CHAPTER IV

GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION

I. GEOGRAPHICAL DISTRIBUTION.[26]

In considering the present distribution of mammals over the globe, we may, in the first place, direct our attention to terrestrial or land types, reserving the consideration of aerial types, like the Bats, and aquatic forms, as exemplified by the Cetaceans, Sirenians, and Seals, to separate sections.

Among terrestrial forms each species has a certain definite area of distribution in space, which may be of very wide extent, or may be confined to a restricted region. This distributional area is, however, always connected, or continuous; that is to say, that although we may have a single species inhabiting two continents, like the Lion in Asia and Africa, or dwelling both on a continent and adjacent continental islands, like the Javan Rhinoceros of India, Java, and Borneo, yet we shall always find that such areas, if not still connected, show evident signs of having been so connected in comparatively late geological epochs; and we never find instances of the same species inhabiting totally disconnected areas, such as India and South America. As examples of mammals with a wide distribution we may mention the Lion and the Leopard, which are now found throughout Africa, and also occur in India, as well as in the intervening areas of Arabia and Persia. In the case of the former species, palæontology further teaches us that its distribution in the last geological epoch was even more extensive, since we have good evidence to show that it formerly ranged over the greater part of Europe, including the British Isles. The Jackal affords another well-known instance of a species common to India and Africa. The American Puma, again, may be cited as an example of a mammal having a very wide range in latitude, since it is found from Patagonia in the south to Canada in the north. As instances of wide range in the opposite direction we have only to mention the Reindeer and the Elk or Moose, found in the northern regions of both the Old and New Worlds, which are only separated from one another by the narrow channel of Behring Strait.

Of mammals with extremely restricted distributional areas, we may mention many of the Insectivora, such as the Desman of the Pyrenees, and some of the Madagascar types of this order, the Lemurs from the same island, some of the species of Marmots, the remarkable bear-like _Æluropus_ of Eastern Tibet, one species of Zebra, and other Ungulates from Africa.

The distribution of a genus (except of course when the genus is represented only by a single form) is very generally more extensive than that of a species; and this may be markedly the case when there are only some two or three species in a genus. In genera, moreover, we meet with what is known as discontinuous distribution, that is, where the distributional area of one or more species is totally separated from that of others. The best instance of this occurs in the case of the Tapirs, where we find one species inhabiting the Malayan Peninsula, and no others anywhere in the world, with the exception of South America. The explanation of such an apparently anomalous feature in distribution is to be found in the past history of the globe, which shows us that Tapirs once existed in China, Europe, and North America, and, therefore, indicates that the existing isolated species are the sole survivors of a group once spread over a large portion of the earth’s surface. In regard to generic distribution it must, however, be mentioned that this depends to a great extent on the limits which we are disposed to assign to genera themselves.

As the distributional area of a genus generally exceeds that of a species, so that of a family, or group of genera, is larger than that of a single genus; and similarly the distribution of an order, or assemblage of families, usually occupies a larger area than that of a single family. Thus, for instance, the genus _Thylacinus_, represented only by the so-called Tasmanian Wolf or Thylacine, is now entirely restricted to Tasmania; but the family _Dasyuridæ_, to which that genus belongs, ranges all over Australia, while the order Marsupialia, which includes the _Dasyuridæ_, is found both in Australia and America, and in past epochs was probably spread over the entire globe.

A remarkable feature in connection with the distribution of the terrestrial Mammalia is the circumstance that, with the exception of certain species introduced by human agency, and small forms which can easily have been transported on floating timber or other similar means, they are totally absent from what are known as oceanic islands—that is islands arising from great depths in the ocean, mainly composed of coral or volcanic rocks, and showing no signs of having ever been connected with the existing continents, or the larger and so-called continental islands. The obvious explanation of this feature is, that from their total isolation these islands have never been able to receive a mammalian fauna from the great continental areas on which mammalian life was probably first developed.

As an intermediate step between these islands which are practically void of mammalian life and the continents which teem with such a variety of forms, are certain larger islands and portions of continents containing a mammalian fauna more or less markedly distinct from that of the whole of the other regions of the globe. The best instance of this is Australia, which, with the exception of one dog—the Dingo—and certain _Muridæ_ and Bats, has no mammals except Monotremes and Marsupials. The latter are, moreover, perfectly distinct from those of America, which, if we exclude the islands in the neighbourhood of Australia, is the only other region which now possesses any Marsupials at all. Here also we have a ready and full explanation which accords with all the facts; since it is evident that Australia has been isolated from the Asiatic continent from some very remote geological epoch, at which period it is probable that Monotremes and Marsupials were the dominant if not the sole representatives of the Mammalia then existing. Consequently Australia has never been able to receive an influx of the Eutherian orders, which have probably swept away all the Marsupials except the small American Opossums from the rest of the globe. Again, the large island of Madagascar, which has a fauna of an African type, but still very markedly different from that of the mainland, may be considered to have been connected with the latter at a time when the Eutheria had become the dominant forms, but has been separated for a sufficiently long period to have enabled a large number of its species and genera to have become distinct from those of the adjacent continent. Similarly, there is evidence to show that South America was probably cut off for a considerable period from the northern half of the American continent, in consequence of which its lowly organised fauna of Edentates were enabled to attain such a remarkable development in the later geological periods.

In contrast to the mammalian fauna of islands of the preceding type is, or rather was, that of the British Islands, which in the early historic and prehistoric periods was identical with that of the Continent. This leads to the inference that at a comparatively late epoch there was a direct land communication between Britain and the Continent, which is shown by geological evidence to have actually been the case.

The above instances are sufficient to show what an important influence the date of separation of islands from the adjacent continents has had upon their existing mammalian fauna, and how largely the present distribution of mammalian life is bound up with the past history of our globe. We must, however, not omit to mention another very important agency of past times which has likewise had great influence on the present distribution of the various faunas of the northern hemisphere. This is the so-called glacial epoch, which took place immediately before the establishment of the present condition of things, and appears to have been the cause of the extinction of many of the larger mammalian types which formerly inhabited Europe, and whose retreat to the warmer regions of the south was apparently cut off by the Mediterranean.

