find the bear, fox, wolf, elk, reindeer, ptarmigan, etc., in the northern parts of both.... The reason why the same animals are to be found in the northern parts is the nearness of the two continents. They are so near as to be within the power of accident to bring the animals, especially the large ones, from one continent to the other either on the ice or even by water. But the continents diverging from each other southward, so as to be at a very considerable distance from each other even beyond the flight of birds, is the reason why the quadrupeds are not the same."
There is no doubt, in fact, that the ocean is the most insuperable of all barriers to the dispersal of mammals. In a less degree mountain ranges and deserts are also barriers. The Desert of Sahara is a striking instance to the point; it separates two exceedingly different faunas.
A third cause of more or less limited range is the barrier due to competition. If the ground is already taken up, there is no room for new immigrants. There is obviously a limit to the number of Antelopes or Deer that can graze upon a given tract of grassy plain. These two groups of Ungulates illustrate the matter well: the Antelopes are African and Indian, especially the former, while Africa has no Deer at all; America, on the other hand, has plenty of Deer but no Antelopes, save the Prong-horn. The more nearly akin the two species or groups of species are, the fiercer will be the competition; for a near kinship will at least often imply similar habits, the need for similar food, and other likenesses which will prevent both from successfully occupying the same tract of country. The remarkable fauna of Australia is believed to afford an example of this. In that country the prevalent inhabitants are the Marsupials. The Monotremes are found there also, and nowhere else save in New Guinea and Tasmania. The remaining mammals are inconspicuous; they embrace a few Rodents and Bats, and the doubtfully indigenous Dingo-dog. Now the Marsupials are fitted to every variety of life. We have the grazing Kangaroos and Wallabies, the burrowing Wombats, the arboreal Phalangers, and the carnivorous Dasyures. In the second place, it is an unquestioned fact that the Marsupials are an older race than are the existing Eutherian mammals; they were the dominant mammals during the Secondary epoch. At that time they were more widely distributed than at present. In most parts of the world they are now absent, since they have been successfully ousted by the more highly organised groups of Eutheria. But at that period, when the higher Eutheria were in the ascendant, Australia and the islands to the north became cut off from Asia, and thus became freed from inroads of Eutheria, which were partly prevented by the physical barrier of the sea from effecting a settlement, and partly perhaps prevented owing to the ground being already taken up by the Marsupials. Likeness of habit gave the older inhabitants victory in the struggle for existence.
The general statements that have been here made are in accord with current opinion upon the factors of geographical distribution. But the past range of animals appears to be less consonant with the received views. In the Tertiaryperiod, groups of animals had often a far wider range than at present. To-day the Rhinoceroses are limited to Asia and Africa, and to quite limited parts of the former continent. In the past, these animals were abundant in Europe and North America. Wild Horses now have a range which is not widely different from that of the Rhinoceroses, save that they extend into the more northern regions of Asia. Their remains are abundant both in North and South America. The Hippopotamus, now confined to Africa, once ranged over Europe, Madagascar, and India. There were plenty of American and European Lemurs. Elephants were nearly world-wide in their range; and, in short, restricted distribution seems to be on the whole a characteristic of animals of the present day.
These statements, however, though perfectly true, must not lead to erroneous inferences. It is rather impressed upon the reader, in books which contain sections dealing with geographical distribution, that animals on the whole occupy more restricted areas at present than in the past. There are, however, plenty of examples of groups of extinct creatures which had, so far as we know, quite a restricted range. Thus the Toxodonts were purely South American, as were the Glyptodonts and some other forms. And, on the other hand, the Cervidae of to-day are as widely, if not more widely, distributed than at any other time. The Hares and Rabbits are now nearly universal in range; the Cats almost so. We meet with Bovidae, even excluding the Sheep and Goats, in all the four quarters of the globe, excluding only South America and, of course, Australia. The Camelidae are still common to both the Old and the New Worlds.
During certain periods of the Tertiary epoch it is true that there was more similarity between Europe and North America than there is at present. It would have been quite necessary to unite both into a Holarctic area, such as is now insisted upon by many; but the reasons for this union would then have been stronger. The fact is, however, that the closer resemblances were due to the larger number of families of animals which existed then than now; these have decayed away from both continents, and allowed the unlikenesses between the mammalian fauna of both to become evident. But the likenesses which still survive have led many to associate the two regions closely together.
So far as the history of a genus or family or larger divisioncan be traced, it results as a conclusion that from a given area of origin the group in question migrated in all directions where possible to a varying degree; it then died out in intervening tracts, or was left only in a certain part of its former and more extensive area of range.
