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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 Nearctic4—Amiurw'(Siljxri.diei); Fundulus (Cyprinodontidae); Sclerognathus (Cyprinidae); and Lepidosteus (Ganoidei). A much larger number are Neotropical; and several Neotropical genera, as Heros and Pcecilia, 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); jfllurickthys (Siluridse); Chalcinopsis (Characniidae); Characodon, Belonesox, Pseudoxiphophorus, Pfatypcecilus, Mollienesia, and Xiphophorus (Cyprinodontidae). A few peculiar Antillean forms are also present; as Agonostoma (Mugilidae); Qambusia and Girardinuus (Cyprinodontidae). The other families represented are Percidae (1 genus); Pristopomatidas (2 gen.); Gobiidae (1 gen.); Clupeidae (2 gen.) ; and Gymnotidae (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 Cyprinodontidae, represented by 12 genera; and the genus Heros (Chromidae) has here its maximum development, containing between thirty and forty species. Dr. Glinther 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 Kica, 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.
Insects.—The butterflies of various parts of Central America and Mexico, having been largely collected, offer us some valuable evidence as to the relations of this sub-region. Their general character is wholly Neotropical, about one half of the
South American genera being found here. There are also a few peculiar genera, aa,Drucina (Satyridae); Microtia (Nymphalidae); Eumatus (Lycaenidae); and Eucheira (Pieridae). Clothilda (Nymphalidae) is confined to this sub-region and the Antilles. The majority of the genera range over the whole sub-region from Panama to Mexico, but there are a considerable number, comprising many of the most characteristic South American forms, which do not pass north of Costa Rica or Nicaragua. Such are Lycorea, Iluna, Thyridia, Callithomia, Oleria and Ceratina, —all characteristic South American groups of Danaidae; Pronophila and Dynastor (Satyridae); Protogonius, Pycina, Prepona, Nica, Ectima and Colcenis (Nymphalidae); Eurybia and Methonella (Nemeobiidae); Hades, and Panthemos (Erycinidae).
Coleoptera.—These present some interesting features, but owing to their vast number only a few of the more important families can be noticed.
Cicindelidae.—The only specially Neotropical genera recorded as occurring in this sub-region, are Ctenostoma and Hiresia, both reaching Mexico.
Carabidae.—Several genera are peculiar. Moldbrus is found in all parts of the sub-region, while Onychopterygia, Phymatocephalus, and Anisotarsus are Mexican only. There are about 20 South American genera, most of which extend to Mexico, and include such characteristic Neotropical forms as Agra, Callida, Coptodera, Pachyteles, Ardistomus, Aspidoglossa, Stenocrepis, and Pelecium.
Lucanidae.—Of this important family there is, strange to say, not a single species recorded in Gemminger and Harold's catalogue up to 1868! It is almost impossible that they can be really absent; yet their place seems to be, to some extent, supplied by an unusual development of the allied Passalidse, of which there are five South American and six peculiar genera.
Cetoniidae.—All the larger South American genera extend to Mexico, which country possesses 3 peculiar forms, Ischnoscelis, Psilocnemis, and Dialithus; while Trigorwpeltastes is characteristic, having 4 Mexican, 1 Brazilian, and 1 North American
Buprestidae.—In this family there are no peculiar genera. All the large South American groups are absent, the only important and characteristic genus being Stenogaster.
Longicorns.—This important group is largely developed, the country being well adapted to them; and their distribution presents some features of interest.
In the Prionidse there are 6 peculiar genera, the largest being Holoiwtus with 3 species; two others, Derotrachics and Mallaspis, are characteristic; 3 more are common to South America, and 1 to Cuba. The Cerambycidae are much more numerous, and there are 24 peculiar genera, the most important being Sphenothecus, Entomosterna, and Cyphosterna; while Crioprosopus and Metaleptus are characteristic of the sub-region, although extending into South America; about 12 Neotropical genera extend to Mexico or Guatemala, while 12 more stop short, as far as yet known, at Nicaragua. Lamiidse have a very similar distribution; 13 genera are peculiar, the most important being Monilema, Hamatoderus, and Carneades, while Phcea and Lagochirus are characteristic. About sixteen typical Neotropical genera extend to Mexico, and 15 more only reach Nicaragua, among which are such important genera as Anisopus, Lepturgus, and Callia.