_Zoological Regions._—Zoologists are now generally agreed in dividing the land surfaces of the globe into a number of zoological regions or provinces, characterised by a more or less distinctly marked general _facies_ of their fauna as a whole. Some of these regions are much more distinctly defined than the others; and in the majority of cases there is a kind of neutral ground or No-man’s-land at the junction between any two of these regions. It must also be remembered that in the Old World proper as we go back in time we find a gradual assimilation in the mammalian faunas of the different regions, indicating that originally there was one large fauna of a generally similar type occupying the greater portion of this area. Thus we find that Hippopotami, Giraffes, Kudus, Elands, and other types of Antelopes now restricted to Africa, formerly extended to Europe and India, while there is also evidence to show that the group of large anthropoid Apes, now found only in Africa and the Bornean region, were likewise spread over a large part of the south-western half of the Old World. Moreover, while at the present day there is a marked connection between the mammals of the northern regions of both the Old and New Worlds, in the Tertiary period it appears that the fauna of the whole of North America was much more nearly allied to that of the central regions of the Old World than is now the case. Thus in the Tertiary rocks of America we meet with remains of what we are accustomed to regard as such essentially Old World genera as Horses and Rhinoceroses. On the other hand there are no traces in America of the existence at any period of Apes, Giraffes, Hippopotami, or Hyænas, while that continent has yielded evidence of groups of Ungulates totally unrepresented in the eastern hemisphere.

The chief zoological regions of the globe, proposed by Mr. Sclater in 1857, and now recognised by the majority of authorities, are six in number, and are named as follows. Firstly, the Palæarctic region, embracing the whole of Europe, Persia, Northern Arabia, and all of Asia northward of the line of the Himalaya proper, Japan, that part of Africa lying northward of the Sahara Desert, and the oceanic islands of the North Atlantic. Secondly, the Ethiopian region, which comprises all Africa lying to the south of the Sahara, the southern part of Arabia, Madagascar, and the Mascarene Islands. Thirdly, the Oriental or Indian region, which is taken to include India south of the Himalaya, and to the north-west as far as Beluchistan, the Malay peninsula, southern China, Sumatra, Java, Borneo, and the Philippines. Fourthly, the Australasian region, which is usually defined as being bounded to the north-west by the deep sea channel lying between Borneo and Celebes known as Wallace’s line, and is taken to include Celebes, Lumbok, New Guinea, Australia, Tasmania, New Zealand, and the host of oceanic islands in the South Pacific. Several writers, however, prefer to regard Celebes and some of the adjacent islands as representing a transitional Austro-Malayan region. Fifthly, the Nearctic region, comprising Greenland and North America as far south as the north of Mexico. And sixthly, the Neotropical region, which embraces the remaining portion of the American continent and the West Indies.

Various minor modifications of this scheme have been proposed. Thus some writers are disposed to raise India to the rank of a distinct primary region, while others propose the same for New Zealand. The Palæarctic and Nearctic regions have a large number of common types, more especially among the mammals, and Dr. A. Heilprin[27] has expressed his opinion that they should be regarded as a single primary region under the name of the Holarctic. The same writer would also separate the South Pacific Islands as constituting a Polynesian region.

Minor divisions or sub-regions have also been marked out, but it will be unnecessary to indicate their limits in the present work. We may, however, mention the Mediterranean sub-region of the Palæarctic, which includes the peninsular portion of southern Europe, North Africa, Asia Minor, Persia, Afghanistan, Beluchistan, and Northern Arabia, as a good instance of the transition from one region to another, since its fauna has a mingling of Palæarctic, Ethiopian, and Oriental types, the former being, however, the predominant ones.

Of the chief mammalian types characteristic of these various regions only a brief sketch can be given in this work.

_Palæarctic Region._—The Palæarctic region is of enormous extent, and includes countries varying greatly in their flora, climate, and elevation. Thus it embraces the Arctic plains of Siberia, the warm regions of Italy, Southern France, and Northern Africa, the forest-clad slopes of the outer Himalaya, and the lofty arid plains of Turkestan and Tibet, scorched by a burning sun in summer and chilled by a still more terrible cold in winter. Its extreme limits in the west are marked by the Canaries and Azores, and in the east by distant Japan; and yet throughout this vast expanse we find a great uniformity of life, as exemplified by the large number of British genera which occur also in Japan. The mammals which are on the whole the most characteristic of this region are the Sheep and Goats, forming a section of the great family of _Bovidæ_; nearly all the species of which are Palæarctic, although we meet with one Goat (_Capra_) in the Nilgherries of Southern India, and a Sheep (_Ovis_) in the Nearctic region. The Musk Ox (_Ovibos_) is characteristic of the Palæarctic and Nearctic regions. At least one species of Camel is characteristic of this region, and it is not improbable that the second may also have originated in it. There are a few characteristic types of Antelopes, such as the Alpine Chamois (_Rupicapra_), the Saiga of Tartary, and the Chiru (_Pantholops_) of Tibet, each of which is represented by only a single species: and we miss the host of Antelopes so characteristic of the Ethiopian region. Deer (_Cervus_) are abundant, although by no means confined to this region; and the Musk Deer (_Moschus_), the sole representative of the subfamily _Moschinæ_, is exclusively Palæarctic. Monkeys, as a rule, are absent, although we meet with one species of _Macacus_ in Northern Africa and at Gibraltar, and some other types on the southern border of Tibet. The Moles (_Talpa_) are mainly Palæarctic, although one species enters Northern India, while the Desmans (_Myogale_) of the Pyrenees and Southern Russia are unknown beyond the limits of this region. The Water-shrew (_Nectogale_) is likewise a peculiar eastern Palæarctic type. Among the Rodents, the Picas or Tailless Hares (_Lagomys_) and the Dormice (_Myoxus_) are essentially Palæarctic forms, only one species of each being found beyond the limits of the region, and the one extra-Palæarctic species of _Lagomys_ occurring in the cognate Nearctic region. The Mice and Rats are represented by the typical genus _Mus_ and other types, and Hares (_Lepus_) and one species of Squirrel (_Sciurus_) are common. The Carnivora include two species of Bears (_Ursus_), Wolves and Foxes (_Canis_), a Lynx and a few species of Cats (_Felis_), as well as numerous weasels (_Mustela_), and some other types.