Zoological Regions.—Seeing that each species of animal has its own definite range, it is clear that the earth's surface can be apportioned into divisions which are characterised by their animal inhabitants. We shall divide the earth into realms, which are the largest divisions; then into regions; and finally into subregions. It must be borne in mind that the various groups of the animal kingdom are of different ages, geologically speaking, and have therefore had less or more time, as the case may be, to settle down into their present distribution, and that different animals differ greatly in their rate of multiplication, their power of migration, and their susceptibility to the effectiveness of various natural and other barriers to distribution. It is not, therefore, possible to divide the world into realms and regions which shall express the facts of distribution of the entire animal kingdom. Such divisions, which are common in text-books of zoology having but a small section devoted to zoogeography, are at best mere approximations and averages; no good is gained by taking such a comprehensive view of the matter, as the essential object of subdividing the earth's surface is thereby lost sight of. The zoogeographical division of the earth which will be adopted here is that originally recommended by Dr. Blanford, and now accepted by a number of authorities. There are three "realms," to which a fourth may perhaps be added—though on negative grounds, and merely for the purpose of emphasising the parts of the world to which mammals have not gained access. The realms are again divisible into regions, at least in the case of one of them, and the regions may be again separated into more or less distinct subregions or provinces. The three primary divisions or realms which contain mammals are the Notogaean, including Australia and certain islands to the north of it; the Neogaean, or the South American continent and Central America; the Arctogaean, including the continents of North America, Europe, Asia, and Africa, together with the adjacent islands, such as the West Indies, East Indies (exclusive of those which fall withinthe realm of Notogaea), and Madagascar; and finally, the realm of Antarctogaea or Atheriogaea, which embraces New Zealand, the Antarctic continent, and a series of islands such as South Georgia and Kerguelen, and possibly even the extreme south of Patagonia. This latter quarter of the globe will need no further reference, as it has no truly indigenous terrestrial mammalian inhabitants. We cannot include the Bats in this statement, as their distribution is due to different powers of extending their range, and to different barriers from those which govern the range of other groups of mammals.
(1) Notogaea.[42]This realm is characterised by the exclusive possession of the Monotremes:—that is to say, one of the two primary divisions of the Mammalia is absolutely restricted to this area. It contains, moreover, the vast majority of the Marsupials. Further, the realm of Notogaea is to be distinguished by the entire absence of the higher mammals, with the exception of a few small Rodents. (The Bats are ignored for the reasons stated, and the Dingo is believed to have been an importation.) It cannot be disputed that this is a very distinctly-marked area of the earth's surface.
(2) Neogaea. The continent of South America has no Monotremes and only a few Marsupials, all of which, with the exception ofCaenolestes, belong to the Polyprotodont division of that order, and to a peculiar family, Didelphyidae. The recent discovery of other fossil Marsupials, however, to some extent favours Huxley's view that Neogaea and Notogaea form one realm as opposed to the rest of the world. Besides this, Neogaea possesses the Edentata, which are found nowhere else;—that is, the division of the Edentata to which the name is now restricted by some authorities. It is also characterised by the nearly entire absence of the important order of Insectivora; and, as minor marks of distinction, by the absence of Antelopes, Oxen and Sheep, of the Ichneumon tribe, of Horses, and of Lemurs. It has the exclusive possession of the Hapalidae and Cebidae, and of several families of Rodents.
(3) Arctogaea. This vast realm is clearly capable of subdivision into four regions, which will be considered in detail later. In the meantime the points of likeness between these subdivisions is more marked than are either the resemblances or thedifferences of any one of them to either of the two realms which have just been defined. The two realms that have been discussed retain their distinctness from each other and from Arctogaea for a considerable way back into the Tertiary period. It is not until we reach very early Tertiary times that Edentates are met with in North America; and then it cannot be regarded as absolutely settled that the Ganodonta are really the forerunners of the Armadillos, Sloths, etc. Nor do we find Marsupials in Europe until far back in time, and at a corresponding period in North America. Indeed the fauna of South America in late Tertiary times was even more distinct than it is now; for then we had confined to that region the Toxodonts, Glyptodonts,Macrauchenia, and other forms, while in Australia there were still Marsupials. In late Tertiary times Europe and India were by no means so distinct from Africa as they are to-day. North America does not resemble the Old World quite so much as the subdivisions of the Old World resemble each other; but, as will be pointed out later, there are and were very substantial agreements. The Elephants, Rhinoceroses, Giraffe, Hippopotamus,Orycteropus, are now distinctively African or Indian animals; but all these genera, or at least families (in the case of the Giraffe), have occurred in Europe during quite recent times.Lycaonindeed, now confined to Africa, is thought to have had a European origin from its occurrence in caves there. The Hyaena and the Lion, certain members of the Horse tribe, Apes, and other animals, were also but are not now European.
India again, and the Oriental region generally, once possessed the Hippopotamus, the Chimpanzee, Giraffidae, the Antelopes,Cobus,Hippotragus,Strepsiceros, andOrias, which are now purely African animals. It shares at present with the Ethiopian region the Catarhines, including the Anthropoid Apes, the Lemurs, Tragulina (the genusDorcatheriumis also known from fossils in India),Manis,Hyaena, the Cheetah, Elephant, Rhinoceros, and the Ratel. There is, in fact, no order of mammals which is now absent from one of these three regions though present in the others, save the Lemurs, and they occurred in past times in Europe. The Tapir of India is known fossil in Europe, and the latter continent had its Monkeys and even Anthropoids. On the other hand, North America is more distinct. It has no Lemurs, Apes, Elephants, Rhinoceroses, Tapirs, Old World Edentates(Effodientia), Viverridae, Horses, or Antelopes, exceptingAntilocapra, a type of a separate division of Bovidae. But since several of these groups have been represented in recent times, no primary line of division can be profitably drawn.
