to effect an entrance into North America, as is exemplified by the
occurrence there of ground-sloths and glyptodonts. Simultaneously
a large immigration of northern forms took place into Neogaea;
these invaders from Arctogaea, including cats and sabre-toothed
tigers, bears, fox-like dogs, raccoons, llamas, horses, tapirs, deer,
mastodons and perhaps opossums. While representatives of most
of these invaders have persisted to the present day, some groups,
such as horses and mastodons, have entirely disappeared, as has
also a large portion of the autochthonous fauna. Here it may be
well to notice that the evidence for the Insulation of Neogaea during
a large portion of the Tertiary period does not by any means rest
only on that supplied by mammals. C. H. Gilbert and E. C. Starks,[1]
for instance, in a work on the fishes of the two sides of the isthmus
of Darien, wrote as follows: "The ichthyological evidence is over-whelmingly in favour of the existence of a former open communication between the two oceans, which must have become closed at a
period sufficiently remote from the present to have permitted the
specific differentiation of a very large majority of the forms
involved. ... All evidence concurs in fixing the date of that
connexion at some time prior to the Pleistocene, probably in the
early Miocene." This, it will be observed, agrees almost precisely
with the conclusions drawn from the fossil mammalian faunas of
North and South America, which indicate that land-communication
between those two continents was interrupted during a considerable portion of the Tertiary epoch, and only re-established (or [?]
established for the first time) either towards the close of the Miocene
or the early part of the Pliocene epoch.
The South American mammalian fauna, as we now know it, is, then, a complex, consisting of an original autochthonous element and of a large foreign infusion from the north. As to the origin of the latter, there is no difficulty; but some degree of obscurity still prevails with regard to the source of the autochthonous fauna. According to Prof. Eigenmann's interpretation of the evidence of the fresh-water fishes the early Tertiary Atlantic "Hellenis" may have been in contact with Guiana on the one side and tropical Africa on the other. That such a connexion did really exist in Tertiary times is the conclusion reached by Dr C. W. Andrews,[2] as the result of his studies of the Tertiary vertebrate fauna of the Fayum district of Egypt, as expressed in the following passage: "Speaking generally, it appears that (1) probably in Jurassic times Africa and South America formed a continuous land-mass; (2) in the Cretaceous period the sea encroached southwards over this land, forming what is now the South Atlantic. How far this depression had advanced southwards at the end of the Secondary period is not clear, but it appears certain that the final separation of the two continents did not take place till Eocene times, and that there may have been a chain of islands between the northern part of Africa and Brazil which persisted even till the Miocene."
By this route, as was suggested considerably earlier by Prof. W. B. Scott and subsequently by the present writer, Neogaea may have received a considerable portion pi its autochthonous mammal-fauna. Further reference to this point is made later; but it may be added that the evidence of the land-faunas is supplemented by that of the shallow-water marine faunas on the two sides of the Atlantic, which present a striking similarity.
In an address to the British Association at the meeting in 1905 in South Africa Mr G. A. Boulenger expressed himself, however, as by no means satisfied with the evidence of a Tertiary connexion between Africa and South America. "It is undeniable," he observed, "that the hypothesis of a South Atlantic land-communication in the Eocene has much in its favour, and when this is really established, all difficulty in explaining the distribution of the Cichlidae will have disappeared. In the meanwhile ... we must not construct bridges without being sure of our points of attachment." In this connexion it may be mentioned that those who explain the distribution of certain forms of life by the former existence of a land-connexion between the southern continents ^by way of "Antarctica," have attached some importance to the existence of fishes of the genus Galaxias in the freshwaters of New Zealand, Australia, South America and the Cape. This evidence has been shattered by Mr Boulenger's description (in a memoir of the fishes of the Congo) of a marine representative of the genus in question from the Southern Ocean.
For the zoological subregions of Neogaea the reader must refer, as in the case of most of the other regions, to special works on zoological distribution.
As Arctogaea includes the whole of the rest of the land-surface of the globe (with the exception of Antarctica) it is almost impossible to give Arctogaea. any general diagnosis even of its mammalian fauna. It may be mentioned, however, that at the present day monotremes are wholly wanting, while marsupials are represented only by one or two species 01 opossums (Didelphys) in North America and by cuscuses (Phalanger) in the Austro-Malayan subregion. The true or typical Edentata are, if we except late wanderers from Neogaea into North America, absent from this realm at the present date and during the Pleistocene; the alleged occurrence of a ground-sloth in the Pleistocene of Madagascar being probably due to a misinterpretation. On the other hand, this region, and more especially its eastern half, is the great home of the ungulate mammals. Indeed rhinoceroses may be considered absolutely characteristic of Arctogaea, since at one time or another they have ranged over the whole area, except Madagascar, and are quite unknown elsewhere. The modern land Carnivora are likewise an essentially Arctogaeic group, which only found its way into Neogaea at a comparatively recent epoch; and the realm may be said to have been the birthplace of most of the higher groups of placental mammals. The tortoises of the family Trionychidae form an exclusively Arctogaean group, once ranging all over the realm, although long since extinct in Europe.