_Ethiopian Region._—The Ethiopian region is of great interest to the student of mammals, since it is inhabited by a number of forms remarkable for their large size. A considerable portion of the area consists of desert, especially in the north; but there is also a wide extent of grassy plains (veltd), as well as vast tracts of equatorial forests of great density. Perhaps the most striking feature in the Ethiopian fauna is the number of Ungulates, both of the Artiodactyle and Perissodactyle sections. In the former section we have the Giraffes (_Giraffa_) represented by one species, which is the type of a family, and is unknown elsewhere. Equally characteristic are the Hippopotami, which likewise form the type of a family, while the Pigs are represented by the Wart-hogs (_Phacochœrus_) and the River-hogs, forming an aberrant group of the genus _Sus_. The Oxen (_Bos_) are represented by Buffaloes, but there are no species of true Oxen or Bison. The Antelopes attain an extraordinary development, the number of species being estimated at from eighty to ninety, which are referred to a large number of genera, although several of these are more or less ill-defined. Most of these genera are peculiar to this region, but the Gazelles (_Gazella_) are also found in the desert regions of other parts of the Old World, and _Oryx_ ranges into Arabia and Persia. In contrast to this abundance of Antelopes is the total absence of the Deer family, or _Cervidæ_, which are so characteristic of the Palæarctic and Oriental regions. The Chevrotains or _Tragulidæ_ are, however, represented by _Dorcatherium_.[28] In the Perissodactyle section we may notice the presence of two species of _Rhinoceros_, both furnished with two horns, and distinguished from those of the Oriental region by the absence of incisor and canine teeth. The Horse family (_Equidæ_) is also represented by several species, and includes the peculiar group of Zebras, characterised by their beautifully striped skins. Of other Ungulates the Elephants, which, like the Rhinoceroses, are now peculiar to the Ethiopian and Oriental regions, have one species, which is widely different from its Indian congener. The Hyraces are mainly characteristic of this region, although one species occurs in Syria and Palestine. The Carnivora include some forms like the Lion, Leopard, and Jackal, common to the Oriental region, but likewise include certain peculiar types like the Earth-wolf (_Proteles_), which may be regarded as the type of a distinct family, and two species Hyænas, which are referred by some authorities to a distinct genus (_Crocuta_). There is also the Hunting dog (_Lycaon_), and the peculiar group of Foxes known as the Fennecs, together with _Otocyon_. Bears, Wolves, and true Foxes are absent; but Civets, etc., are abundant, although not characteristic of the region. The Primates yield several very characteristic types, such as the Gorilla and the Chimpanzee (_Anthropopithecus_) among the _Simiidæ_, which, with the exception of the Orangs of Borneo, are the only existing large man-like Apes, and the group of Dog-faced Baboons (_Cynocephalus_) in the _Cercopithecidæ_. The genus _Colobus_ is also a group of the latter family, absolutely characteristic of the region. Lemurs, again, occur on the continent of Africa, but the great development of this group is in the adjacent island of Madagascar, where several peculiar genera occur, and where the larger Carnivora and Ungulata are absent. These peculiarities of the fauna of Madagascar apparently point, as previously mentioned, to its separation from the mainland before the latter was overrun by the larger types, and at a time when its chief mammals were Lemurs and Insectivores. There are two genera of Edentates, the Pangolins (_Manis_), and the Aard-vark (_Orycteropus_), the latter being peculiar.

Although the foregoing groups of mammals are now so characteristic of the Ethiopian region, it cannot be too strongly insisted that their restriction to this region is, so to speak, merely a feature of the present day, and that at a late geological epoch nearly all the peculiar genera were represented in India, and many of them also in Europe.

_Oriental Region._—The third or Oriental region is likewise of very considerable extent, and is the only one, in addition to the Ethiopian, which is the home of huge Ungulates, like Elephants and Rhinoceroses, and the large man-like Apes. A large proportion of this extensive area is occupied by tropical and subtropical forests and swamps; these being especially abundant in Burma, Southern China, Siam, and the southern ridges of the Himalaya, collectively constituting the Indo-Chinese sub-region, and also in the Indo-Malayan sub-region of the Malay peninsula and adjacent islands. In the third or Indian sub-region, comprising peninsular India, with the exception of the Carnatic, there are large tracts of open country, including some of the hottest regions in the world, parts of which form plains more or less covered with vegetation during the cooler and rainy seasons, while others are barren rocky table-lands, as in the Deccan, or arid deserts like those of parts of the Punjab and Sind. Finally, in the fourth or Cingalese sub-region, represented by the Carnatic and the island of Ceylon, we find vast areas of luxuriant forest and jungle. In the north-western desert area of the Indian sub-region the fauna includes a mixture of Palæarctic and Ethiopian forms, with those characteristic of the Oriental region.

Among the chief features of the mammalian fauna of this region we may notice the absence of Hippopotami and Giraffes, the greatly diminished number of Antelopes, as compared with those of Africa, and the abundance of Deer and true Pigs. The Antelopes comprise the two peculiar genera _Boselaphus_ (Nilghai) and the typical _Antilope_ (Black-buck), each of which is represented by only a single species, while the Deer belong to the so-called Rusine group, which is markedly different from that to which the Palæarctic Red Deer belongs. True Chevrotains (_Tragulus_) are peculiar to this region. The Oxen include the true Buffalo, differing in many respects from the African species of the same group, and also certain species of true Oxen, such as the Gaour and Banting, belonging to the Bibovine group, which is confined to this region. In the Perissodactyla Horses (_Equus_) are represented only by a single species in the desert area of the Indian sub-region, while the two species of _Rhinoceros_ differ from those of Africa in being furnished with canines and incisors. The Malayan Tapir is the only Old World species of its genus. The Indian Elephant differs, moreover, so markedly from its African ally that some writers regard the two as types of distinct genera. The Carnivora include the Lion, Leopard, Jackal, and Hunting-Leopard, which are common to Africa; but the Tiger is very characteristic of this region, although extending northwards into the Palæarctic. Civets are abundant, comprising some peculiar genera, of which it will suffice to mention the well known _Paradoxurus_. Wolves closely allied to the Palæarctic species occur in Northern India, and there are also Foxes related to the typical species. The Dog-like animals which hunt in packs, and are separated by some writers from _Canis_ under the name of _Cyon_, occur in the present and the Palæarctic region. The striped Hyæna is the Indian representative of its genus. Ratels are common to this and the Ethiopian region, and constitute the genus _Mellivora_. The most striking feature in the Carnivorous fauna of this region, as distinguished from the Ethiopian, is, however, the presence of Bears, some of which belong to the typical genus _Ursus_, while one species is usually generically separated under the name of _Melursus_. Among the Rodents we may especially notice the abundance of the _Muridæ_ and _Sciuridæ_. In the former family we have numbers of true Mice (_Mus_), and also the peculiar genus _Nesocia_ (Bandicoot-Rat), while in the latter both the true Squirrels (_Sciurus_) and the Flying-Squirrels (_Pteromys_) attain great development. The genus (_Pteromys_) is, indeed, mainly characteristic of this region, although in Kashmir and Japan it enters the Palæarctic. The Bats are very numerous, being represented by all the families, with the exception of the _Phyllostomatidæ_, or Vampyres, of South America. Among the Insectivora the genera _Tupaia_ and _Galeopithecus_ (Flying Lemur) are peculiar to this region, although not found in India. Finally, in the Primates we have the genera _Macacus_ and _Semnopithecus_ very abundantly represented, although both also enter the Palæarctic region; but the Anthropoid types are confined to the south-eastern half of the region, and include the Orangs (_Simia_) of Borneo, and the smaller long-armed Gibbons (_Hylobates_), which are abundant in the Malay peninsula, both genera not being found beyond this region. The Lemurs are much less abundant than in the Ethiopian region, but they include the peculiar Tarsier of Sumatra, Borneo, and Celebes (Austro-Malayan region), which differs so markedly in dentition and structure of the feet from all other forms that it has been made the type of a separate family. The Edentates, so poorly represented in the Old World, include only Pangolins (_Manis_), which, as we have already seen, also occur in the Ethiopian region.