Arctogaea as a whole may be characterised by both negative and positive characters. As negative features may be mentioned;—the entire absence of Edentates (Necrodasypusof Filhol is rather doubtful, see p.164, n.), though a few crept up into the Nearctic region from Neogaea during past times; and of Hapalidae, Cebidae, and Marsupials, except an Opossum in North America. This realm has, on the other hand, all the Lemurs, all the Insectivores with the exception of the West IndianSolenodon, all the Proboscidea, Rhinoceroses, Horses, Deer, Antelopes, the last group including the Oxen and a variety of other important families. It is in fact the headquarters of all the Eutheria with the exception of the Edentata and Marsupials.
The subdivisions of this realm have been variously effected. The classical subdivisions are of course those of Mr. Sclater, who would recognise (1) the Nearctic, North America; (2) the Palaearctic, including Europe, Northern Asia, and Japan; (3) the Oriental, including Asia south of the Himalayas and the islands of the Malay Archipelago as far east as the Australian region; and (4) the Ethiopian,i.e.tropical Africa and Madagascar. Some would alter this by uniting America and the north of the Old World into a Holarctic region, separating off the southern parts of the North American continent into a Sonoran region. To some, the claims of Madagascar to form a separate region are convincing. To distinguish the boundaries of the several regions is a difficult task; they dovetail into each other on the frontiers with the complex curves of a puzzle-map. The difficulty has been grappled with by the suggestion of intermediate transitional areas; but this proceeding really doubles the difficulty, for there are then two frontiers to delimit in each case instead of only one. The animal inhabitants must be expected to mingle somewhat at the lines of junction of one region with another.
The Sonoran region does not appear to us to have great claims to recognition. It shows a mingling of southern with northern forms exactly as might be expected. An Armadillo andDidelphyshave, as it is believed, invaded it from the Neogaeic realm; it possesses also the South American genera,Dicotyles,Nasua,Conepatus,Sigmodon. On the other hand, the Sonoran generaAntilocapra,Cynomys,Procyon, and the InsectivoraBlarinaandScapanus, extend further north. Peculiar to this region are only six genera of Rodents, which seems an insufficient reason for raising the Sonoran province to the dignity of a region. Considered from the point of view of numbers of peculiar forms, the Thibetan subregion has more claims to distinction as a region; for confined to that area we have the generaNectogale,Aeluropus,Eupetaurus,Pantholops,Budorcas; while by slightly extending its limits, a number of other peculiar forms might be added. Madagascar has distinctly more claims to regional division. Absolutely confined to it are eleven of the seventeen existing genera of Lemurs, the family Centetidae among the Insectivora, which contains seven genera, and another recently discovered and peculiar genus,Geogale; it has six peculiar genera of Viverridae; it has five peculiar genera of Rodents. In addition to this it is negatively characterised by the absence of the following typical African animals, Felidae, Proboscidea, Rhinocerotidae, Equidae, Monkeys, etc. It seems to be impossible to avoid allowing the rank of a region to this part of the world.
In separating the Nearctic from the Palaearctic region, stress must be laid rather upon the absence of Asiatic and European forms from North America than upon the existence in the northern half of the New World of many peculiar forms. Peculiar to the Nearctic are the Goat genusHaploceros, the RodentsErethizon,Zapus, and the family Haplodontidae. The Mole genusCondylurais also restricted to this part of the New World. Even so it has more peculiar forms than the Sonoran. If we add to this the absence of Horses, Antelopes exceptAntilocapra, Pigs, Hyaenas, etc., there are strong grounds for retaining this division. It must be agreed, however, that it comes rather nearer to the Eurasian district than the latter does to the Oriental.
The Oriental region has many characteristic animals. It has among the Anthropoid Apes the Orangs and Gibbons; of Old World Apes it has confined to its own area the generaSemnopithecusandNasalis. Of Lemurs there areLorisandNycticebus, andTarsius, representing a family of that order, or even a sub-order. The Galeopithecidae are entirely Malayan. There are many Rodent, Carnivorous, and Insectivorous genera; the Rhinoceroses and the Elephant of this region differ from those of Africa.Tragulusconcludes a sample from a very rich list of peculiar forms.
The Ethiopian region has also its Anthropoids, the Gorilla and the Chimpanzee, but they belong to genera or a genus different from those which include the Oriental forms. There are five peculiar genera of Cercopithecidae. The Lemurs restricted to this region areGalago,PerodicticusandArctocebus. The peculiar Insectivorous families Macroscelidae and Chrysochloridae are only found here, besides many other peculiar genera. Africa is especially the home of Antelopes, and the Giraffe is not found now outside its borders. The Elephant and the Rhinoceroses are of different species from those of India. There are many peculiar Rodents and Ungulates.
