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latter family has a single species iu the Indian seas. Among the curiosities of distribution are,—the extensive genus Diagramma, confined to the Pacific with the exception of one species in the Mediterranean ; the single species constituting the family Lophotidse, found only in the Mediterranean and Japan; the small family of Notacanthi, confined to Greenland, the Mediterranean, and West Australia; and the four families, Sternoptychidae, Stomiatidae, Alepocephalidae, and Halosauridae, which are believed to inhabit exclusively the depths of the ocean, and are therefore very rarely obtained.

Fresh-water Fish.—There are 36 families of fishes which inhabit fresh water exclusively, and 5 others, which are both marine and fresh-water. These present many interesting peculiarities of distribution. The Neotropical region is the richest in families, and probably also in genera and species. No less than 22 families inhabit it, and of these 6 are altogether peculiar. The Ethiopian and Nearctic regions each have 18 families, the former with 3, and the latter with 5 peculiar. Several isolated forms, requiring to be placed in distinct families, inhabit the great American lakes; and, no doubt, when the African lakes are equally well known, they will be found also to possess many peculiar forms. The Oriental region comes next, with 17 families, of which 3 are peculiar. The Palaearctic has 12, and the Australian 11 families, each with only 1 altogether peculiar to it.

If we take those regions which are sometimes supposed to be so nearly related that they should be combined, we shall find the fresh-water fishes in most cases markedly distinct The Nearctic and Palaearctic regions, for example, together contain 20 families, but only 11 of these occur in both, and only 5 are exclusive inhabitants of these two regions. This shows an amount of diversity that would not, perhaps, be exhibited by any other class of animals. The Ethiopian and Oriental regions together possess 24 families, only 11 of which are found in both, and only 1 exclusively characteristic of the two. The Australian and Neotropical regions possess together 27 families, of which 7 are found in both, and 3 are exclusively characteristic of the two. This last fact is very interesting: the marine family of Trachinidse possesses a fresh-water genus, AphrUis, one species of which inhabits Tasmania, and two others Patagonia; the Haplochitonidie (2 genera, 3 sp.) are found only in Tierra del Fuego, the Falkland Islands, and South Australia; and the Galaxidffi (1 genus, 12 sp.) inhabit the same regions, but extend to Chili, to New Zealand and to Queensland. We have here an illustration of that connection between South America and Australia which is so strongly manifested in plants, but of which there are only scattered indications in most classes of animals. The dividing line across the Malay Archipelago, separating the Oriental from the Australian regions, and which is so strikingly marked in mammalia and birds, is equally so in fresh-water fishes. No less than six families have their eastern limits in Java and Borneo; while the extensive family of Cyprinidse has no less than 23 genera in Java and Borneo, but not a single species has been found in Celebes or the Moluccas.

The distribution of fresh-water fishes lends no support to the view that the peninsula of India belongs to the Ethiopian region. A large proportion of the Oriental families are common to the whole region; while there is hardly a single example, of a characteristic Ethiopian family or genus extending into the peninsula of India and no further.

Among the special peculiarities of distribution, is the curious fish, forming the family Comephoridae, which is confined to Lake Baikal, among the mountains of Central Asia, 2,000 feet above the sea, and a thousand miles distant from the ocean; yet having its nearest allies in the exclusively oceanic family of the mackerels (Scomberidae). The Characinidae are confined to Africa and South America, distinct genera inhabiting each region. The Salmonidae are confined to the two northern regions, except a single species of a peculiar genus in New Zealand. The genus Osteoglossum has a species in South America, another in the Sunda Islands, and a third in Queensland; while the curious Sirenoidei are represented by single species of peculiar genera in Tropical America, Tropical Africa, and Tropical Australia.

