« EelmineJätka »
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 of 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 Blas. 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 Picarie (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 ;-Parula and Granatellus (Mniotiltidae); Icterus (Icteridae); and Amazilia (Trochilidae). 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 (Procyon). 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
mammals are peculiar, indicates that the separation is not a very recent one.
At the same time, as all the species are very closely allied to those of the opposite coasts when not identical, we may be sure that the subsidence which isolated them is not geologically remote.
Socorro, the largest of the Revillagigedo Islands, is altogether different from the Tres Marias. It is situated a little further south (19 S. Latitude), and about 300 miles from the coast, in deep water. It is about 2,000 feet high, very rugged and bare, and wholly volcanic. No mammalia were observed, and no reptiles but a small lizard, a new species of a genus (Uta) characteristic of the deserts of N.-Western Mexico. The only observed land-shell (Orthalicus undatus) also inhabits N.-W. Mexico. Only 14 species of birds were obtained, of which 9 were land-birds; but of these 4 were new species, one a peculiar variety, and another (Parula insularis) a species first found in the Tres Marias. With the exception of this bird and a Buteo, all the land-birds belonged to different genera from any found on the Tres Marias, though all were Mexican forms. The peculiar species belonged to the genera Harporhynchus (Turdida); Troglodytes (Troglodytidæ); Pipilo (Fringillidae) ; Zenaidura (Columbidæ); and a variety of Conurus holochrous (Psittacida).
The absence of mammals and snakes, the large proportion of peculiar species, the wholly volcanic nature of these islands, and their situation in deep water 300 miles from land,—all indicate that they have not formed part of the continent, but have been raised in the ocean; and the close relation of their peculiar species to those living in N.-Western Mexico, renders it probable that their antiquity is not geologically great.
The Cocos Islands, about 300 miles S.-W. of the Isthmus of Panama, are known to possess one peculiar bird, a cuckoo of the Coccyzus type, which is considered by some ornithologists to constitute a peculiar genus, Nesococcyx.
IV. The West Indian Islands, or Antillean Sub-region. The West Indian islands are, in many respects, one of the most interesting of zoological sub-regions. In position they
form an unbroken chain uniting North and South America, in a line parallel to the great Central American isthmus; yet instead of exhibiting an intermixture of the productions of Florida and Venezuela, they differ widely from both these countries, possessing in some groups a degree of speciality only to be found elsewhere in islands far removed from any continent. They consist of two very large islands, Cuba and Hayti;? two of moderate size, Jamaica and Portorico; and a chain of much smaller islands, St. Croix, Anguilla, Barbuda, Antigua, Guadeloupe, Dominica, Martinique, St. Lucia, St. Vincent, Barbadoes, and Grenada, with a host of intervening islets. Tobago, Trinidad, Margarita, and Curaçao, are situated in shallow water near the coast of South America, of which they form part zoologically. To the north of Cuba and Hayti are the Bahamas, an extensive group of coral reefs and islands, 700 miles long, and although very poor in animal life, belonging zoologically to the Antilles. All the larger islands, and most of the smaller ones (except those of coral formation) are very mountainous and rocky, the chains rising to about 8,000 feet in Hayti and Jamaica, and to nearly the same height in Cuba. All, except where they have been cleared by man, are covered with a luxuriant forest vegetation; the temperature is high and uniform; the rains ample; the soil, derived from granitic and limestone rocks, exceedingly fertile ; and as the four larger islands together are larger than Great Britain, we might expect an ample and luxuriant fauna. The reverse is however the case ; and there are probably no land areas on the globe, so highly favoured by nature in all the essentials for supporting animal life, and at the same time so poor in all the more highly organised groups of animals. Before entering upon our sketch of the main features of this peculiar but limited fauna, it will be well to note a few peculiarities in the physical structure of the islands, which have an important bearing on their past
1 This name will be used for the whole island of St. Domingo, as being both shorter and more euphonious, and avoiding all confusion with Dominica, one of the Lesser Antilles. It is also better known than “ Hispaniola," which is perhaps the most correct name.
history, and will enable us to account for much that is peculiar in the general character of their natural productions.
