independent region, is best ranked (on Prof. Max Weber's scheme) as a transitional zone between the Nearctic subregion of the Holarctic region of Arctogaea, on the one hand, and Neogaea on the other. Sonoran transition-zone. Its fauna is indeed essentially a mixture of Nearctic and Neotropical types (inclusive of those originally indigenous to Neogaea and such as are properly immigrants from the north) together with a few more or less wholly endemic forms. Marsupials are represented by opossums (Didelphys), and armadillos by Tatusia. Peccaries (Dicotyles, or "Tagassu") make their appearance, but the fork-antlered deer (Mazama) are mainly of the northern type. The pronghorn antelope, representing the family Antilocapndae, may be regarded as mainly a Sonoran type; and the same may be said with regard to the pocket-gophers (Geomyidae) and kangaroo-rats (Heteromyidae), some of the genera of which are peculiar to this area. Among cricetine rats, Rhithrodontomys, Sigmodon and Neofiber are characteristically Sonoran. In the Carnivora the three genera of skunks, Mephitis, Conepatus and Spilogale are represented, as are the three raccoon genera Procyon, Nasua and Bassariscus; the third in each case being mainly confined to this zone. Scalops and Notiosorex among the Insectivora are almost exclusively Sonoran, while Blarina and Scapanus here attain their maximum development.
Omitting all reference to birds, it may be mentioned that among reptiles the poisonous lizards (Helodermatidae) and the chelonian family Dermatemydidae, with two or three genera, are almost or quite exclusively Sonoran; while such southern types as iguanas become more numerous than in the north. The Mississippi alligator is also Sonoran, although with a congener in China. Among Batrachians the salamanders of the genera Cryptobranchus (if distinct from the Old World Megalobatrachus), Amphiuma, Typhlomolge, Necturus and Siren, together with Scaphiopus and certain other genera of the frog and toad groups, are exclusively American and mainly Sonoran. The axolotls, or tiger-salamanders (Amblystoma), are also a group attaining a great development in this zone, although also ranging to the northward, and likewise recurring in south-eastern Asia.
So far as birds and mammals are concerned, the Antarctic continent can scarcely represent a distinct zoological province; its fauna consisting mainly of certain peculiar generic groups of sealsAntarctica. (which are at least as much inhabitants of the pack-ice as of the continent) together with a number of species of penguins a group common to the extremities of the three great southern continents and certain other birds.
The zoological provinces already discussed are based on the present distribution of mammals and birds (see further Mammalia and Bird). The distribution of reptiles and batrachians, like that of fresh-water fishes, by no means accords with that indicated by mammals and birds, Reptilian and Amphibian regions.probably owing to the earlier radiation, or rather radiations, of the former groups, and different zoological provinces have been proposed to indicate their distribution, as is more fully recorded in the articles Batrachia and Reptiles.
From the evidence of batrachian distribution Dr H. Gadow adopts the view that the world may be divided into a northern and a southern half, for the former of which the name Arctogaea is adopted, while Notogaea is used for the latter. It would, however, be much better if entirely new terms were proposed, since the use of the former in a sense different from that in which they are now employed in the case of mammals is liable to create confusion. Notogaea, which (in this sense) includes Australia, Polynesia and the Neotropical region, is characterized by the presence of that family of frogs known as the Cystignathidae, combined with the preponderance of the section Arcifera, the representatives of which form nearly 90 per cent, of the Anura (frogs and toads) inhabiting this half of the globe. Arctogaea, on the other hand, is characterized by the absence of Cystignathidae, and is divisible into two main provinces, or regions, respectively termed Periarctic and Palaeotropical. Of these latter, the Penairtic province is characterized by the presence of salamanders and their allies (Urodela), which are indeed almost peculiar to the area. It is divisible into the (1) Western Palaearctic, (2) Eastern Palaearctic and (3) Nearctic subregions, of which the first two approximately correspond to the Palaearctic subregion plus the Mediterranean transition-zone of the mammalian scheme, while the third represents the combined Nearctic and Sonoran areas. The Palaeotropical region has few salamanders or newts; but possesses caecilians (Apoda) which are wanting in the Periarctic; and includes the Ethiopian, Oriental, Malagasy and Austro-Malayan areas of the system based on mammalian distribution, together with Melanesia. Whether the region should be broken up into the four above-named divisions, or regarded as indivisible, may be a matter of opinion; but if such divisions be adopted, they have no approach to the corresponding mammalian areas, the Oriental tract not even possessing a peculiar subfamily. It is thus evident, as might have been expected, that the zoological provinces indicated by the distribution of batrachians are in no wise comparable to those based on mammalian evidence.