_Australasian Region._—With the fourth or Australasian region we come to a mammalian fauna so peculiar that we have no difficulty whatever in defining it from all the other regions of the globe, although it should be observed that in the Austro-Malayan islands we have a partial mingling of the Australasian and Malayan faunas. If we exclude Celebes from this region we find that, with the exception of a Pig in New Guinea, of the Dingo in Australia, of numerous Mice and Rats (_Muridæ_), and Bats, there are no Eutherian mammals throughout the area. The mammals of this region are restricted to the Australian mainland, the island of Tasmania, New Guinea, and the Aru islands, the whole area of New Zealand having been totally devoid of mammalian life until introduced by man. The whole of the Monotremata, constituting the subclass Prototheria, and all the Marsupials, exclusive of the few outlying forms ranging into the transitional Austro-Malayan area, and with the exception of the American family of the Opossums (_Didelphyidæ_), are absolutely confined to this region.

_Celebes._—The mammals of Celebes—the typical representative of the Austro-Malayan transitional region or sub-region—include the peculiar Ape known as _Cynopithecus_, _Tarsius_ (also Oriental), the Anoa, and the single species of _Babirusa_. Several other types of placental mammals are found in this transitional area, while the Marsupials are represented by _Phalanger_ and _Petaurus_.

_Nearctic Region._—The two remaining regions we have to consider are comprised in the New World. The first of these is the Nearctic, which, as already mentioned, has a fauna showing such a strongly marked relationship to that of the Palæarctic region, that it has been proposed to unite the two regions. Among types common to these two regions we may mention closely allied species of true Deer (_Cervus_) as exemplified by the Red Deer and the Wapiti; the allied Bisons of the two regions; the Reindeer and Elk common to both; as well as nearly related, and in some cases identical, species of Cats, Lynxes, Bears, Wolves, Foxes, Beavers, Squirrels, Marmots, and Hares. The Glutton or Wolverene, and the Musk Ox is also common to the Arctic portions of the two regions. The Ungulates are very poorly represented, but we have, in addition to the forms already mentioned, one species of the Palæarctic genus _Ovis_, namely the Bighorn, and the Prong-buck (_Antilocapra_), which is quite peculiar. There are, however, no Perissodactyla. The Raccoons and Coatis (_Procyonidæ_) constitute a family represented out of the New World only by the aberrant Cat-Bear (_Ælurus_) of Nipal. The characteristic American feline known as the Puma extends over this region; but there are no Edentates, and the Marsupials are represented only by a single species of Opossum. Rodents are extremely numerous, and comprise several characteristic types, which alone would tell us what part of the globe we were visiting. The most distinctive are the Pouched Rats (_Geomyidæ_), and the Beaver-like rodents known as the _Haplodontidæ_. True Rats and Mice (_Mus_), which are represented throughout the Old World, are totally wanting in the New, where they are replaced by the Vesper-mice, which may be included in the European genus _Cricetus_, although often separated as _Hesperomys_. This feature alone would seem to justify the distinction of the Nearctic from the Palæarctic region. The Musquash (_Fiber_) is a genus of Nearctic rodents unknown in the Old World. Among other characteristic genera we may mention, in the Carnivora, the Skunk (_Mephitis_) and the American Badger (_Taxidea_). Primates are absent from the entire region.

_Neotropical Region._—The last of the six main regions is the Neotropical, including Mexico, South America, and the West Indies. A very large extent of this area is occupied by forests, which are described as being denser and more luxuriant than those of any other part of the globe. Alternating with these forest areas are the vast grassy plains known in different regions as llanos, savannas, and pampas. The back-bone of the region is formed by the great chain of the Andes. Next to the Australasian, this region is perhaps better characterised by its mammalian fauna than any of the others. Commencing with the Ungulates, we find a total absence of Antelopes, Sheep, and Oxen, and also of all Perissodactyles except Tapirs. Deer are, however, represented, although by peculiar forms (_Cariacus_) unknown beyond the New World. The Peccaries (_Dicotyles_), which are often made the type of a distinct family, take the place of the Old World Pigs, while the Llamas and Alpacas (_Auchenia_) are the substitutes for the Palæarctic Camels. The Carnivora include several Cats (_Felis_), among which the Puma and the Jaguar are the most noticeable; and there are also Raccoons, Coatis, Foxes, and one species of Bear. Insectivora are totally wanting; but the Bats are characterised by the presence of the Vampyres (_Phyllostomatidæ_), which are almost restricted to this region. The Rodents likewise include three families unknown elsewhere, namely the Chinchillas and Viscacha (_Chinchillidæ_), the Agouties (_Dasyproctidæ_), and the Cavies (_Caviidæ_); while a large number of the _Octodontidæ_ are Neotropical, all the other forms being Ethiopian. In the Primates, again, we have all the forms quite peculiar to this region, and constituting two families, viz. the _Cebidæ_ or Prehensile-tailed Monkeys, and the _Hapalidæ_, or Marmosets, both of which differ decidedly in their dentition, as well as in other features, from the Old World Monkeys. Lemuroids are unknown. Perhaps, however, the mammals which may be considered as most characteristic of the Nearctic region are the numerous Edentates, which form three families, mostly confined to it. These comprise the _Bradypodidæ_ or Sloths, which solely inhabit the forest region; the _Myrmecophagidæ_ or Anteaters; and the _Dasypodidæ_ or Armadillos, of which one species has crept northward as far as Texas. Almost equally characteristic are the numerous Opossums, the majority of which belong to the genus _Didelphys_. Finally, it should be observed that the West Indies are distinguished from the rest of the region by the absence of Primates, Carnivora, and Edentates.

_Aquatic Mammals._—Many mammals grouped for the present purpose as terrestrial pass a great portion of their lives in brooks, lakes, or rivers, and, being dependent upon such waters for obtaining their subsistence, are necessarily confined to their vicinity; but the truly aquatic mammals, or those living constantly in the water, and unable to move their quarters from place to place by land, are the orders Cetacea and Sirenia, with which may also be grouped the Seals, forming the Pinniped division of the order Carnivora.

