This sub-region is of comparatively small extent, consisting of the irregular neck of land, about 1,800 miles long, which connects the North and South American continents. Almost the whole of its area is mountainous, being in fact a continuation of the great range of the Rocky Mountains. In Mexico it forms an extensive table-land, from 6,000 to 9,000 feet above the sea, with numerous volcanic peaks from 12,000 to 18,000 feet high; but in Yucatan and Honduras, the country is less elevated, though still mountainous. On the shores of the Caribbean Sea and Gulf of Mexico, there is a margin of low land from 50 to 100 miles wide, beyond which the mountains rise abruptly; but on the Pacific side this is almost entirely wanting, the mountains rising almost immediately from the sea shore. With the exception of the elevated plateaus of Mexico and Guatemala, and the extremity of the peninsula of Yucatan, the whole of Central America is clothed with forests; and as its surface is much broken up into hill and valley, and the volcanic soil of a large portion of it is very fertile, it is altogether well adapted to support a varied fauna, as it does a most luxuriant vegetation. Although many peculiar Neotropical types are absent, it yet possesses an ample supply of generic and specific forms; and, as far as concerns birds and insects, is not perhaps inferior to the richest portions of South America in the number of species to be found in equal areas.
Owing to the fact that the former Republic of Mexico comprised much territory that belongs to the Nearctic region, and that many Nearctic groups extend along the high-lands to the capital city of Mexico itself, and even considerably further south, there is much difficulty in determining what animals really belong to this sub-region. On the low-lands, tropical forms predominate as far as 28° N. latitude; while on the cordilleras, temperate forms prevail down to 20°, and are found even much farther within the tropics.
Mammalia.—Very few peculiar forms of Mammalia are restricted to tropical North America; which is not to be wondered at when we consider the small extent of the country, and the facility of communication with adjacent sub-regions. A peculiar form of tapir (Elasmognathus bairdi) inhabits Central America, from Panama to Guatemala, and, with Myxomys, a genus of Muridæ, are all at present discovered. Bassaris, a remarkable form of Procyonidæ, has been included in the Nearctic region, but it extends to the high-lands of Guatemala. Heteromys, a peculiar genus of Saccomyidæ or pouched rats, inhabits Mexico, Honduras, Costa Rica, and Trinidad. Five genera of monkeys extend here,—Ateles, Mycetes, Cebus, Nyctipithecus, and Saimiris; the two former alone reaching Mexico, the last only going as far as Costa Rica. Other typical Neotropical forms are Galera, the tayra, belonging to the weasel family; Nasua, the coatimundi; Dicotyles, the peccary; Cercolabes, the tree porcupine; Dasyprocta, the agouti; Cælogenys, the paca; Cholœpus, and Arctopithecus, sloths; Cyclothurus, an ant-eater; Tatusia, an armadillo; and Didelphys, oppossum. Of Northern forms, Sorex, Vulpes, Lepus, and Pteromys reach Guatemala.
Birds.—The productiveness of this district in bird life, may be estimated from the fact, that Messrs. Salvin and Sclater have catalogued more than 600 species from the comparatively small territory of Guatemala, or the portion of Central America between Mexico and Honduras. The great mass of the birds of this sub-region are of Neotropical families and genera, but these are intermingled with a number of migrants from temperate North America, which pass the winter here; with some northern forms on the high-lands; and with a considerable number of peculiar genera, mostly of Neotropical affinities.
The genera of birds peculiar to this sub-region belong to the following families:—Turdidæ (2 genera); Troglodytidæ (1 gen.); Vireonidæ (1 gen.); Corvidæ (2 gen.); Ampelidæ (1 gen.); Tanagridæ (1 gen.); Fringillidæ (2 gen.); Icteridæ (1 gen.); Formicariidæ (2 gen.); Tyrannidæ (2 gen.); Cotingidæ (1 gen.); Momotidæ (1 gen.); Trogonidæ (1 gen.); Trochilidæ (14 gen.); Conuridæ (1 gen.); Cracidæ (2 gen.); Strigidæ (1 gen.); in all 37 genera of land-birds. The Neotropical families that do not extend into this sub-region are, Pteroptochidæ; the sub-family Furnariinæ of the Dendrocolaptidæ; the sub-family Conophaginæ of the Tyrannidæ; the sub-family Rupicolinæ of the Cotingidæ; Phytotomidæ; Todidæ; Opisthocomidæ; Chionididæ; Thinocoridæ; Cariamidæ; Psophiidæ; Eurypygidæ; Palamedeidæ; and Struthionidæ. On the other hand Paridæ, Certhiidæ, Ampelidæ, and Phasianidæ, are northern families represented here, but which do not reach South America; and there are also several northern genera and species, of Turdidæ, Troglodytidæ, Mniotiltidæ, Vireonidæ, Fringillidæ, Corvidæ, Tetraonidæ, and Strigidæ, which are similarly restricted. Some of the most remarkable of the Neotropical genera only extend as far as Costa Rica and Veragua,—countries which possess a rich and remarkable fauna. Here only are found an umbrella bird, (Cephalopterus glabricollis); a bell bird (Chasmorhynchus tricarunculatus); and species of Dacnis (Cerœbidæ), Buthraupis, Eucometis, Tachyphonus (Tanagridæ), Xiphorhynchus (Dendrocolaptidæ); Hypocnemis (Formicariidæ); Euscarthmus (Tyrannidæ); Attila (Cotingidæ); Piprites (Pipridæ); Capito, Tetragonops (Megalæmidæ); Selenidera (Rhamphastidæ); Neomorphus (Cuculidæ); Monasa (Bucconidæ); many genera of Trochilidæ; and Nothocercus (Tinamidæ); none of which extend further north. A considerable number of the peculiar genera noted above, are also found in this restricted area, which is probably one of the richest ornithological districts on the globe.