If we draw a line immediately south of St. Croix and St. Bartholomew, we shall divide the Archipelago into two very different groups. The southern range of islands, or the Lesser Antilles, are, almost without exception, volcanic ; beginning with the small detached volcanoes of Saba and St. Eustatius, and ending with the old volcano of Grenada. Barbuda and Antigua are low islands of Tertiary or recent formation, connected with the volcanic islands by a submerged bank at no great depth. The islands to the north and west are none of them volcanic; many are very large, and these have all a central nucleus of ancient or granitic rocks. We must also note, that the channels between these islands are not of excessive depth, and that their outlines, as well as the direction of their mountain ranges, point to a former union. Thus, the northern range of Hayti is continued westward in Cuba, and eastward in Portorico; while the south-western peninsula extends in a direct line towards Jamaica, the depth between them being 600 fathoms. Between Portorico and Hayti there is only 250 fathoms; while close to the south of all these islands the sea is enormously deep, from more than 1,000 fathoms south of Cuba and Jamaica, to 2,000 south of Hayti, and 2,600 fathoms near the south-east extremity of Portorico. The importance of the division here pointed out will be seen, when we state, that indigenous mammalia of peculiar genera are found on the western group of islands only; and it is on these that all the chief peculiarities of Antillian zoology are developed.
Mammalia.—The mammals of the West Indian Islands are exceedingly few, but very interesting. Almost all the orders most characteristic of South America are absent. There are no mionkeys, no carnivora, no edentata. Besides bats, which are abundant, only two orders are represented; rodents, by peculiar forms of a South American family; and insectivora (an order entirely wanting in South America) by a genus belonging to a family largely developed in Madagascar and found nowhere else. The early voyagers mention “Coatis” and “ Agoutis " as being
found in Hayti and the other large islands, and it is not improbable that species allied to Nasua and Dasyprocta did exist, and have been destroyed by the dogs of the invaders ; though, on the other hand, these names may have been applied to the existing species, which do bear some general resemblance to these two forms.
The Chiroptera, or bats, are represented by a large number of species and by several peculiar genera. The American family of Phyllostomidæ or vampires, has six genera in the Antilles, of which three, Lonchorina, Brachyphylla, and Phyllonycteris, are peculiar, the latter being found only in Cuba. The Vespertilionidæ have four genera, of which one, Nycticellus, is confined to Cuba. There are six genera of Noctilionidæ, of which one, Phyllodia, is confined to Jamaica.
The Insectivora are represented by the genus Solenodon, of which two species are known, one inhabiting Cuba the other Hayti. These are small animals about the size of a cat, with long shrew-like snout, bare rat-like tail, and long claws. Their peculiar dentition and other points of their anatomy shows that they belong to the family Centetidæ, of which five different genera inhabit Madagascar; while there is nothing closely allied to them in any other part of the world but in these two islands.
Seals are said to be found on the shores of some of the islands, but they are very imperfectly known.
The rodents belong to the family Octodontidæ, or, according to some authors, to the Echimyidæ, both characteristic South American groups. They consist of two genera, Capromys, containing three or four species inhabiting Cuba and Jamaica ; while Plagiodontia (very closely allied) is confined to Hayti. A peculiar mouse, a species of the American genus Hesperomys, is said to inhabit Hayti and Martinique, and probably other islands. A Dasyprocta or agouti, closely allied to, if not identical with, a South American species, inhabits St. Vincent, St. Lucia, and Grenada, and perhaps St. Thomas, and is the only mammal of any size indigenous to the Lesser Antilles. All the islands in which sugar is cultivated are, however, overrun with European rats and mice, and it is not improbable that these may have