The Neotropical, which is the only region of the Neogæic realm, comprises the West Indian islands, all of Central and South America and the lowlands of Mexico, extending a short distance into southeastern Texas. Of its four subregions, the most typical is (1) the Brazilian, which includes not only Brazil, but all of South America east of the Andes and as far south as Paraguay, and is a vast area of tropical forests. (2) The Chilian subregion takes in the west coast, the high Andes and the southern end of the continent, south of the Brazilian subregion; it is a country chiefly of open plains and high mountains, and a few deserts, of which South America has less than any other continent, except Europe, which has none. (3) The Central American subregion reaches from the Isthmus of Panama to Mexico, the lowlands of which are included and even a small portion of southeastern Texas. (4) The West Indian subregion includes all the islands of that archipelago, except Trinidad, which is a fragment of the continent, detached at a comparatively recent date; the southern extremity of Florida also belongs to this subregion.

The two subregions into which continental South America is divided are not altogether satisfactory and will doubtless require change when the distribution of South American mammals has been more accurately determined.

“Richness combined with isolation is the predominant feature of Neotropical Zoölogy, and no other region can approach it in the number of its peculiar family and generic types” (Wallace). Just as North America has received many immigrants from the Old World, so it has sent many migrants into South America, materially changing the character of the Neotropical mammalian fauna, but these intruders may be readily identified and almost seem to be out of place in their new surroundings. Not all of these northern migrants were able to maintain their footing in the southern continent and several became extinct during and at the close of the Pleistocene epoch, as was even more markedly the case with the southern forms which invaded the northern continent.

There are two families of monkeys in the forested areas of South America, both very different from those of the Old World. One of these families, the marmosets (Hapalidæ), differs from all other monkeys in several particulars, most obvious of which are the long claws on the feet and the non-opposable thumb. The second family (Cebidæ) comprises forms which are superficially much more like those of the eastern hemisphere, but many of them have prehensile tails, which are used as efficient grasping organs.

Insectivora are entirely absent from the South American continent, but some shrews (Blarina) have entered Central America from the north and a very curious genus is represented by one species in Cuba (Solenodon cubanus) and another in Hayti (S. paradoxus). These remarkable animals are, strange to relate, most nearly allied to the tenrecs (Centetes) of Madagascar and by some authorities are placed in the same family.

The Carnivora are quite numerous and varied and rather peculiar, but they all belong to northern families and are the more or less modified descendants of northern immigrants. The dogs (Canidæ) belong to genera not represented elsewhere and form a considerable assemblage of interesting types. There are no true wolves or foxes, but several species of fox-like wolves (Cerdocyon), with bushy tails, are common, especially in the plains regions. The Bush-Dog (Icticyon venaticus), a small, short-legged animal, is very peculiar. The musteline or weasel family (Mustelidæ) is rather scantily represented. There are no badgers and but few skunks (Spilogale and Conepatus); weasels are absent, but their place is taken by the Grison (Galera vittata) and Tayra (Tayra tayra) and in the far south Lyncodon patagonicus. These animals are peculiar in having a lighter colouration on the back than on the belly. There are two or three species of otter (Lutra). The raccoons (Procyon) have a very wide range in South America, as in the northern continent, and the curious, long-snouted coatis (Nasua), which just enter the Sonoran region, are typically Neotropical. The Spectacled Bear (Tremarctos ornatus) is the only member of the family that occurs in South America and is confined to the highlands of Peru and Chili. The cat family is quite numerously represented; the Jaguar (Felis onca), which ranges from Texas to Patagonia, is a large spotted cat, rivalling the Leopard in size and ferocity; the Ocelot (F. pardalis, Arkansas to Paraguay) is smaller and streaked and blotched rather than spotted. The pumas differ little from those of North America, and there are many small cats, spotted, clouded and of solid colour, but no lynxes, which are essentially northern types.

Hoofed animals are not numerously represented in South America. The only existing Perissodactyla of the western hemisphere are the tapirs (Tapirus) of Central and tropical South America, a very remarkable contrast to the ancient faunas, especially of the northern continent, as will be shown in the sequel. The Artiodactyla are more varied, though very scanty in comparison with those of the Old World; even North America, which has but a poor representation of these animals, is much richer than the southern continent, where, indeed, all the hoofed animals are the descendants of comparatively recent immigrants from the north and none are truly autochthonous. Members of three different artiodactyl suborders occur in the Neotropical region; the peccaries (Tagassu) extend through Central and South America to Paraguay, though also entering the Sonoran region in Texas. Most interesting are the members of the camel family, which are very distinct from the true Camel of Asia. Tierra del Fuego and the Patagonian plains support great herds of the Guanaco (Lama huanacus), which extends along the Andes to Ecuador and Peru, where it is associated with the Vicuña (L. vicunia), a smaller and more slenderly built species. The Vicuña does not range south of Bolivia. Just as the mountain systems of North America carry the Boreal and Transition faunas through nearly the whole breadth of the Sonoran region, so the high Andes afford a pathway by which the mammals of the south temperate zone extend their range to the equator.

