An analogous statement may be made with regard to the sea-cows, or Sirenia, which appear to be derivates from the great herbivorous order of Ungulata, and might consequently be included in that group, as indeed has been already done in Dr Max Weber’s classification. It is with the proboscidean suborder of the Ungulata to which the Sirenia are most nearly related; the nature of this relationship being described by Dr Andrews as follows:—
“In the first place, the occurrence of the most primitive Sirenians with which we are acquainted in the same region as the most generalized proboscidean, Moeritherium, is in favour of such a view, and this is further supported by the similarity of the brain-structure and, to some extent, of the pelvis in the earliest-known members of the two groups. Moreover, in the anatomy of the soft-parts of the recent forms there are a number of remarkable points of resemblance. Among the common characters may be noted the possession of: (1) pectoral mammae; (2) abdominal testes; (3) a bifid apex of the heart; (4) bilophodont molars with a tendency to the formation of an additional lobe from the posterior part of the cingulum. The peculiar mode of displacement of the teeth from behind forwards in some members of both groups may perhaps indicate a relationship, although in the case of the Sirenia the replacement takes place by means of a succession of similar molars, while in the Proboscidea the molars remain the same numerically, but increase greatly in size and number of transverse ridges.”
These and certain other facts referred to by the same author point to the conclusion that not only are the Sirenia and the Proboscidea derived from a single ancestral stock, but that the Hyracoidea—and so Arsinöitherium—are also derivatives from the same stock, which must necessarily have been Ethiopian.
Of the other suborders of ungulates, the Toxodontia and Litopterna are exclusively South American, and while the former may possibly be related to the Hyracoidea and Barypoda, the latter is perhaps more nearly akin to the Perissodactyla. The Amblypoda, on the other hand, are perhaps not far removed from the ancestral Proboscidea, which depart comparatively little from the generalized ungulate type. The latter is represented by the Eocene Condylarthra, which undoubtedly gave rise to the Perissodactyla and Artiodactyla, and probably to most, if not all, of the other groups. The Condylarthra, in their turn, approximate closely to the ancestral Carnivora, as they also do in some degree to the ancestral Primates. As regards the latter order, although we are at present unacquainted with all the connecting links between the lemurs and the monkeys, there is little doubt that the ancestors of the former represent the stock from which the latter have originated. C. D. Earle, in the American Naturalist for 1897, observes that “so far as the palaeontological evidence goes it is decidedly in favour of the view that apes and lemurs are closely related. Beginning with the earliest known lemur, Anaptomorphus, this genus shows tendencies towards the anthropoids, and, when we pass up into the Oligocene of the Old World, Adapis is a decidedly mixed type, and probably not far from the common stem-form which gave origin to both suborders of the Primates. In regard to Tarsius, it is evidently a type nearly between the lemurs and apes, but with many essential characters belonging to the former group.”
Distribution.—For an account of the “realms” and “regions” into which the surface of the globe has been divided by those who have made a special study of the geographical distribution of animals, see [Zoological Distribution]. For the purposes of such zoo-geographical divisions, mammals are much better adapted than birds, owing to their much more limited powers of dispersal; most of them (exclusive of the purely aquatic forms, such as seals, whales, dolphins and sea-cows) being unable to cross anything more than a very narrow arm of the sea. Consequently, the presence of nearly allied groups of mammals in areas now separated by considerable stretches of sea proves that at no very distant date such tracts must have had a land-connexion. In the case of the southern continents the difficulty is, however, to determine whether allied groups of mammals (and other animals) have reached their present isolated habitats by dispersal from the north along widely sundered longitudinal lines, or whether such a distribution implies the former existence of equatorial land-connexions. It may be added that even bats are unable to cross large tracts of sea; and the fact that fruit-bats of the genus Pteropus are found in Madagascar and the Seychelles, as well as in India, while they are absent from Africa, is held to be an important link in the chain of evidence demonstrating a former land-connexion between Madagascar and India.
There is another point of view from which mammals are of especial importance in regard to geographical distribution, namely their comparatively late rise and dispersal, or “radiation,” as compared with reptiles.
