88 NEARCTIC AND ORIENTAL REGIONS CHAP. patus, Sigmodon. On the other hand, the Sonoran genera Antilocapra, Cynomys, Procyon, and the Insectivora Blarina and Scapanus, 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 genera Nectogale, 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 genus Haploceros, the Rodents Erethizon, Zapus, and the family Haplodontidae. The Mole genus Condylura is 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 except Antilocapra, 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 genera Semnopithecus and Nasalis. Of Lemurs there are Loris and Nycticebus, and Tarsius, 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. THE ETHIOPIAN REGION 89 Tragulus concludes 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 are Galago, Perodicticus and Arctocebus. 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. CHAPTER III THE POSSIBLE FORERUNNERS OF THE MAMMALIA 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. The very fact that a genus Tritylodon, 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 the 1 A series of papers in the Phil. Trans. for 1888-96, of which a useful abstract by Professor Osborn was published in the American Naturalist, 1898, p. 309; see also Cambr. Nat. Hist. viii. 1901, p. 303. CHAP. III THE THEROMORPHA 91 ground 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. 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 in Dicynodon. 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 teethi 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 goodsized 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 the 1 Cf. vol. viii. p. 82. 92 TEETH OF THEROMORPHA CHAP. maxillae and the premaxillae above, is a sine qua non for 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 in Tritylodon and Diademodon the 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 reptile Cynognathus and the mammal Myrmecobius there 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. Cynognathus and 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 in Tritylodon, where they exactly resemble those of certain Multituberculata, and whose structure originally led to the placing of Tritylodon among 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 in Trirhachodon |