The relationship of Mammals to Vertebrates lying below them in the scale, their origin in fact, is a much-debated question, with many attempted solutions. To enter into this large question in detail would involve a great deal of useless statement of arguments founded upon misleading or upon quite inaccurate "facts." It will perhaps be sufficient if we reflect here the current view most in vogue at the present,i.e.that which would refer the Mammalia to reptiles belonging to the extinct Permian and Triassic group of the Theromorpha (also called Anomodontia). These have been explored lately to a very large extent, and chiefly by Professor Seeley.[43]The very fact that a genusTritylodon, only known by the forepart of the skull, has been called Mammalian and Anomodont by various authors, shows at least the difficulty of differentiating the two groups when the material for study is imperfect. As a matter of fact these Theromorpha are without doubt reptiles; they show, for example, a lower jaw formed out of several distinct pieces, of which the articular articulates with a fixed quadrate on the skull. They possess the characteristic reptilian bones, the "transverse," the pre- and post-frontals, and there are various other points of structure which leave no room for doubt as to their truly reptilian nature. There are, however, numerous indications of an evolution in the mammalian direction in all parts of the skeleton, to the more important of which some reference will be made here. It may be as well to clear theground by mentioning the fact that among the Theromorpha four distinct types of reptiles are included, which are considered to form four orders,i.e.the Pareiasauri, the Theriodontia, the Anomodontia (Dicynodontia), and the Placodontia.
The first of these divisions includes what seem to be basal forms. These reptiles show numerous points of likeness to the Amphibian Labyrinthodonts.[44]On the other hand the third division, that of the Dicynodontia, are highly-specialised Theromorpha, from which no further evolution would appear to have been possible. Thus the dentition was either completely lost, or reduced to tusks as inDicynodon. We need not therefore concern ourselves in the present volume with these Anomodonts. It is with the Theriodonts that our business lies. The very name, be it observed, is aptly chosen on the hypothesis to be explained here; but it is not only in the teeth that these reptiles show likenesses to the Theria or Mammals, but in almost every feature of their organisation. Unlike other reptiles, the Theromorpha in general were lifted comparatively high above the ground on legs of fair length and of mammalian relationship in the position of the segments of the limbs. The typical reptile grovels upon the earth with legs sprawling out, as indeed the very name suggests. One bar to the Theriodonts being on the direct line of mammalian ancestry has been urged as a preliminary difficulty, and that is their large size. The earliest undoubted mammals were small creatures, comparable to a Rat or a Mouse in size; whereas a good-sized Bear or a Wolf is a better standard of size for some of the best-known genera of Theriodonts. It has, however, been quite permissibly suggested that living in company with these large Theriodonts were less obtrusive genera, from which the mammals might have sprung. It is so familiar a fact that a given group of animals generally contains giants, dwarfs, and members of intermediate size, that this suggestion may almost be accepted as a fact. It need at least present no difficulties to us in our comparisons.
The most salient "mammalian" feature of the Theriodonts is the heterodonty of the teeth, the pattern of the "molars," and the limited number which constitute the series. The fact, too, that they are limited to the dentary bones below and to themaxillae and the premaxillae above, is asine qua nonfor mammalian comparison. In the more basal Theromorpha the teeth are not so limited in position. Finally, to complete the remarkable mammalian resemblance of the teeth of these reptiles, it must be mentioned that inTritylodonandDiademodonthe roots of the molars, as we may fairly term them, though not actually divided after the mammalian fashion, were deeply marked by a groove, which suggests an incipient division or a fusion of two distinct roots. Some of these facts of structure may now be considered in further detail. As to the incisors and canines, it is sufficient to say that the numbers of the former, and the shape of the latter, are in perfect consonance with a derivation of the Mammalia from this group. The molar series can be divided into premolars and molars, at least in so far as regards their shape; for the anterior teeth are often smaller and less complicated than those which follow, as is often the case with the two series in mammals. The molar series also consist of teeth in close apposition to each other and separated from the canines by a diastema, which is a character of mammalian teeth. The fact that in the reptileCynognathusand the mammalMyrmecobiusthere are nine of these molar teeth in each half of each jaw is perhaps not a point upon which it is desirable to dwell with too much weight; but the general fact that the molars are further reduced in some genera of Theriodontia than in that which has been mentioned, is clearly a matter of significance when the ancestry of the mammals is under consideration.