As regards terrestrial mammals (with which alone we are at present concerned), one of the most striking features in their distribution is their practical absence from oceanic islands; the only species found in such localities being either small forms which might have been carried on floating timber, or such as have been introduced by human agency. This absence of mammalian life in oceanic islands extends even to New Zealand, where the indigenous mammals comprise only two peculiar species of bats, the so-called Maori rat having been introduced by man.
One of the leading features in mammalian distribution is the fact that the Monotremata, or egg-laying mammals, are exclusively confined to Australia and Papua, with the adjacent islands. The marsupials also attain their maximum development in Australia (“Notogaea” of the distributionists), extending, however, as far west as Celebes and the Moluccas, although in these islands they form an insignificant minority among an extensive placental fauna, being represented only by the cuscuses (Phalanger), a group unknown in either Papua or Australia. Very different, on the other hand, is the condition of things in Australia and Papua, where marsupials (and monotremes) are the dominant forms of mammalian life, the placentals being represented (apart from bats, which are mainly of an Asiatic type) only by a number of more or less aberrant rodents belonging to the mouse-tribe, and in Australia by the dingo, or native dog, and in New Guinea by a wild pig. The dingo was, however, almost certainly brought from Asia by the ancestors of the modern natives; while the Papuan pig is also in all probability a human introduction, very likely of much later date. The origin of the Australasian fauna is a question pertaining to the article [Zoological Distribution]. The remaining marsupials (namely the families Didelphyidae and Epanorthidae) are American, and mainly South and Central American at the present day; although during the early part of the Tertiary period representatives of the first-named family ranged all over the northern hemisphere.
The Insectivora (except a few shrews which have entered from the north) are absent from South America, and appear to have been mainly an Old World group, the only forms which have entered North America being the shrew-mice (Soricidae) and moles (Talpidae). The occurrence of one aberrant group (Solenodon) in the West Indies is, however, noteworthy. The family with the widest distribution is the Soricidae, the Talpidae being unknown in Africa. The tree-shrews (Tupaiidae) are exclusively Asiatic, whereas the jumping-shrews (Macroscelididae) are equally characteristic of the African continent. Madagascar is the sole habitat of the tenrecs (Centetidae), as is Southern Africa of the golden moles (Chrysochloridae). It is, however, important to mention that an extinct South American insectivore, Necrolestes, has been referred to the family last mentioned; and even if this reference should not be confirmed in the future, the occurrence of a representative of the order in Patagonia is a fact of considerable importance in distribution.
The Rodentia have a wider geographical range than any other order of terrestrial mammals, being, as already mentioned, represented by numerous members of the mouse-tribe (Muridae) even in Australasia. With the remarkable exception of Madagascar, where it is represented by the Nesomyidae, that family has thus a cosmopolitan distribution. Very noteworthy is the fact that, with the exception of Madagascar (and of course Australia) the squirrel family (Sciuridae) is also found in all parts of the world. Precisely the same may be said of the hares, which, however, become scarce in South America. On the other hand, the scaly-tailed squirrels (Anomaluridae), the jumping-hares (Pedetidae), and the strand-moles (Bathyergidae) are exclusively African; while the sewellels (Haplodontidae) and the pocket-gophers (Geomyidae) are as characteristically North American, although a few members of the latter have reached Central America. The beavers (Castoridae) are restricted to the northern hemisphere, whereas the dormice (Gliridae) and the mole-rats (Spalacidae) are exclusively Old World forms, the latter only entering the north of Africa, in which continent the former are largely developed. The jerboa group (Dipodidae, or Jaculidae) is also mainly an Old World type, although its aberrant representatives the jumping-mice (Zapus) have effected an entrance into Arctic North America. Porcupines enjoy a very wide range, being represented throughout the warmer parts of the Old World, with the exception of Madagascar (and of course Australasia), by the Hystricidae, and in the New World by the Erethizontidae. Of the remaining families of the Simplicidentata, all are southern, the cavies (Caviidae), chinchillas (Chinchillidae), and degus (Octodontidae) being Central and South American, while the Capromyidae are common to southern America and Africa, and the Ctenodactylidae are exclusively African. The near alliance of all these southern families, and the absence of so many Old World families from Madagascar form two of the most striking features in the distribution of the order. Lastly, among the Duplicidentata, the picas (Ochotonidae or Lagomyidae) form a group confined to the colder or mountainous regions of the northern hemisphere.