As regards reptiles, Dr H. Gadow has remarked that since the various orders have come into existence at very different geological periods, and have each followed their own line of dispersal, no scheme of zoological distribution can be formulated for the class as a whole. In the case of the crocodilian order little information of importance can be gathered from its present distribution, seeing that representatives of all the principal groups occur together in the older Tertiary deposits of Europe. It is, however, of some interest to note that caimans (Caiman) are restricted to Neogaea (in the mammalian sense); while the long-snouted Gavialis and Tomistoma are equally confined to the Oriental region. An important feature in the present distribution of chelonians is the restriction of the section Pleurodira to the southern hemisphere (inclusive of all the three southern continents, as well as Madagascar), and thus in no wise corresponding to the "batrachian Notogaea." The value of this feature in chelonian distribution is to some extent discounted by the occurrence of Pleurodira in the northern hemisphere during the Eocene period.
In regard to lizards (Lacertilia), Dr Gadow remarks that their distribution does not support the division of the land-areas of the globe into a northern and a southern half; the marked distinctness of the lacertian fauna of the New from that of the Old World pointing in the same direction. On the other hand, lizards countenance the view "that the Palaeotropical region is but the tropical and therefore richer continuation of the now impoverished Palaearctic subregion."
The present distribution of snakes, according to the same naturalist, indicates that the Ophidia are a group of late radiation, while it further suggests that some of the divisions adopted in classification are not natural ones. Perhaps the most important fact is that the two families, Boidae (boas and pythons) and Typhlopidae (blind burrowing-snakes), which alone retain vestiges of hind-limbs, occur all over the tropical zone, while certain allied families are found in portions of the same. The restriction of true vipers (Viperinae) to the Palaeotropical and Periarctic areas of the batrachian distributional scheme is a noteworthy fact. The pit-vipers (Crotalinae), however, may perhaps be presumed to have originated in the Palaearctic area, whence they reached and spread all over America, although they were unable to enter either Africa or Australia. The absence of all venomous snakes (which abound in Africa and India) from Madagascar, is a fact harmonizing with mammalian distribution, so far as the peculiarity of the fauna of that island is concerned.
For a fuller account of the distribution of fishes, both fresh-water and marine, see Ichthyology; here it will suffice to refer to a zoo-geographical scheme, based on the present distribution of the freshwater families, adopted by Dr A. C. L. G. Günther. According to this scheme,the land-surface of the earth is divided into three parallel zones, the Northern, the Equatorial and the Southern. Distribution of fresh-water fishes.The Northern zone is characterized by the presence of sturgeons (Acipenseridae), the paucity of cat-fishes (Siluridae), and the abundance of carps (Cyprinidae), the salmon-tribe (Salmonidae), and pike (Esocidae). This zone falls naturally into an eastern and a western section. In the first, forming the Palaearctic, or Europe-Asiatic region, bony-pikes (Lepidosteidae) are lacking, while loaches (Cobitidae) and barbels (Barbus) abound. In the second section, constituting the Nearctic or North American region, bony-pikes, bow-fins (Amiidae), and the family Catostomatidae, all of which are unknown in the Palaearctic area, are present, while loaches and barbels are lacking.
The Equatorial zone is divisible into two provinces: an Eastern, characterized by the presence of carps and of the family (Anabantidae) typified by the well-known "climbing-perch," and a Western province, in which these two groups are wanting. The Eastern province includes an Indian (Oriental) region, in which lung-fishes (Dipnoi) are absent, while serpent-heads (Ophiocephalidae), Mastacembelidae and loaches are numerous; and an African (Ethiopian) region, distinguished by the presence of lung-fishes, bichirs (Crossopterygii), and proboscis-fishes (Mormyridae), the abundance of chromids (Chromididae) and characinids (Characinidae), and the lack of loaches. The Western Equatorial province is likewise divisible into two regions, namely, a Tropical American (Neotropical), distinguished by the possession of lung-fishes and electric eels and the abundance of chromids and characinids, and a Tropical Pacific (Australian and Hawaiian) characterized by the presence of Dipnoi (widely different, however, from those of Africa and South America, which are nearly related), and the lack of chromids and characinids.
Lastly, the Southern zone is characterized by the absence of carps and the scarcity of cat-fishes, while the salmon tribe is replaced by the Haplochitomdae and pike by the Galaxiidae (since this classification was proposed a marine Galaxias has been discovered). This zone includes only a single (Antarctic) region, embracing Tasmania and the south-eastern point of Australia, New Zealand and Patagonia and Tierra del Fuego.
It will be seen that the present distribution of fishes, although showing certain features in common, by no means accords as a whole with that of mammals. Indeed, it is suggestive of the period of the earth's history when there was an extensive and more or less continuous belt of equatorial land.