The most interesting fact about the molar series in the Theriodontia is that we meet with the two types of molars that occur in the mammals.Cynognathusand other genera have molars which consist of a main cusp, and of one cusp before and one after the main cusp; in fact these teeth are triconodont as in certain early mammals, a state of affairs which is believed by the "trituberculists" (see p.56) to have preceded the tritubercular tooth. There are also "multitubercular" teeth, especially well developed inTritylodon, where they exactly resemble those of certain Multituberculata, and whose structure originally led to the placing ofTritylodonamong the mammals of that group. If there is any question about the mammalian nature of this fossil, there remain several other Theriodontia in which the multituberculism is well marked. It is so inTrirhackodonand inDiademodonfor instance. This incidentally lends some support to the idea that the Mammalia have been evolved from two sources, a way of looking at the origin of the group that will coincide with the views of some authors like the late Dr. Mivart, and will at the same time reconcile the trituberculists and the multituberculists. For we should then assume that the Eutheria and Triconodontia had originated from some such form asCynognathus; and the Multituberculata and the existing Monotremes from some form likeDiademodon. It is not of great use to point out thatDiademodonis really of the trituberculate pattern, because in its molars, though multituberculate, the trituberculate main cones can be recognised; for that state of affairs could just as well have been brought about by a reduction from the multituberculate type. The skull of these Theriodonts shows some well-marked approximations to the mammalian type. There is in the first place a commencing consolidation and reduction of the individual bones, which is so distinguishing a feature of the mammalian skull as opposed to the skull of lower vertebrates. InCynognathusthe postorbital is fused with the jugal, and the supratemporal with the squamosal, forming apparently one bone. In the lower jaw the splenial is often reduced to the thinness of paper, thus indicating a commencing disappearance. In many Theromorpha the squamosal shares largely in the formation of the articular facet for the lower jaw, obviously an important mammalian characteristic; this is brought about by the reduction of the quadrate, which latter bone, moreover, acquires in certain particulars the appearance of the mammalian malleus, with which it is, according to many, homologous. But this subject has been already dealt with on page26. A very pronounced likeness to the mammalian skull is that there are two occipital condyles. That this has been brought about by the further development of a tripartite condyle such as occurs in tortoises, and that by the suppression of the basi-occipital part, does not affect the resemblance to the mammalian skull; in fact it explains the origin of two condyles from the typical reptilian single condyle, and disposes of the necessity for believing, with Huxley and others, the Amphibia to be on the main line of mammalian evolution on account of their two condyles. The general aspect of the skull inCynognathushas beencompared to that "ofThylacinusorDissacus." No one can examine the actual sketches of the skull of that Theriodont without endorsing that opinion. As a curious detailed point of likeness to certain Mammalia may be mentioned "a small descending process of the malar bone, which may be a diminutive representative of the descending element of the malar seen inElotherium,Nototherium,Diprotodon,Macropus, certain Edentata, such asGlyptodon,Megatherium,Mylodon,Bradypus, but unparalleled so far as I am aware in fossil reptiles." (Osborn.) The zoologist cannot help being impressed with the significance of small details of similarity, which do not seem to be due in any way to surrounding conditions of life, and thus referable to mere convergence, like the fish-like form of Whales and Seals.
The rest of the skeleton of the Theriodontia is by no means so well known as the skull and teeth. But from what is known, other mammalian characters can be pointed out. Perhaps the most striking mammalian feature is to be found in the scapula ofCynognathus. It is in this creature somewhat narrow and elongated; but it has a well-marked spine, ending in a hooked acromion. Now it is to be noted in support, so far, of the diphyletic origin of mammals, that in the Monotreme, as in Whales indeed, the spine forms the anterior border of the scapula, and is coincident with it, there being thus no prescapula at all in the Monotreme, and only a trace of it in certain Whales.[45]Whether the multituberculateTritylodonorDiademodonhad a scapula after the Monotreme pattern is not known; but it is clear that the scapula of the triconodontCynognathusis quite after the pattern of the Eutherian scapula. Furthermore, Professor Seeley is of opinion that the coracoid was relatively small, and indeed smaller than the same bone in Edentates, anda fortiorithan in Monotremes. Another fact of structure which points also, possibly, in the direction of a diphyletic origin for the Mammalia, is the double-headed ribs ofCynognathus. As is well known, the ribs of the Monotremata have only the central head, the capitulum.
As a general mark of affinity with mammals the reduction of the intercentra inCynognathusmay be noted, and also the existence of a small though perfectly obvious obturator-foramen, separating the pubis from the ischium. There are further detailswhich tend in the same direction. And we shall probably not go far wrong in the present state of our knowledge if we assign the origin of the mammals to some type which would be included in the order Theriodontia or at least in the sub-class Theromorpha.