The suborder Pecora of the Artiodactyla is represented in the Neotropical region only by the deer family (Cervidæ), of which there are several genera (or subgenera), all of them North American as distinguished from the Old World type, but some are so peculiar that they must have had a relatively long South American ancestry. The Virginia Deer (Odocoileus virginianus) of the northern United States is a comparatively large animal, becoming much smaller in Florida and the Southwest. The type extends through Mexico and Central America to Guiana and Peru, the Neotropical forms being so small and having such weak antlers that they are referred to separate species. Another type is the Marsh Deer (Blastoceros paludosus) of eastern South America, which has short, stout antlers, each beam with two double bifurcations; there are other species of the same genus, such as the Pampas Deer of Argentina (B. bezoarticus). In the Andes of Peru and Chili and the forests of western Patagonia are two species of a genus which bears the preposterous name of Hippocamelus and in which the antlers are simply forked. The vernacular name of these animals is “Huemul.” Peculiarly Neotropical are the little brockets, which hardly exceed a height of two feet at the shoulder, with simple spike-like antlers not more than three inches long; the genus, Mazama, has several species, one of which occurs as far north as the state of Puebla in Mexico. “The smallest of all deer is the Chilian pudu (Pudua pudu), a creature not much larger than a hare, with almost rudimentary antlers” (Lydekker). Old World types of deer, such as the Wapiti, Moose and Caribou, of the Boreal and Transition zones of North America, are entirely absent from the Neotropical region.

South America has an astonishingly rich and varied assemblage of rodents, both indigenous and immigrant, but the former are much the more important, varied and abundant. Of the four divisions of the order, all of which are represented, three are immigrants from the north and the fourth is autochthonous, but this far outnumbers the other three combined. The hares and rabbits have but very few species, one of which occurs in Brazil and is separated by a very wide interval from the one in Costa Rica, while the pikas are absent. Of the squirrel division, only the true squirrels are found, and of these there are many species, the ground-squirrels, marmots, prairie-dogs and beavers all being lacking. In the same way the rat and mouse division is represented by a single family. The vesper or white-footed mice (Sitomys) have invaded the southern continent and a number of peculiar genera have arisen there, but all of northern ancestry, such as the groove-toothed mice (Rheithrodon) and the fish-eating rats (Ichthyomys). The voles, or meadow-mice, the muskrats, jumping mice, kangaroo-rats and pocket-gophers of the northern continent are all absent. While the immigrant suborders have thus but one family each in South America, the case is very different with the fourth or porcupine group, of which that continent is to-day, as it has been for ages past, the headquarters. No less than six families and twenty-nine genera are known, all of the genera and four of the families being restricted to the Neotropical region. Contrast this assemblage with the extreme scantiness of this group in North America, where but a single genus, the Short-tailed or Canada Porcupine (Erethizon) represents it, and that is a late immigrant from the south.

It would lead us too far to attempt a description of this horde of curious and interesting rodents, so only a few of the more striking and characteristic forms can be mentioned. There are two genera of porcupines (Coendou and Chætomys), both arboreal, which belong in the same family as the North American Erethizon, but are distinguished by their long, prehensile tails, which they use, as monkeys and opossums do, for grasping and climbing. The very large family of the Octodontidæ has 17 Neotropical genera and four others are found in Africa. The Degu (Octodon) of Chili, Bolivia and Peru has the appearance of a large rat with tufted tail; the tuco-tucos (Ctenomys) are extremely abundant burrowers in Patagonia, where they honeycomb the ground over wide areas. The spiny rats (Echimys and Loncheres) are so called from their appearance, not because they are related to the true rats; they have numerous horny spikes through the fur of the back. The Coypu (Myocastor) is a large, aquatic animal, remotely like the northern Muskrat, and the Hutias (Capromys and Plagiodontia) are arboreal and found only in Cuba, Hayti and Jamaica. The chinchillas (Chinchilla and Lagidium) of the Andes and the Vizcacha (Viscaccia) of the Argentine plains have somewhat the appearance of hares, but with long and bushy tails. The cavies, to which the familiar, misnamed Guinea-Pig (Cavia porcellus) belongs, are a very characteristic family; besides the true cavies, it includes the Patagonian Mara (Dolichotis), a large, long-legged, long-eared, short-tailed creature, and the Water-Hog, or Carpincho (Hydrochærus), an aquatic animal, as its name implies, and much the largest of existing rodents; it occurs in the warmer regions, south to Argentina. The heavy Paca (Agouti) and the slender-limbed Agouti (Dasyprocta) make up another family. Altogether, this assemblage of the porcupine-like suborder of rodents is a very remarkable one and in no other region of the earth is anything like it to be found.