For the marine Cetacea, animals mostly of large size and endowed with powers of rapid locomotion, there are obviously no barriers to universal distribution over the surface of the earth covered by sea, except such as are interposed by uncongenial temperature or absence of suitable food. Nevertheless it was thought some years ago that the fact of a Whale or a Dolphin occurring in a sea distant from that in which it had usually been found was sufficient justification for considering it as a distinct species and imposing a new name upon it. There are now, however, so many cases known in which Cetaceans from the northern and southern seas, from the Atlantic and Pacific Oceans, present absolutely no distinguishing external or anatomical characters upon which specific determination can be based that the opposite view is gaining ground; and, since some species are undoubtedly very widely distributed, being in fact almost cosmopolitan, there seems little reason why many others should not be included in the same category. The evidence is satisfactory enough in those instances in which the intermediate regions are inhabited by the same forms;—the cases of “continuous areas” of distribution. In those in which the areas of distribution are apparently discontinuous, there may be more room for doubt; but it must not be forgotten that the negative evidence is here of much less value than in the case of land animals, since the existence of Cetaceans in any particular part of the ocean may be easily overlooked. The great Sperm Whale (_Physeter macrocephalus_) is known to be almost cosmopolitan, inhabiting or passing through all the tropical and temperate seas, although not found near either pole. At least three of the well-known species of Rorqual (_Balænoptera_) of the British coasts are represented in the North Pacific, on the South American shores, and near New Zealand, by species so closely allied that it is difficult to point out any valid distinctive characters, though it may perhaps be desirable to wait for a more exhaustive examination of a large series of individuals before absolutely pronouncing them to be specifically identical. There is nothing yet known by which we can separate the “Humpback Whales” (_Megaptera_) of Greenland, the Cape of Good Hope, and Japan. The same may be said of the common Dolphin of the European seas (_Delphinus delphis_) and the so-called _D. bairdi_ of the North Pacific and _D. forsteri_ of the Australian seas. The Pilot Whale (_Globicephalus melas_) and the _Pseudorca_ of the North Atlantic and of New Zealand are also, so far as present knowledge enables us to judge, respectively alike. Many other similar cases might be given. Captain Maury collected much valuable evidence about the distribution of the larger Cetacea, and, finding Right Whales (_Balæna_) common in both northern and southern temperate seas, and absent in the intermediate region, laid down the axiom that “the torrid zone is to the Right Whale as a sea of fire, through which he cannot pass.” Hence all cetologists have assumed that the Right Whale of the North Atlantic (_B. biscayensis_), that of the South Seas (_B. australis_), and that of the North Pacific (_B. japonica_), are necessarily distinct species. The anatomical structure and external appearance of all are, however, so far as yet known, marvellously alike, and, unless some distinguishing characters can be pointed out, it seems scarcely justifiable to separate them from geographical position alone; as, though the tropical seas may be usually avoided by them, it does not seem impossible, or even improbable, that some individuals of animals of such size and rapid powers of swimming may have at some time traversed so small a space of ocean as that which divides the present habitual localities of these supposed distinct species. If identity or diversity of structural characters is not to be allowed as a test of species in these cases, as it is usually admitted to be in others, the study of their geographical distribution becomes an impossibility.

Although many species are thus apparently of such wide distribution, others are certainly restricted; thus the Arctic Right Whale (_Balæna mysticetus_) has been conclusively shown to be limited in its range to the region of the northern circumpolar ice, and no corresponding species has been met with in the southern hemisphere. In this case, not only temperature, but also the peculiarity of its mode of feeding, may be the cause. The Narwhal and the Beluga have a very similar distribution, though the latter occasionally ranges farther south. The common Hyperoödon is restricted to the North Atlantic, never entering, so far as is yet known, the tropical seas. Other species are exclusively tropical or austral in their range. One of the true Whalebone Whales (_Neobalæna marginata_) has only been met with hitherto in the seas round Australia and New Zealand; and a large Ziphioid (_Berardius arnouxi_) only near the last-named islands.

The Cetacea are not limited to the ocean, or even to salt water, some entering large rivers for considerable distances, and others being exclusively fluviatile. One species of _Platanista_ is extensively distributed throughout nearly the whole of the river systems of the Ganges, Brahmaputra, and Indus, ascending as high as there is water enough to swim in, but apparently never passing out to sea. The individuals inhabiting the Indus and the Ganges must therefore have been for long ages isolated without developing any definite distinguishing anatomical characters; for those by which the supposed _P. indi_ was formerly separated from _P. gangetica_ have been shown by Anderson to be of no constant value. _Orcella fluminalis_ appears to be limited to the Irawaddy river, and at least two distinct species of Dolphin belonging to different genera are found in the waters of the upper Amazon. A _Neomeris_ has been found in the great Chinese river, the Yang-tsi-Kiang, nearly a thousand miles from the sea. It is remarkable, however, that none of the great lakes or inland seas of the world are, according to our present knowledge, inhabited by Cetaceans. A regular seasonal migration has been observed in many of the oceanic Cetacea, especially those inhabiting the North Atlantic, but further observations upon this subject are still much needed.

The great difference in the manner of life of the Sirenia, as compared with that of the Cetacea, causes a corresponding difference in their geographical distribution. Slow in their movements, and feeding exclusively upon vegetable substances, water-grasses, or fuci, the Sirenia are confined to rivers, estuaries, or coasts where these grow, and are not denizens of the open sea, although of course there is a possibility of accidental transport by the assistance of oceanic currents across considerable distances. Of the three genera existing within historic times, one (_Manatus_) is exclusively confined to the shores of the tropical Atlantic and the rivers entering into it, individuals scarcely specifically distinguishable being found both on the American and the African side of the ocean. The Dugong (_Halicore_) is distributed in different colonies, at present isolated, throughout the Indian Ocean from Arabia to North Australia. The _Rhytina_ or Northern Sea-Cow was, for some time before its extinction, limited to a single island in the extreme north of the Pacific Ocean.

The Pinnipeds, although capable of traversing long reaches of ocean, are less truly aquatic than the last two groups, always resorting to the land or to extensive ice-floes for the purpose of breeding. The geographical range of the various species is generally more or less restricted, usually according to climate, as they are mostly inhabitants either of the Arctic or Antarctic seas and adjacent temperate regions, very few being found within the tropics. For this reason the northern and the southern species are for the most part quite distinct. In fact, the only known exception is the case of a colony of the Sea-Elephant (_Macrorhinus leoninus_), the general range of which is in the southern hemisphere, inhabiting the coast of California. Even in this case a different specific name has been given to the northern form; but the characters by which it is distinguished are not of great importance, and probably, except for the abnormal geographical distribution, would never have been noticed. The most remarkable circumstance connected with the distribution of the Pinnipeds is the presence of members of the suborder in the three isolated great lakes or inland seas of Central Asia—the Caspian, Aral, and Baikal; these forms, notwithstanding their long isolation, having varied but slightly from species now inhabiting the Polar Seas.