The animals that we considered in the last chapter, though showing certain unmistakable likenesses to the mammals, are nevertheless unquestionably not mammals but reptiles. In the Triassic strata, however, we first meet with the remains of undoubted mammals. The Mammalia first appeared upon the earth in a tentative and hesitating way: they had not cast off many of the characters of their supposed reptilian forefathers; they shrank from observation and destruction by their small size, and apparently, so far at any rate as their teeth afford a clue, by an omnivorous diet. The world abounded at that period in large and carnivorous reptiles, which may indeed have been the principal enemies with which the first mammals had to cope. These early mammals lingered on to so late a period as the Eocene; but the majority of the genera were Triassic, Jurassic, and Cretaceous. Certain of the primitive mammalian forms have been referred to the Marsupials, and their resemblances to the Monotremata have also been pointed out. The current view of the present time, however, is that they form a special order, which may possibly have embraced the ancestors of both Marsupials and Monotremes; for it is reasonable to explain in this way the combination of characters of these two orders which they present. For this group the name Allotheria has been proposed by Marsh, and Multituberculata by Cope; the latter term is the less suitable, in that the Monotremata (Ornithorhynchus) are also "multituberculate." The group is known in a very imperfect fashion. The remains are but few and fragmentary; and for the most part we have only a few teeth to speculate upon. This is natural enough, for the harder teeth might easily be supposed tohave resisted the decay which would more readily affect the softer bones. Where there are bones it is frequently the lower jaw alone which has been preserved for us—a bone which has also been preserved in the case of some of the contemporary Marsupials.
It has been pointed out (from the observation of dead dogs floating in canals) that the lower jaw is occasionally detached from the carcase. It is the most readily separable part which contains a skeleton. It may be, therefore, that the remains of these early mammals, floating down some river to the sea, may have lost their jaws while in the river, or at furthest in the shallow waters of the sea, the rest of the carcase floating out to a greater distance, and being finally entombed in the stomach of some carnivorous fish, or in the mud at the bottom of a deep ocean, which has never since seen the light.
The characters of this group are really more those of the Monotremata than of the Marsupials. The undoubted likeness which their molar teeth show to the temporary teeth of the Platypus have already been commented upon. Like the Monotremes the Allotheria appear to have possessed a large and independent coracoid; the evidence for this rests upon the discovery of the lower end of a scapula ofCamptomus, a Cretaceous genus from North America upon which there is a distinct facet for the articulation of what can have been nothing else than a coracoid. On the other hand they differ from the Monotremata by the presence of incisor teeth which were Rodent-like in form, and not very different from those of certain Marsupials. This point of difference cannot be regarded as of very first-rate importance; no one would relegate the Sloth and the Armadillo to different orders on account of their tooth differences, which are about on a par with those to which we have just referred. It seems indeed likely that it will be ultimately necessary to rub out the boundary line which now divides the Allotheria and the Monotremata.
The Plagiaulacidae are unquestionably mammals, and they are placed by most naturalists in this at present uncertain group of Multituberculata, which will be retained here in deference to the distinguished authorities who have instituted the group, though there are but few characters by which it can be defined. This family though appearing in the Trias, extends down in time to the Eocene. The type-genus, that which has given its name tothe family, isPlagiaulax. As it is not Triassic, the consideration of its characters will be deferred until later.Microlestesis a Rhaetic genus, known from rocks in Germany and England; but it is entirely based upon molar teeth.M. antiquushas a two-rooted molar of an elongated form with a row of tubercles on either side of a median groove, which traverses the long axis of the tooth. To some extent the teeth of the ancient form resemble those ofOrnithorhynchus.Microlesteshas been sometimes spoken of as a Marsupial, but Mr. Tomes[46]has found that it does not show one very universal character of the Marsupial teeth: it has not those continuations of the dentinal tubes which traverse the enamel in all Marsupials that have been examined with the sole exception of the Wombat.
The rarity of the remains of mammals in these earliest rocks of the Secondary epoch has been accounted for in another way from that which has been suggested above. It may be that the group Mammalia was not evolved in Europe at all, and that the stray remains which have been found in that continent represent the fragmentary remnants of a few scattered immigrants which heralded the later invasion of more numerous genera during the Jurassic period.
The Mammals of the Jurassic Period.—Some of the Allotheria or Multituberculata described in the last section occur in the rocks of this early part of the Secondary epoch. They are doubtful in position, as already stated; some of them indeed, as for instanceTritylodonandDromatherium, are possibly not mammals at all, while the remainder probably belong to a non-existent order of mammals. Along with these dubious creatures are the fragmentary remains of small animals which are not merely mammals, but in all probability definitely Marsupials. It is true that here again we have little beyond lower jaws and teeth to deal with; so that there may be less certainty in referring them to the Marsupials than appears to be the opinion of the majority of Palaeontologists.
Professor Osborn in fact considers that the Mesozoic mammals consist of three groups: (1) The Multituberculata, including the Bolodontidae, Stereognathidae, Plagiaulacidae, Polymastodontidae, and possibly the Tritylodontidae (which, however, are regarded by him and by others as more probably reptiles of theTheromorphous group). (2) The Triconodonta, which were Marsupials, though in all probability with a complete succession of teeth and with an allantoic placentation. This group will include the generaPhascolotheriumandAmphilestes, as well asTriconodonandSpalacotherium. Finally we have (3) the Trituberculata (or Insectivora Primitiva) with the generaAmphitherium,Peramus,Amblotherium,Stylacodon, andDryolestes.