If Madagascar be excepted, the Ethiopian region (or Ethiopia) is the most distinct of all the regions of Arctogaea. So distinct is it that, on the evidence of the distribution of moths, Ethiopian region. Dr H. S. Packard[3] has suggested that it should be separated from Arctogaea to form a realm by itself, under the name of Apogaea. The mammalian fauna, even exclusive of the Tertiary one of Egypt, does not, however, countenance such a separation. By Sclater and Wallace, Madagascar was included in the Ethiopian region, but that island was subsequently made a region by itself by Dr Blanford. This separation of Madagascar to form a Malagasy region has met with general acceptance; but in the opinion of Mr R. I. Pocock,[4] who bases his conclusion on the distribution of trapdoor-spiders (which in other respects accords curiously well with that of mammals), it is not justified. The mammalian evidence appears, however, to be overwhelmingly strong in its favour; and it also receives support from reptilian distribution. All are agreed that the Ethiopian region should exclude that part of Africa which lies, roughly speaking, north-ward of the tropic of Cancer. By Sclater and Wallace the region was taken to include that portion of Arabia lying to the south of the same tropic; but Mr Pocock[5] has pointed out that this separation of Arabia into two portions is not supported by the distribution of scorpions, and he would refer the whole of it to the Mediterranean transitional region. The occurrence of a tahr-goat (Hemitragus) in Oman lends some support to this proposal since that genus has no representative in Africa, and occurs else-where only in the Himalaya and the mountains of southern India. Other writers have not accepted Mr Pocock's emendation; and the reference of the northern half of Arabia to the Mediterranean and of the southern half to the Ethiopian region is usually followed. The area is admittedly a meeting-ground of at least two faunas.
Discoveries in the Fayum district of Egypt have conclusively proved that during the early (Eocene) part of the Tertiary period Ethiopia was a great centre of development, and subsequently of dispersal, instead of having received (as was formerly supposed) the whole of its higher modern mammalian fauna from the north. In this Ethiopian centre were developed the ancestors of the elephants (Proboscidea) and of the hyraxes (Hyracoidea); the latter group being represented by species of much larger size than the existing forms, some of the former of which ranged into southern Europe during the later Tertiary. It was also the home of a peculiar subordinal group of ungulates (Barypoda), typified by Arsinoitherium, and may likewise have been the birthplace of the swine (Suidae) as the earliest known representative of that group (Geniohyus) occurs in the Fayum Eocene. The hippopotamuses (Hippopotamidae), which appear to be descended from the Tertiary Anthracotheriidae, may likewise be of Ethiopian origin, and the same may turn out to be the case with the giraffe group (Giraffidae) although definite evidence with regard to the latter point is wanting.
The occurrence of an ostrich-like flightless bird in the Fayum Eocene—the oldest known representative of that group—is suggestive that the Ratitae originated in Ethiopia, which would accord well with their distribution both in the present and the past. A giant land-tortoise (Testudo) is likewise known from the Fayum beds, and as it is allied to the species recently or still inhabiting Madagascar and the Mascarene islands, there is a strong probability that Ethiopian Africa was likewise the centre of development and dispersal of that group.
Turning to its existing mammalian fauna, Ethiopia possesses a number of peculiar family or generic groups, and is also nearly equally well characterized by the absence of others. As remarked by Wallace, one of its characteristics is the great number of species of large size. Among the Primates, it is the home of the typical group of the Negroid branch of the human species, whose northern limits coincide approximately with the boundary of the region itself, being replaced in northern Africa by races of the Caucasian stock. Gorillas and chimpanzees (Anthropopithecus) are peculiar to the region, as are also baboons (Papio and Theropithecus), if southern Arabia be included. Monkeys abound, and although in most cases nearly allied to those of the Oriental region, are generically
- ↑ Mem. Californian Academy, vol. iv . (1904).
- ↑ Catalogue of the Tertiary Vertebrata of the Fayum (London, 1906).
- ↑ Science, ser. 2, vol. xix. p. 221 (1904). Dr Packard groups Notogaea and Neogaea in a single realm under the name Antarctogaea. Some other writers, such as Dr H. Gadow, take Notogaea to include all the three southern continents, and employ the term Arctogaea for the rest of the world.
- ↑ Proc. Zool. Soc, London, 1903, pp. 340-368.
- ↑ Natural Science, vol. iv., pp. 353-364 (1894).