With the exception of one genus of armadillos, which has invaded Texas, the entire order of the Edentata is at present confined to the Neotropical region, the so-called edentates of the Old World now being removed to other orders. The Edentata, which were once far more varied and abundant than they now are, comprise three groups of animals so bizarre and strange that they seem more like fabulous creatures than actual, living mammals. One group, or suborder, is that of the sloths (Tardigrada), arboreal, shaggy animals, with short, almost monkey-like head and no tail; their very long legs and hook-like feet make them nearly helpless on the ground, but are very useful for hanging from the branches of the trees, in which the creatures live. Indeed, the sloths are the only mammals which habitually hang in a suspended position. Two genera of sloths inhabit the tropical forests, between which the most obvious difference is that in one (Bradypus) the forefoot has three toes, and in the other (Cholœpus) two.

The suborder of the anteaters (Vermilingua) is more varied, and is the only one of the order to which the term “edentate” applies strictly, for they alone in the order are altogether toothless. The great Ant-Bear (Myrmecophaga jubata), which may reach a total length of seven feet, has an extravagantly long, slender and nearly cylindrical head, long, shaggy, black and white hair and an immense, bushy tail; the forefeet are armed with huge, sharp-pointed claws, which are used for tearing open ant-hills, and when occasion arises, as formidable weapons of defence, for the Ant-Bear can successfully repulse even the Jaguar. In walking, the claws are curved inward and the preposterous beast rests his weight upon the outside edges of the forefeet, while the hind feet apply the sole to the ground, as does a bear or raccoon. The Collared Anteater (Tamandua) is much smaller and mainly arboreal in habits. It has a short-haired, black body, with a white stripe down the back, white neck and limbs, a colour-pattern which gives to the animal the appearance of wearing a close-fitting black jacket; the long tail, which has some cross bars, is short-haired, very different from the extremely bushy tail of the Ant-Bear. The little Two-toed Anteater (Cyclopes didactylus), hardly larger than a rat, is exclusively arboreal and has a prehensile tail, like so many other South American mammals. Sloths and anteaters are forest animals and are not found west of the Andes or south of Paraguay.

The third existing suborder of edentates is that of the armadillos (Dasypoda), which have a very complete armour of bony scutes, ossifications in the skin, covered with scales of horn. They are all more or less burrowers in habit and omnivorous in diet, eating roots, insects, worms, etc.; the extraordinary rapidity with which they burrow into the ground is almost their only way of escape from pursuit, but in one genus, Tolypeutes, the animal can roll itself into a ball, completely protected by mail all around. The armadillos are much more varied than the anteaters or sloths and have a wider geographical range, extending from Texas to Patagonia. The head, which is long-snouted, is protected by a shield made up of numerous horn-covered plates of bone, and the tail is encased in a tubular sheath of more or less regular rings, each ring of bony plates and horny scales. The body-shield, or carapace, which covers the back and sides, consists of an anterior and posterior buckler, in which the plates are immovably attached to one another by their edges, and between the two is a series of movable, overlapping bands, the number of which varies in the different genera. In the little Pichiciago (Chlamydophorus truncatus) the head and back are covered with four-sided plates of horn, the bony scutes being small and thin and much reduced. The carapace has no bucklers, but about 20 transverse rows of plates, and is attached along only the middle line of the back and beneath it the body is covered with silky, white fur; the rump is covered with a solid shield of bone, placed nearly vertically and covered with thin scales, and is notched below for the tail, altogether a most exceptional arrangement. Seven or more distinct genera of armadillos are found in the Neotropical region and they display a great range in size; the Giant Armadillo of Brazil (Priodontes) is a yard or more in length, while the little Zaëdyus of Patagonia is smaller than a rabbit and, least of all, the Pichiciago is but five inches long.

Two families of marsupials occur in South America. The opossums are much more numerous and varied than in North America; three genera and a large number of species, some not larger than mice, range through the forested parts of the continent. Of particular interest is the little Cænolestes, which has two species, with two enlarged lower front teeth, the sole survivors of a group which is abundantly represented in the Tertiary deposits of Patagonia.

The fauna of the Central American subregion is less rich and characteristic than that of the Brazilian and is, to a certain extent, transitional to that of the Sonoran region of North America, several genera proper to the latter region extending into it, which are not known to pass the Isthmus of Panama, such as shrews, a fox and one of the pocket-mice. The West Indian islands are exceedingly poor in mammals, a great contrast to the East Indian, or Malay, Archipelago; only a few rodents, insectivores and bats occur in them.