We shall take these three groups in order. The Multituberculata have already been to some extent defined, if such a word can be used to express the summation of the very scanty information at our disposal. Of this group,Plagiaulaxis a genus which occurs in the Purbeck beds; it is only known by lower jaws implying an animal of the size of a Rat or rather smaller. The jaws have in front a large incisor which looks Rodent-like, and also like those of the Diprotodont Marsupials; but it is held that these teeth did not grow from persistent pulps, and there is in any case no anterior thickened coating of enamel. Canines are absent; the diastema is followed by four premolars increasing progressively in size and possessing somewhat complicated grinding surfaces. These surfaces are formed by several obliquely-set ridges. The succeeding teeth are termed molars on account of their difference in structure, and there are but two of them on each side. The molars are of a pattern common in the Multituberculata; the centre is hollowed, and the raised rim is beset by tubercles. Other Jurassic genera of Multituberculates areBolodon,Allodon, andStereognathus. All of these possess the same multituberculate molars.
Of the Triconodonta the type-genus isTriconodon. This genus is better known than most Jurassic mammals, since both the upper and the lower dentition have been described. It appears to have possessed the typical Eutherian dentition of forty-four teeth, to which a fourth molar is added in some species. The great difference between the molars and premolars argues a complete tooth-change. The genus is American as well as European.
Spalacotheriumhas more molars, five or six.
Phascolotherium bucklandi, on the other hand, is a much older type in the form of its teeth. There are, however, not so many of them as inAmphitherium;Phascolotheriumhas but two premolars and five molars, making a total of forty-eight teeth. The teeth are of the triconodont form, the three cusps being in line, and the middle one the largest.
Amphilesteshas teeth of the same pattern but has more of them, the premolars and molars being respectively four and five. All these animals had the lower jaw inflected. Whether they are all Marsupials or not, it is clear thatPhascolotheriumandAmphilestesshould be united and placed away fromAmphitheriumon account of the more primitive form of their teeth.
We next come to the Trituberculata.
Among the most celebrated of these remains are a few jaws discovered in the Stonesfield slates near Oxford, and examined by Buckland, Cuvier, and some of the most eminent naturalists of the beginning of the last century. These jaws have been lately submitted to a careful re-examination by Mr. Goodrich,[47]who has increased our knowledge of the subject by exposing from the rocky matrix in which the jaws lie fresh details of their structure; it is probable therefore that now all that there is to be learnt from these specimens has been recorded.
Amphitherium prevostiiwas a creature about the size of a Rat. Its jaw was first brought to Dean Buckland about the year 1814, and described six years later. Buckland thought the jaw to be that of an Opossum, an opinion in which Cuvier concurred. The jaw, however, is marked by a groove running along its length, and this groove was regarded by de Blainville as evidence of the composition of the jaw out of more than one element, which would naturally lead to its being regarded as the jaw of a reptile.[48]This species and another named after Sir Richard Owen have a dental formula which, like that of the Marsupials, is large as compared with that of the Placental mammals; it runs: I 4, C 1, Pm 5, M 6—i.e.64 teeth altogether. This is a larger number than we find in any existing Marsupial. But as in Marsupials, and in certain Insectivora also, the angle of the jaw is inflected. These teeth are of the tritubercular pattern with a "heel." They are in fact closely like those of the livingMyrmecobius; but not, it should be remarked, unlike those of certain Insectivora.
The Mammals of the Cretaceous Period.—At one time there was a totally inexplicable gap between the Jurassic and the basal Eocene, a series of strata which occupy an enormous expanse of time in the history of the earth having appeared tobe devoid of mammalian remains. This gap, however, has been filled up by the discovery of mammalian remains in the North American Laramic formation, which seems to be clearly of Cretaceous age. Furthermore, it is held by some that the Purbeck beds are more properly to be placed with the Cretaceous, which would then necessitate the consideration under the present heading of some of the types already dealt with; and if, as is suggested in the following section, the lowest so-called Eocene beds are really referable to the Cretaceous, there is no lack of mammalian remains in that period. And, moreover, it was in that case the Cretaceous period which witnessed the evolution of the existing orders of Placental mammals. Otherwise the mammalian remains of the Cretaceous agree with those of the Jurassic. We find remains of the Multituberculata in fragments of Plagiaulacidae and Polymastodontidae.Ptilodusis a genus which has two premolars; andMeniscoessusis another multituberculate from the same Laramic formation. The other detached fragments of mammals are thought by Osborn to represent both Placentals and Marsupials.