II. GEOLOGICAL DISTRIBUTION.

_Geological Sequence._—In order to understand the geological distribution, or in other words the distribution in time of mammals, it is necessary to be acquainted with the chief divisions, or time-periods, of the strata constituting the crust of the globe. These are shown in the following table, which commences with the uppermost or most recent beds and ends with the lowest and oldest.

I. CAINOZOIC OR TERTIARY— 1. Pleistocene—River alluvia, etc. 2. Pliocene—Suffolk Crag. 3. Miocene—Hempstead Beds of Hampshire. 4. Eocene—Paris Gypsum and London Clay.

II. MESOZOIC OR SECONDARY— 1. Cretaceous—Chalk, Greensands, etc. 2. Jurassic—Oolites and Lias. 3. Triassic—Red Marls, Dolomites, etc.

III. PALÆOZOIC OR PRIMARY— 1. Permian—Beds overlying the Coal. 2. Carboniferous—Coal-measures, etc. 3. Devonian—Old Red Sandstone. 4. Silurian—Wenlock Limestone, etc. 5. Cambrian—Llanberis Slate, etc. 6. Archæan—Gneiss and other schists.

The names in the first column indicate the primary divisions or life-periods, while those in the second column are the great systems, each of which is again divided into minor groups, the popular names of a few of these minor groups being given in the third column. There are at present no means of arriving at any satisfactory conclusion as to the absolute length of time indicated by either the primary or secondary divisions; but there is little doubt that the whole of the Tertiary period is only equal to a fraction of the Mesozoic as regards its duration, while it is probable that the duration of the Mesozoic epoch was largely exceeded by that of the Palæozoic.

_Mesozoic Mammals._—The earliest date at which mammals are at present known is in the upper part of the Triassic period, which forms the base of the great Mesozoic epoch; and from this date they are represented more or less abundantly in various horizons of the Jurassic and Cretaceous.

The very rapid advances in our knowledge of these forms which have been made in the last few years, especially in consequence of the explorations of rich fossiliferous beds in North America, have not only completely changed the present aspect of the science, but give such promise for the future, that any sketch which we may now attempt of this branch of the subject can only be regarded as representing a transient phase of knowledge. It will be well, however, to gather together in this place the leading facts now ascertained with regard to the most ancient forms, as, owing to the uncertainty of their relationship with any of the existing orders, they will be most conveniently treated of separately, while the ascertained facts relating to the geological history of the forms more nearly allied to those now living will be more appropriately described under the account of the different groups into which the class may now be divided.

The remains of mammals which existed anterior to the Tertiary period hitherto discovered nearly all belong to creatures of very small size, many of the largest scarcely exceeding the common Polecat or Squirrel. Some are known only by a few isolated teeth, others by nearly complete sets of these organs, and the majority by more or less nearly perfect specimens of the rami of the lower jaw. It is a very curious circumstance that this part of the skeleton alone has been preserved in such a large number of instances. Only very rarely has a nearly complete cranium been found; and there is no satisfactory evidence of the structure of the vertebral column of any single individual, and only one known case of a complete limb.[29] The species already described from European strata are numerous, although the number of genera and species has lately been reduced. Of these by far the greater number have been found at a single spot near Swanage in Dorsetshire, in a bed of calcareous mud only forty feet long, ten feet wide, and averaging five inches in depth. The marvellous results obtained by the exploration by Mr. S. H. Beckles of this small fragment of the earth’s surface show by what accidents, as it were, our knowledge of the past history of life has been gained, and what may still remain in store where little thought of at present. A bed, apparently equally rich, has been discovered in the Jurassic of Wyoming, North America, the contents of which have been made known by Professor Marsh, while another fertile source of these remains occurs in the Laramie beds of the Upper Cretaceous of the United States.[30]

The whole of the Mesozoic mammals at present known may be divided into two great groups, the one characterised by a type of dentition more or less clearly resembling that found among the existing Polyprotodont Marsupials, while the other presents an altogether peculiar modification, recalling in some respects that of the Diprotodont Marsupials, although differing so decidedly as to show that the owners of this form of dentition cannot be included in that group.

[Illustration: FIG. 24.—Frontal and oral aspects of the cranium of _Tritylodon longævus_; from the Karoo system of Basuto-land, South Africa. ⅔ natural size. (After Owen.)]

_Multituberculata._—The name Multituberculata has been proposed for the group exhibiting the type of dentition last mentioned, and is generally adopted, although the term Allotheria has been also suggested. The essential characteristic of the dentition of this group is the presence of a single scalpriform incisor on each side of the lower jaw (Fig. 25) and of one larger incisor, and in some instances of one or two smaller ones in each premaxilla (Fig. 24). These incisors are separated by an interval or diastema from the first of the premolars. The true molars, and in some instances the premolars (Fig. 24), are characterised by having longitudinal rows of tubercles separated by one or more grooves; there being either two or three of these rows in the upper molars of those forms in which these teeth are known, while there are, at least usually, only two in those of the lower jaw. In other cases the premolars are of a secant type, with a highly convex cutting-edge, and usually either serrated or obliquely grooved (Figs. 25, 26). From a certain resemblance between these secant premolars and those of some of the smaller _Macropodidæ_ it was at one time considered that we had in these mammals representatives of Diprotodont Marsupials. The great difference in the structure of the molar teeth of these forms, coupled with the circumstance that when the number of upper incisors is reduced below three it is the second in place of the first which becomes enlarged and opposed to the incisor of the lower jaw, seems to prevent the acceptation of this view. Moreover, in their peculiar structure the molars seem, on the whole, to make a nearer approximation to the teeth of _Ornithorhynchus_ than to any other known mammal; and it has accordingly been suggested that the Multituberculata may really represent an order of Prototheria. Some support is afforded to this suggestion by certain fragmentary bones from the Cretaceous of the United States, which are regarded by Marsh as parts of a coracoid and interclavicle. The peculiar character of the whole dentition of these forms indicates that if they are really Prototherians they cannot be regarded as primitive and ancestral types.

[Illustration: FIG. 25.—The right ramus of the mandible of _Plagiaulax beklesi_; from the Purbeck of Swanage. Twice natural size. _i_, Incisor; _m_, molar; _b_, coronoid process; _c_, condyle. (After Owen.)]

