The Tertiary Mammalia of North America at present described belong to from eighty to one hundred genera, while those of Europe are nearly double that number; yet only eighteen genera are common to the two faunas, and of these eight are living and belong chiefly to the Pliocene period. Taking first, the genera which in America do not go back beyond the Pliocene period (ten in number), we find that eight of them in Europe go back to the Upper Miocene. These are Felis, Pseudælurus, Hipparion, Cervus, Mastodon, Elephas (in India), Castor and Hystrix; while another, Canis, goes back to the Upper Eocene and the tenth, Equus, confined to the newer Pliocene or perhaps to the Post-Pliocene in America, extends back to the older Pliocene in Europe. Of the seven European genera which are confined to the Miocene period in America, three, Hyænodon, Anchitherium, and Lophiodon go back to the Eocene in Europe; three others, Machairodus, Rhinoceros, and Aceratherium, are also of Miocene age in Europe; Amphicyon goes back to the Lower Miocene of Europe. Lophiotherium belongs to the Eocene of both countries.
If we turn now to families instead of genera, we find that the same general rule prevails. Mustelidæ (weasels), Ursidæ (bears), true Equidæ (horses), and Bovidæ (oxen &c.), go no further back in America than the Pliocene, while they all go back to the Miocene in Europe. Suidæ (swine) and Anoplotheridæ (extinct) are found in the American Miocene and in the European Eocene. Anchitheridæ (extinct) reach the Upper Eocene in America, while in Europe they range through Upper, Middle, and Lower Eocene. Cervidæ (deer) alone are Miocene in both countries. There remain two families in which America has the preeminence. Camelidæ (camels) were wonderfully developed in the American Pliocene and Miocene periods, abounding in genera and species; whereas in Europe the group only exists in the Post-Pliocene or Lower Pliocene, with one Upper Miocene species of Camelus in N. India. The Anthracotheridæ (extinct), found only in the Upper Miocene of France and India, reach even the Lower Eocene in America.
These facts may be due, in part, to a want of strict co-ordination between the Tertiary deposits of Europe and North America,—in part to the imperfection of the record in the latter country. Yet it does not seem probable that they are altogether due South America and well marked differences to imperfect knowledge; yet we find such important families as the Civets, Hyænas, Giraffes, and Hippopotami absent from America, with the Weasels, and Antelopes almost so; while America possesses almost all the Camelidæ, two peculiar orders, Dinocerata and Tillodontia, and four remarkably peculiar families, Limnotheridæ, Lemuravidæ, Oreodontidæ and Brontotheridæ. If then the facts at present known represent approximately the real time-relations of the groups in question on the two continents, they render it probable that weasels, bears, true horses, swine, oxen, sheep and antelopes, originated on the Old World continent, and were transmitted to America during some part of the Miocene period; while camels originated in the New World, and somewhere about the same time passed over to Europe. Of the extinct families common to the two hemispheres, the Anthracotheridæ alone seem to have had an American origin. Of the genera common to the two countries, almost all seem to have had a European origin, the only genera of equal date being the two rhinoceroses and three Anchitheridæ; but if the Brontotheridæ are allied to the Rhinocerotidæ, these latter may have originated in America, although now an exclusively Old World type. These conclusions are not improbable when we consider the much greater size of the Old World continents, extending far into the tropics and probably always more or less united to the tropical areas; while the evidence of the extinct mammalia themselves shows, that South America has been for the most part isolated from the northern continent, and did not take part in the development of its characteristic Tertiary fauna.
Before speculating further on this subject, it will be well to lay before our readers a summary of South American palæontology, after which we shall be in a better position to draw correct inferences from the whole body of the evidence.
South America.
Unfortunately, our knowledge of the interesting fossil fauna of this continent, is almost wholly confined to the Post-Pliocene and Pliocene periods. A few remains have been discovered in deposits believed to be of Eocene age, but nothing whatever representing the vast intervening period, so rich in peculiar forms of animal life both in North America and Europe.
Fauna of the Brazilian caves.—What we know of the Post-Pliocene period is chiefly due to the long-continued researches of Dr. Lund in the caves of Central Brazil, mostly situated in a district near the head waters of the San Francisco river in the Province of Minas Geraes. The caves are formed in limestone rocks, and are so numerous that Dr. Lund visited thousands, but only sixty contained bones in any quantity. These caves have a floor of reddish earth, often crowded with bones. In one experiment, half a cubic foot of this earth contained jaws of 400 opossums, 2,000 mice, besides remains of bats, porcupines and small birds. In another trial, the whole of the earth in a cavern was carried out for examination, amounting to 6,552 firkins; and, from a calculation made by measured samples, it was estimated to contain nearly seven millions of jaw-bones of cavies, opossums, porcupines, and mice, besides small birds, lizards, and frogs. This immense accumulation is believed to have been formed from the bodies of animals brought into the cavern by owls; and, as these are unsocial birds, the quantity found implies an immense lapse of time, probably some thousands of years. More than 100 species of Mammalia, in all, were obtained in these caves. Some were living species or closely allied to such; but the majority were extinct, and a considerable number, about one-fourth, belonged to extinct genera, or genera not now inhabiting South America. Stone implements and human remains were found in several of the caves with extinct animals. The following enumeration of these remains is from the corrected list of M. Gervais.
Primates.—Extinct species of Cebus, Callithrix, and Jacchus—South American genera of monkeys; with an extinct genus, Protopithecus—an animal of large size but belonging to the American family Cebidæ.
Chiroptera.—Species belonging to the South American Phyllostomidæ, and to two South American genera of other families.
Carnivora.—Five species of Felis, some allied to living animals, others extinct; a species of the widespread extinct genus Machairodus; and a small species referred to Cynælurus, the genus containing the hunting leopard now found only in Africa and India. Canidæ are represented by Canis and Icticyon (a living Brazilian species of the latter genus), and the extinct genus Speothos. Mustelidæ are represented by extinct species of the South American genera Mephitis and Galictis. Procyonidæ, by a species of Nasua. Ursidæ, by Arctotherium, a genus closely resembling, if not identical with, that containing the "spectacled bear" of Chili.