The Mammals of the Tertiary Period.—Unless the lowest beds of the earliest Tertiary period, the Eocene, such as the Torrejon of North America, should be in reality referred to the Cretaceous, there is no evidence that the modern groups of Mammalia existed before the present epoch of the earth's history. It is probable, however, that the Eutheria as a group were Mesozoic. The fossil jaws that have been considered in the last chapter may quite probably be primitive Eutherians, or even divisible, as believed by Professor Osborn, into Marsupials and Insectivores. In the Tertiary, however, apart from the question as to the nature of the Puerco and Torrejon formations, and as to certain South American strata whose fossil contents have been investigated by Professor Ameghino, we find the first traces of mammals definitely referable to existing orders, or to be distinctly compared with existing orders. Since, however, representatives of types which have obvious relationships to modern types appear in considerable profusion in the very earliest strata of the Eocene, it seems clear that much remains to be discovered in beds earlier than these. Confining ourselves, however, to facts and to comparisons which can be made on more than a few lower jaws and scattered teeth, which is practically all that wepossess of earlier mammals, we must arrive at the general conclusion that two of the existing larger groups of the Eutherian, non-Marsupial, mammals were differentiated at quite the beginning of the Eocene, and were represented by forms from which it is possible to derive at least the existing Carnivora, Insectivora, Artiodactyla, and Perissodactyla. These were the Creodonta and the Ungulate Condylarthra. In addition to these we may enumerate as very early types the Lemuroidea, represented by such forms asIndrodonin the New World, and (though later) byNecrolemur, etc., in the Old World, and the Edentata, if we are to allow as their ancestors the Ganodonta.
The early Eocene strata also contain representatives of at least one order, the Amblypoda, which increased subsequently, but has died out without descendants, unless we are to believe with some that the Elephants are to be derived from these Eocene "pachyderms." In later Eocene times the great majority of the existing orders, and even subdivisions of orders, are to be met with; and there are in addition such totally extinct orders as the Typotheria, Ancylopoda, and Tillodontia. Coupled with this gradual specialisation in the orders of Eutherian mammals, there is naturally a vast increase in the number of generic and family types. This culminates perhaps in the Miocene, from which time there has been a gradual decline in mammalian variety, so that it is justly said that we live now in an epoch which is impoverished of mammals. This gradual decay has persisted until to-day, as is witnessed by the extinction of the Rhytina and the Quagga, and the growing rarity of the White Rhinoceros and the American Bison.
The early Eutherian stock consisted of small mammals with small heads and slender, long tails. The limbs were pentadactyle, ensheathed in claws or broader hoofs. The fore-limbs may have been partly prehensile. The teeth were forty-four, completely differentiated into incisors, canines, molars, and premolars; and there appears to have been a complete diphyodontism. The canines were not greatly enlarged, and no diastema separated any of the teeth. The molars were bunodont or of a more cutting pattern, with some five or six tubercles. These animals were, moreover, very small-brained. This early stock is represented by Creodont and Condylarthrous animals, the exact boundaries between which are hardly marked in thevery early types. Professor Osborn has argued that from this early Eutherian stock there were two waves of progress, or, as he expresses it, "two great centres of functional radiation."[49]
The first was largely ineffective, the second has produced all the Eutherian orders of to-day. These two divisions are termed by him "Mesoplacentalia" and "Cenoplacentalia." The first division embraces the Amblypoda and their descendants the Coryphodonts and Dinocerata, many of the Condylarthra, the bulk of the Creodonts and the Tillodonts. These creatures persisted for a time, but died out in the Miocene. They were mainly distinguished by the smallness of their brain; the great specialisation of structure which they exhibit having left that organ unaffected, and therefore tending in the long run to render them unable to cope with changes in the inorganic and organic world. The successful division of the primitive Eutheria comprises the groups which exist at the present day, and is not connected directly with those small-brained Mesoplacentals; it has apparently originated, however, from the least specialised of their ancestors. Professor Osborn thinks, moreover, that the Lemurs and the Insectivores are persistent descendants of the earlier wave of Eutherian life. It appears in fact as if Nature had created the existing Ungulate, Unguiculate, and other types on a defective plan, and, instead of mending them to suit more modern requirements, had evolved an entirely new set of similarly-organised types from some of the more ancient and plastic forms remaining over. The Marsupials may be the only group of the early wave remaining, and they have been able to hold their own for the geological reason that Australia was early cut off from communication with the rest of the world. That they are disappearing seems to be shown by their gradual diminution as we pass from Australia towards the continent of Asia, through the islands of the Malay Archipelago. Competition has here decimated them, as it may do in the remote future in Australia.
It is often said, but with some looseness of statement, that ancient quadrupeds are huger than their modern representatives. This statement is partly true in fact, but largely wrong in implication. For it suggests that—and the suggestion is often expressed in books that are not authoritative—huge animalshave left a dwarfish offspring; that there were giants of old, and that there is a puny race to-day. As a matter of fact, the study of the gradual evolution of the early Tertiary Mammalia into their descendants of later times shows very plainly the truth of this interesting generalisation: That the primitive types were all small creatures, and that in those instances where we can trace a pedigree, there was a gradual increase in size up to a point where greater increase led to extinction. We point out later on a number of facts illustrating this matter in detail. It has been ascertained, for instance, that the pedigree of the Horses, the Camels, the Rhinoceroses, and many other groups, commences with small forms and culminates in large ones. It may be urged that such animals as the Tapir are to-day smallish forms, and that related to them in the past were the gigantic Titanotheres; but in this and similar cases it will be found that the extinct giants were not in the direct line of pedigree, but represented side-branches which waxed huge on their own account and then disappeared.