It would be beyond the scope of the present work to describe in detail, or even to mention the names of all the members of this group, and it will therefore suffice to refer to a few of the principal types. Of the forms with tubercular premolars the best known is the genus _Tritylodon_ (Fig. 24), which occurs typically in beds of Lower Mesozoic in South Africa, but is also known from the Trias of Stuttgart. In the Stonesfield Slate, near Oxford, which belongs to the lower part of the Jurassic system, and is separated from the Trias by the intervening Lias, a fragmentary jaw with three teeth (Fig. 27) appears to indicate an allied type, the teeth having three longitudinal ridges separated by grooves. In the Purbeck beds of Dorsetshire, forming the top of the Jurassic system, we find another member of this group, which has been described as _Bolodon_, closely allied to which is _Allodon_ of the Upper Jurassic of the United States.

[Illustration: FIG. 26.—The imperfect right ramus of the mandible of _Plagiaulax minor_; from Swanage. Four times natural size. _p_, Premolars; _m_, molars. (After Lyall.)]

The first discovery of the remains of Mesozoic mammals was made in the Keuper or Upper Trias of the Rhætian Alps in Bavaria. In 1847 Professor Pleininger of Stuttgart, while sifting some sand from the Keuper of Diegerloch and Steinenbronn, found, among an immense mass of teeth, scales, and unrecognisable fragments of skeletons of fish and saurians, two minute teeth, each with well-defined, enamelled, tuberculated crowns and distinct roots, plainly showing their mammalian character. These were considered by their discoverer to indicate a predaceous and carnivorous animal of very small size, to which he gave the name of _Microlestes antiquus_. Subsequently Mr. C. Moore discovered in a bone bed of Rhætic (topmost Trias) age, filling a fissure in the Mountain Limestone at Holwell, near Frome in Somersetshire, various isolated teeth with their crowns much worn, but apparently including both upper and lower molars and a canine, which are assigned by Sir R. Owen to Pleininger’s genus _Microlestes_, and described specifically as _M. moorei_. Under the name of _Hypsiprymnopsis rhæticus_, Professor Boyd Dawkins described a single tooth with two roots discovered in the Rhætic Marlstone at Watchet in Somersetshire. Sir R. Owen referred the latter tooth to _Microlestes_, and if its describer is right in regarding it as a much worn premolar of the type of those of _Plagiaulax_ (Fig. 25) there would be evidence that _Microlestes_ was closely allied to the latter, from the molars of which those of _Microlestes_ are scarcely distinguishable.

[Illustration: FIG. 27.—_Stereognathus oölithicus_. Fragment of jaw with three teeth (_a_, _b_, _c_), in matrix; from the Stonesfield Slate. Natural size. (After Owen.)]

_Plagiaulax_, of the Dorsetshire Purbeck (Figs. 24, 25), is at once distinguished from _Tritylodon_ by its secant premolars, which, as already mentioned, recall those of some of the _Macropodidæ_, although readily distinguished by the convexity of the cutting edge and their oblique grooving. This remarkable and highly specialised type has been the occasion of one of the most interesting discussions on the inferences which may be drawn as to the affinities and habits of an otherwise unknown animal from the structure of a small portion of its organisation which occurs in the annals of natural history—a discussion carried on with great ability, ingenuity, and wealth of illustration on both sides. Dr. Falconer maintained that it was more nearly allied to the Rat-Kangaroo (_Potorous_ or _Hypsiprymnus_) than to any other existing form, and that, as it is known that these animals feed upon grass and roots, “it may be inferred of _Plagiaulax_ that the species were herbivorous or frugivorous. I can see nothing in the character of their teeth,” he adds, “to indicate that they were either insectivorous or omnivorous.” Sir R. Owen, on the other hand, from the same materials came to the conclusion that “the physiological deductions from the above-described characteristics of the lower jaw and teeth of _Plagiaulax_ are that it was a carnivorous Marsupial. It probably found its prey in the contemporary small insectivorous mammals and Lizards, supposing no herbivorous form like _Stereognathus_ to have co-existed during the Upper Oolitic period.”

It is impossible here to give at any length the arguments by which these opposing views are respectively supported, but it may be indicated that the first-mentioned is strongly countenanced by the consideration of the following facts: (1) all existing Marsupials may be divided, so far as their dentition is concerned, into two groups—(_a_) those which have a pair of large more or less procumbent incisors close to the symphysis of the lower jaw, and rudimentary or no canines (diprotodont dentition), and (_b_) those which have numerous small incisors and large pointed canines (polyprotodont dentition); (2) the vast majority of the former group are purely vegetable feeders, and almost all of the latter are carnivorous or insectivorous; and (3) _Plagiaulax_, so far as its structure is known, shows an analogy with the former group; and, as we have no sure basis for inferences as to the habits of an unknown animal, but the knowledge of the habits of such as are known, we have no grounds for supposing that its habits differed from those forms having an analogous type of dental structure.[31]

Allied types, such as _Ctenacodon_, are also met with in the Upper Jurassic of North America; and the _Plagiaulacidæ_ also persisted into the lower part of the Eocene division of the Tertiary period; _Neoplagiaulax_ being a Tertiary form common to Europe and the United States, while _Liotomus_ and _Ptilodus_ are at present known only from the latter country.

The present group is also represented in the upper Cretaceous of the United States by _Selenacodon_ (_Meniscoëssus_ in part), _Cimoliomys_, etc. _Polymastodon_, of the Lowest or Puerco Eocene of New Mexico is the largest known form, and is characterised by the presence of only one premolar and the elongated molars. The angle of the mandible is inflected after the Marsupial fashion.

_Polyprotodont Types._—The second type of mammalian dentition found in the Mesozoic period resembles that occurring among recent Polyprotodont Marsupials—that is to say there are at least three lower incisors, the canines are well developed, and the premolars and molars are cuspidate, the number of the latter reaching in some cases to seven or eight. There has been much discussion as to the taxonomic position of these forms, and while the majority of writers admit the Marsupial affinities of at least a moiety, it has been contended that others indicate distinct ordinal groups more or less closely allied to the Insectivora. At present, however, there is no decisive evidence to support such a view. Important proof of the Marsupial affinity of one of these forms is afforded by the replacement of the teeth, which appears to be of the same nature as in the existing Marsupials, that is to say, the last premolar alone is preceded by a milk-tooth.

The most generalised forms appear to be _Dromatherium_ and _Microconodon_, from Lower Mesozoic beds in the United States, of which enlarged views of the teeth are given in Fig. 4 (1, 2), p. 31. Professor Osborn points out the extremely simple character of these teeth, and it is quite possible that these forms may prove to be _Prototheria_. There are three premolars and seven molars in the lower jaw of _Dromatherium_.

[Illustration: FIG. 28.—Reversed view of the left ramus of the mandible of _Triconodon mordax_; from the Purbeck of Swanage. Natural size. (After Owen.)]

