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Europe and North America; while a still more ancient form of large size is found in the Lower Eocene of France and England, indicating an immense antiquity for this group of Mammalia. There are many other extinct forms connecting these with the Palæotheridæ, already noticed in chapter vi. (vol. i. pp. 119-125).
FAMILY 45.—RHINOCEROTIDÆ. (1 Genus, 9 Species.)
Living Rhinoceroses are especially characteristic of Africa, with Northern and Malayan India. Four or perhaps five species, all two-horned, are found in Africa, where they range over the whole country south of the desert to the Cape of Good Hope. In the Oriental region there are also four or five species, which range from the forests at the foot of the Himalayas eastwards through Assam, Chittagong, and Siam, to Sumatra, Borneo and Java. Three of these are one-horned, the others found in Sumatra, and northwards to Pegu and Chittagong, two-horned. The Asiatie differ from the African species in some dental characters, but they are in other respects so much alike that they are not generally considered to form distinct genera. In his latest catalogue however (1873), Dr. Gray has four genera, Rhinoceros (4 species), and Ceratorhinus (2 species), Asiatic; Rhinaster (2 species), and Ceratotherium (2 species), African.
Extinct Rhinocerotida.-Numerous species of Rhinoceros ranged over Europe and Asia from the Post-pliocene back to the Upper Miocene period, and in North America during the Pliocene period
only. The hornless Acerotherium is Miocene only, in both countries. Other genera are, Leptodon from Greece, and Hyracodon from Nebraska, both of Miocene age. More than 20 species of extinct rhinoceroses are known, and one has even been found at an altitude of 16,000 feet in Thibet.
FAMILY 46.-HIPPOPOTAMIDÆ. (1 Genus, 2 Species.)
The Hippopotamus inhabits all the great rivers of Africa; a distinct species of a smaller size being found on the west coast, and on some of the rivers flowing into Lake Tchad.
Fossil Hippopotami.—Eight extinct species of Hippopotamus are known from Europe and India, the former Post-pliocene or Pliocene, the latter of Upper Miocene age. They ranged as far north as the Thames valley. An extinct genus from the Siwalik Hills, Merycopotamus, according to Dr. Falconer connects Hippopotamus with Anthracotherium, an extinct form from the Miocene of Europe, allied to the swine.
FAMILY 47.-SUIDÆ. (5 Genera, 22 Species.)
The Swine may be divided into three well-marked groups, from peculiarities in their dentition. 1. The Dicotylinæ, or
peccaries (1 genus, Dicotyles). These offer so many structural differences that they are often classed as a separate family. 2. The true swine (3 genera, Sus, Potamochoerus, and Babirusa); and, 3. The Phacocherinæ, or wart hogs (1 genus, Phacocherus). These last are also sometimes made into a separate family, but they are hardly so distinct as the Dicotylinæ.
The Peccaries (2 species), are peculiar to the Neotropical re-. gion, extending from Mexico to Paraguay. They also spread northwards into Texas, and as far as the Red River of Arkansas, thus just entering the Nearctic region; but with this exception swine are wholly absent from this region, forming an excellent feature by which to differentiate it from the Palæarctic.
Sus (14 species), ranges over the Palæarctic and Oriental regions and into the first Australian sub-region as far as New Guinea ; but it is absent from the Ethiopian region, or barely enters it on the north-east. Potamochorus (3 species ?), is wholly Ethiopian (Plate V. vol. i. p. 278). Babirusa (1 species), is confined to two islands, Celebes and Bouru, in the first Australian sub-region.
Phacochorus (2 species), ranges over tropical Africa from Abyssinia to Caffraria.
Dr. J. E. Gray divides true swine (Sus) into 7 genera, but it seems far better to keep them as one.
Fossil Suidæ.—These are very numerous. Many extinct species of wild hog (Sus), are found in Europe and North India, ranging back from the Post-pliocene to the Upper Miocene formations. In the Miocene of Europe are numerous extinct genera, Bothriodon, Anthracotherium, Palæochorus, Hyotherium, and some others; while in the Upper Eocene occur Cebochærus, Choropotamus, and Acotherium,—these early forms having more resemblance to the peccaries.
None of these genera are found in America, where we have the living genus Dicotyles in the Post-pliocene and Pliocene deposits, both of North and South America; with a number of extinct genera in the Miocene. The chief of these are, Elotherium, Perchorus, Leptochorus, and Nunohyus, all from Dakota, and Thinohyus, from Oregon. One extinct genus, Platygonus, closely allied to Dicotyles, is found in the Post-pliocene of Nebraska
Oregon, and Arkansas. Elotherium is said to be allied to the peccary and hippopotamus. Hyopotamus, from the Miocene of Dakota, is allied to Anthracotherium, and forms with it (according to Dr. Leidy) a distinct family of ancestral swine.
It thus appears, that the swine were almost equally well represented in North America and Europe, during Miocene and Pliocepe times, but by entirely distinct forms; and it is a remarkable fact that these hardy omnivorous animals, should, like the horses, have entirely died out in North America, except a few peccaries which have preserved themselves in the sub-tropical parts and in the southern continent, to which they are comparatively recent emigrants. We can hardly have a more convincing proof of the vast physical changes that have occurred in the North American continent during the Pliocene and Post-pliocene epochs, than the complete extinction of these, along with so many other remarkable types of Mammalia.
According to M. Gaudry, the ancestors of all the swine, with the hippopotami and extinct Anthracotherium, Merycopotamus, and many allied forms,—are the Hyracotherium and Pliolophus, both found only in the London clay belonging to the Lower Eocene formation.
FAMILY 48.—CAMELIDÆ. (2 Genera, 6 Species).
The Camels are an exceedingly restricted group, the majority of the species now existing only in a state of domestication. The genus Camelus (2 species), is a highly characteristic desert form
of the Palæarctic region, from the Sahara to Mongolia as far as Lake Baikal. Auchenia (4 species), comprehending the Llamas and Alpacas, is equally characteristic of the mountains and deserts of the southern part of South America. Two species entirely domesticated inhabit the Peruvian and Bolivian Andes; and two others are found in a wild state, the vicuna in the Andes of Peru and Chili (Plate XVI. vol. ii. p. 40), and the guanaco over the plains of Patagonia and Tierra del Fuego.
Extinct Camelidæ.—No fossil remains of camels have been found in Europe, but one occurs in the deposits of the Siwalik Hills, usually classed as Upper Miocene, but which some naturalists think are more likely of Older Pliocene age. Merycotherium, teeth of which have been found in the Siberian drift, is supposed to belong to this family.
In North America, where no representative of the family now exists, the camel-tribe were once abundant. In the Post-pliocene deposits of California an Auchenia has been found, and in those of Kansas one of the extinct genus Procamelus. In the Pliocene period, this genus, which was closely allied to the living camels, abounded, six or seven species having been described from Nebraska and Texas, together with an allied form Homocamelus. In the Miocene period different genera appear,—Pebrotherium, and Protomeryc,—while a Procamelus has been found in deposits of this age in Virginia.
In South America a species of Auchenia has been found in the caves of Brazil, and others in the Pliocene deposits of the pampas, together with two extinct genera, Palæolama and Camelotherium.
We thus find the ancestors of the Camelidæ in a region where they do not now exist, but which is situated so that the now widely separated living forms could easily have been derived from it. This case offers a remarkable example of the light thrown by palæontology on the distribution of living animals; and it is a warning against the too common practice of assuming the direct land connection of remote continents, in order to explain similar instances of discontinuous distribution to that of the present family.