Reptiles.—These are much less known than the preceding classes, but they afford several peculiar and interesting forms. Snakes are perhaps the least remarkable; yet there are recorded 4 peculiar genera of Calamariidæ, 1 of Colubridæ, 1 of Homalopsidæ, 3 of Dipsadidæ; while Boa and Elaps are in common with South America. Lizards are much more specially developed. Chirotes, one of the Amphisbænians, is confined to Mexico and the southern part of the Nearctic region; Heloderma forming a peculiar family, Helodermidæ, is Mexican only; Abronia and Barissia (Zonuridæ) are also Mexican, as is Siderolampus belonging to the Scincidæ, while Blepharactitis (same family) inhabits Nicaragua; Brachydactylus, one of the geckoes, is from Costa Rica; while Phymatolepis, Lamanctus, Corytheolus, Cachrix, Corythophanes and Chamæleopsis, all belonging to the Iguanidæ, are confined to various parts of the sub-region. In the same family we have also the Antillean, Cyclura, and the Nearctic Phrynosoma and Tropidolepis, as well as the wide-spread American genus Anolius.
Among the tortoises, Staurotypus, allied to Chelydra, is found in Mexico and Guatemala; and another genus, Claudius, has been lately described from Mexico.
Amphibia.—These are chiefly Batrachians; Rhinophryna (forming a peculiar family) being confined to Mexico; Triprion, a genus of Hylidæ, inhabiting Yucatan, with Leyla and Strabomantis (Polypedatidæ) found only in Costa Rica and Veragua, are peculiar genera. The Salamandridæ, so abundant in the Nearctic region, are represented by a few species of Amblystoma and Spelerpes.
Fresh-water fish.—Since the British Museum catalogue was published, a valuable paper by Dr. Günther, in the Transactions of the Zoological Society for 1868, furnishes much additional information on the fishes of Central America. In that part of the region south of Mexico, 106 species of fresh-water fishes are enumerated; and 17 of these are found in streams flowing into both the Atlantic and Pacific Oceans. On the whole, 11 families are represented among the fresh-water fish, and about 38 genera. Of these, 14 are specially Nearctic,—Amiurus (Siluridæ); Fundulus (Cyprinodontidæ); Sclerognathus (Cyprinidæ); and Lepidosteus (Ganoidei). A much larger number are Neotropical; and several Neotropical genera, as Heros and Pœcilia, are more largely developed here than in any other part of the region. There are also a considerable number of peculiar genera;—Petenia, Theraps, and Neotrophus (Chromides); Ælurichthys (Siluridæ); Chalcinopsis (Characniidæ); Characodon, Belonesox, Pseudoxiphophorus, Platypœcilus, Mollienesia, and Xiphophorus (Cyprinodontidæ). A few peculiar Antillean forms are also present; as Agonostoma (Mugilidæ); Gambusia and Girardinuus (Cyprinodontidæ). The other families represented are Percidæ (1 genus); Pristopomatidæ (2 gen.); Gobiidæ (1 gen.); Clupeidæ (2 gen.); and Gymnotidæ (1 genus).
On the whole the fish-fauna is typically Neotropical, but with a small infusion of Nearctic forms. There are a considerable proportion of peculiar genera, and almost all the species are distinct from those of other countries. The predominant family is that of the Cyprinodontidæ, represented by 12 genera; and the genus Heros (Chromidæ) has here its maximum development, containing between thirty and forty species. Dr. Günther considers that a number of sub-faunas can be distinguished, corresponding to some extent, with the islands into which the country would be divided by a subsidence of about 2,000 feet. The most important of these divisions is that separating Honduras from Costa Rica, and as it also divides a very marked ornithological fauna we have every reason to believe that such a division must have existed during the latter portion of the tertiary epoch. We shall find some farther evidence of this division in the next class.