=Centers of Distribution.=--We may assume, in regard to any species, that it has had its origin in or near that region in which it is most abundant and characteristic. Such an assumption must involve a very large percentage of error or of doubt, but in considering the mass of species, it may represent essential truth. In the same fashion we may regard a genus as being autochthonous or first developed in the region where it shows the greatest range or variety of species. Those regions where the greatest number of genera are thus autochthonous may be regarded as centers of distribution. So far as the marine fishes are concerned, the most important of these supposed centers are found in the Pacific Ocean. First of these in importance is the East-Indian Archipelago, with the neighboring shores of India. Next would come the Arctic Pacific and its bounding islands, from Japan to British Columbia. Third in importance in this regard is Australia. Important centers are found in temperate Japan, in California, the Panama region, and in New Zealand, Chili, and Patagonia. The fauna of Polynesia is almost entirely derived from the Indies; and the shore fauna of the Red Sea, the Bay of Bengal, and Madagascar, so far as genera are concerned, seems to be not really separable from the Indian fauna generally. [Illustration: FIG. 174.--Globefish, _Tetraodon setosus_ Rosa Smith. Clarion Island, Mexico.] I know of but six genera which may be regarded as autochthonous in the Red Sea, and nearly all of these are of doubtful value or of uncertain relation. The many peculiar genera described by Dr. Alcock, from the dredgings of the _Investigator_ in the Bay of Bengal, belong to the bathybial or deep-water series, and will all, doubtless, prove to be forms of wide distribution. In the Atlantic, the chief center of distribution is the West Indies; the second is the Mediterranean. On the shores to the northward or southward of these regions occasional genera have found their origin. This is true especially of the New England region, the North Sea, the Gulf of Guinea, and the coast of Argentina. The fish fauna of the North Atlantic is derived mainly from the North Pacific, the differences lying mainly in the relative paucity of the North Atlantic. But in certain groups common to the two regions the migration must have been in the opposite direction, exceptions that prove the rule. =Distribution of Marine Fishes.=--The distribution of marine fishes must be indicated in a different way from that of the fresh-water forms. The barriers which limit their range furnish also their means of dispersion. In some cases proximity overbalances the influence of temperature; with most forms questions of temperature are all-important.