Fossil Fishes.—Fishes have existed from a very remote era, and it is remarkable that the first whose remains have been discovered belong to the Ganoidei, a highly developed group which has continued to exist down to our times, and of which the sturgeon is the best known example. We may therefore be sure that the Upper Silurian rocks in which these are found, although so very far back in geological history, do not by any means lead us to the time when the primitive fish-type appeared upon the earth. In the Carboniferous and Permian formations numerous remains of fishes are found, allied to the Lepidosteus or Gar-pike of North America. The next group in order of appearance, are the Plagiostomata, containing the existing Sharks and Rays. Traces of these are found in the highest Silurian beds, and become plentiful in the Devonian and Carboniferous formations and in all succeeding ages, being especially abundant in Cretaceous aud Eocene strata. The Holocephali appear first in the Oolitic period, and are represented by the living Chimaeridae. The Dipnoi, to which belong the Lepidosiren and Ceratodus, are believed to have existed in the Triassic period, from the evidence of teeth almost identical with those of the existing Australian fish. All the ancient fossil fishes belong to the above-mentioned groups, and many of them have little resemblance to existing forms. The Teleostean fishes, which form the great bulk of those now living, cannot be traced back further than the Cretaceous period, while by far the larger number first appear in the Tertiary beds. The Salnionidae, Scopelidae, Percidae, Clupeidae, Scombresocidae, Mugilidae, and Siluridae, or forms closely allied to them, are found in the Cretaceous formation. In the Eocene beds we first meet with Squammipennes, Cyprinidie, Pleuronectidae, Characinidae, Muraenidae, Gadidae, Pediculati, Syngnathidae, and Hippocampidae.

Most of these fossils represent marine fishes, those of freshwater origin being rare, and of little importance as an aid in determining the causes of the distribution of living forms. To understand this we must look to the various changes of the land surface which have led to the existing distribution of all the higher vertebrates, and to those special means of dispersal which Mr. Darwin has shown to be possessed by all fresh-water productions.

CHAPTER XXI.

THE DISTRIBUTION OF 80ME OF THE MORE IMPORTANT FAMILIES AND GENERA OF INSECTS.

Although insects are, for the most part, truly terrestrial animals, and illustrate in a very striking manner the characteristic phenomena of distribution, it is impossible here to treat of them in much detail. This arises chiefly from their excessive numbers, but also from the minuteness and obscurity of many of the groups, and our imperfect knowledge of all but the European species. The number of described species of insects is uncertain, as no complete enumeration of them has ever been made; but it probably exceeds 100,000, and these may belong to somewhere about 10,000 genera—many times more than all vertebrate animals together. Of the eight Orders into which Insecte are usually divided, only two—the Coleoptera and Lepidoptera —have been so thoroughly collected in all parts of the globe that they can be used, with any safety, to compare their distribution with that of vertebrate animals; and even of these it is only certain favourite groups which have been so collected. Among Lepidoptera, for example, although the extensive group of Butterflies may be said, in a general sense, to be thoroughly well known—every spot visited by civilized man having furnished its quota to our collections—yet the minute Tineidse, or even the larger but obscure Noctuidse, have scarcely been collected at all in tropical countries, and any attempt to study their geographical distribution would certainly lead to erroneous results. The same thing occurs, though perhaps in a less degree, among the Coleoptera. While the Carabidse, Bupreatidae, and Longicorns of the Tropics, are almost as well known as those of the Temperate Zones, the Staphylinidae, the smaller Elateridae, and many other obscure and minute groups, are very imperfectly represented from extra-European countries. I therefore propose to examine with some care the distribution of the Butterflies, and the Sphingina among Lepidoptera, and the following large and well-known families of Coleoptera:—Cicindelidse, Carabidae, Lucanida, Cetoniidse, Buprestidse, and the three families of Longicorns. These families together contain over 30,000 species, classed in nearly 3,000 genera, and comprise a large proportion of the best known and most carefully studied groups. We may therefore consider, that a detailed examination of their distribution will lead us to results which cannot be invalidated by any number of isolated facts drawn from the less known members of the class.

Eange of Insects in Time.—In considering how much weight is to be given to facts in insect distribution, and what interpretation is to be put upon the anomalies or exceptional cases that may be met with, it is important to have some idea of the antiquity of the existing groups, and of the rate at which the forms of insect life have undergone modification. The geological record, if imperfect in the case of the higher animals, is fragmentary in the extreme as regards indications of former insect life; yet the positive facts that it does disclose are of great interest, and have an important bearing on our subject. These facts and the conclusions they lead to have been discussed in our first volume (p. 166), and they must be carefully weighed in all cases of apparent conflict or incongruity between the distribution of insects and that of the higher animals.

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