A common form in the Purbeck of Dorsetshire is _Triconodon_ (_Triacanthodon_), in which the formula of the lower teeth is _i_ 3, _c_ 1, _p_ 4, _m_ 3-4. A lower jaw is shown in Fig. 28, and an enlarged view of a molar tooth in Fig. 4 (5). The molar teeth consist of three flattened cones placed in the same antero-posterior line, those of the upper and lower jaw being alike. _Priacodon_, of the Jurassic of the United States, is probably inseparable from _Triconodon_. In the genus _Phascolotherium_ (Fig. 29) of the Lower Jurassic Stonesfield Slate, the lower teeth may be classified as _i_ 4, _c_ 1, _p_ 3, _m_ 4, the premolars and molars being much alike. The molars approximate to the type of those of _Triconodon_, but the anterior and posterior cones are relatively smaller. Like that of the last-named genus, the mandible of _Phascolotherium_ is remarkable for the extremely low position of its articular condyle. In _Amphilestes_ (Fig. 30) of the Stonesfield Slate the molars appear to be of the same general type as those of _Phascolotherium_, but are more numerous, although their exact number cannot be determined. A somewhat different type of lower molar is displayed by the genus _Amblotherium_, of the Dorsetshire Purbeck, to which _Amphitherium_ of the Stonesfield Slate was probably allied. This type of tooth is shown in Fig. 4 (8, 9, 12) p. 31, and, as there stated, represents that modification of the tritubercular type known as the tubercular sectorial. The three primitive tritubercular cusps form what is known as the blade of the tooth, behind which there is the talon or hypocone. A similar form of molar occurs in the existing Opossums and Bandicoots. The number of lower teeth in _Amblotherium_ is _i_ 4, _c_ 1, _p_ 4, _m_ 7-8. Numerous allied types, such as _Achyrodon_ and _Dryolestes_ occur in the Upper Jurassic of Europe or the United States, while from only one side of the jaw being exposed in each case so-called genera like _Stylodon_ and _Stylacodon_ have been formed upon specimens showing the opposite side to that which is exposed in the types of _Amblotherium_ and _Amphitherium_. The only parallel among existing forms to the excessive number of molar teeth found in these Mesozoic genera occurs in the Marsupial genus _Myrmecobius_, of which a description is given in a succeeding chapter. Jaws more or less closely resembling those described under the names mentioned above are also found in the uppermost Cretaceous of the United States. A feature common to these Mesozoic mammals and _Myrmecobius_ and some other existing forms is the presence of a narrow channel on the inner side of the mandibular ramus known as the mylohyoid groove (Fig. 29).

[Illustration: FIG. 29.—Inner view of the right ramus of the mandible of _Phascolotherium bucklandi_; from the Stonesfield Slate. The outline shows the natural size. _i_, Incisors (one missing); _c_, canine; _p_, premolars; _m_, molars. The mylohyoid groove is seen near the lower border. (After Owen.)]

[Illustration: FIG. 30.—Reversed inner view of the left ramus of the mandible of _Amphilestes broderipi_; from the Stonesfield Slate. Twice natural size. The restoration of the anterior teeth is conjectural, and the condyle is placed too high. (After Owen.)]

The last type of molar dentition occurring among the Mesozoic Mammalia is that found in the lower jaws (Fig. 31), upon which the genus _Spalacotherium_ was established, the upper jaws, described as _Peralestes_, being apparently referable to the same animal. Upper and lower teeth of this form are represented in Fig. 4 (6, 7), p. 31, where they are described as typical examples of the tritubercular type of molars, the upper teeth having one inner and two outer cusps, and the reverse condition obtaining in the lower ones. This type of molar presents a marked resemblance to that found in the existing Insectivorous genus _Chrysochloris_; the number of lower teeth in _Spalacotherium_ is, however, _i_ 3, _c_ 1, _p_ + _m_ 10, by which it is widely distinguished from all the Insectivora. _Menacodon_, of the Upper Jurassic of the United States, appears to be allied to _Spalacotherium_.

[Illustration: FIG. 31.—Part of the left ramus of the mandible, viewed from the outer side, of _Spalacotherium tricuspidens_; from the Purbeck of Swanage. Twice natural size. (After Owen.)]

_Tertiary Mammals._—The more important types of Tertiary mammals will, as already mentioned, be noticed under the heads of the groups to which they are severally allied; but a few general remarks on this subject may be advantageously recorded in this chapter. In the first place, it may be observed that the comparatively scanty evidence of mammalian life hitherto yielded by the Cretaceous, coupled with the number and variety of forms approximating to the existing groups found even in the lowest Tertiary, indicates a great imperfection of the geological record. At present, indeed, we have no decisive evidence of the existence of any members of the Eutherian subclass previously to the Tertiary; but it can hardly be doubted that in some part of the world they had made their appearance before that epoch. The Eutherian mammals of the lowest Eocene, both in Europe and the United States, are of an extremely generalised type; and although many of them approximate to existing groups, they show such a combination of characters, now restricted to individual groups, as to indicate that several of the various orders into which the subclass is now divided were at that period very intimately connected. A marked feature of these early Eutherians is the prevalency of trituberculism in the dentition, not less noteworthy being the frequent occurrence of pentadactylism in the feet, while many of the individual bones were devoid of the grooves and ridges found in those of later types. By the time that we reach the upper division of the Eocene period, such as the horizon of the well-known gypsum of the Paris basin, nearly all the chief groups of mammals had become clearly differentiated from one another, although their representatives were usually more generalised than their existing allies. From this date to the later geological periods there is a gradual approximation to the types of mammalian life existing at the present day.

In addition to the features of trituberculism and pentadactylism so characteristic of the oldest known Eutherians, we may notice some other points in connection with the earlier types. Thus the older Tertiary mammals, as we have already stated, had relatively smaller and simpler brains than the later types, so that a gradual evolution in this respect may be traced from the Eocene to the Pleistocene. Again, there is a great tendency among the Eocene forms to a retention of the typical Eutherian dental formula noticed on page 25, and also to the absence of an interval, or diastema, in the dental series. Concomitantly with this feature we may notice the short crowns and simpler structure of the molar teeth of the earlier Ungulates as compared with those of to-day, of which details will be given in a later chapter. Another instance of the more generalised characters of the earlier mammals is afforded by the absence or slight development of horns, antlers, and tusks among the Ungulata. Thus the earlier Rhinoceroses were hornless, and the Deer either without antlers or with antlers of a very simple kind, while the male Swine were not furnished with the formidable tusks of the existing Wild Boars. Finally, all, or nearly all of the mammals, from the lowest Eocene of Rheims present the peculiarity of having a vertical perforation in the astragalus.

The intimate connection existing during the Middle Tertiary between many families of mammals now widely distinguished from one another may be more conveniently noted when we come to the consideration of the families in question.