The land-shells are not sufficiently known to furnish any corresponding results. They are however mostly of South American genera, and have comparatively little affinity for those of the Antilles.
Relations of the Mexican sub-region to the North and South American Continents.—The sudden appearance of numerous South American forms of Edentata in temperate North America, in Post-Tertiary times, as narrated in Chapter VII., together with such facts as the occurrence of a considerable number of identical species of sea fish on the two sides of the Central American isthmus, render it almost certain that the union of North and South America is comparatively a recent occurrence, and that during the Miocene and Pliocene periods, they were separated by a wide arm of the sea. The low country of Nicaragua was probably the part submerged, leaving the highlands of Mexico and Guatemala still united with the North American continent, and forming part of the Tertiary " Nearctic region." This is clearly indicated both by the many Nearctic forms which do not pass south of Nicaragua, of which the turkeys (Meleagris) are a striking example, and by the comparative poverty of this area in typical Neotropical groups. During the Miocene period there was not that marked diversity of climate between North and South America that now prevails; for when a luxuriant vegetation covered what are now the shores of the Arctic Ocean,the country south of the great lakes must have been almost or quite tropical. At an early Tertiary period, the zoological differences of the Nearctic and Neotropical regions were probably more radical than they are now, South America being a huge island, or group of islands—a kind of Australia of the New World, chiefly inhabited by the imperfectly organized Edentata; while North America abounded in Ungulate and Carnivora, and perhaps formed a part of the great Old World continent There were also one or more very ancient unions (in Eocene or Miocene times) of the two continents, admitting of the entrance of the ancestral types of Quadrumana into South America, and, somewhat later, of the Camelidse; while the isthmus south of Nicaragua was at one time united to the southern continent, at another made insular by subsidence near Panama, and thus obtained that rich variety of Neotropical types that still characterises it. When the final union of the two continents took place, the tropical climate of the lower portions of Guatemala and Mexico would invite rapid immigration from the south; while some northern forms would extend their range into and beyond the newly elevated territory. The Mexican sub-region has therefore a composite character, and we must not endeavour too rigidly to determine its northern limits, nor claim as exclusively Neotropical, forms whieh are perhaps comparatively recent immigrants; and it would perhaps be a more accurate representation of the facts, if we were to consider all the highlands of Mexico and Guatemala above the limits of the tropical forests, as still belonging to the Nearctic region, of- which the whole country so recently formed a part. The long-continued separation of North and South America by one or more arms of the sea, as above indicated, is further rendered necessary by the character of the molluscan fauna of the Pacific shores of tropical America, which is much more closely allied to that of the Caribbean sea, and even of West Africa, than to that of the Pacific islands. The families and many of the genera are the same, and a certain proportion ot very closely allied or identical species, shows that the union of the two oceans continued into late Tertiary times. When the evidence of both land and sea animals support each other as they do here, the conclusions arrived at are almost as certain as if we had (as we no doubt some day shall have) geological proof of these successive subsidences.
Islands of the Mexican Sub-region.—The only islands of interest belonging to this sub-region, are Tres Marias and Socorro, recently investigated by Col. Grayson for some of the American Natural History societies.
Tres Marias consist of four small islands lying off the coast of north-western Mexico, about 70 miles from San Bias. The largest is about 15 miles long by 10 wide. They are of horizontally stratified deposits, of moderate height and flat-topped, and everywhere covered with luxuriant virgin forests. They appear to lie within the 100 fathom line of soundings. Fifty-two species of birds, of which 45 were land-birds, were collected on these islands. They consisted of 19 Passeres; 11 Picarise (7 being humming-birds); 10 Accipitres; 2 parrots, and 3 pigeons. All were Mexican species except 4, which were new, and presumably peculiar to the islands, and one tolerably marked variety. The new species belong to the following genera;—Panda and Granatellus (Mniotiltidae); Icterus (Icteridse); and Amazilia (Trochilidse). A small Psittacula differs somewhat from the same species on the mainland.
There are a few mammalia on the islands; a rabbit (Lepus) supposed to be new; a very small opossum (Didelphys), and a racoon (Procyori). There are also several tree-snakes, a Boa, and many lizards. The occurrence of so many mammalia and snakes is a proof that these islands have been once joined to the mainland; but the fact that some of the species of both birds and