Fig. 251.—Flying-fish, Cypsilurus heterurus (Rafinesque). Family Exocætidæ Woods Hole, Mass.
Fig. 252.—The Schoolmaster Snapper, a Perch-like fish. Family Lutianidæ. Key West.
Tertiary Fishes.—With the Eocene or first period of the Tertiary great changes have taken place. The early families of bony fishes nearly all disappear. The herring, pike, smelt, salmon, flying-fish, and berycoids remain, and a multitude of other forms seem to spring into sudden existence. Among these are the globefishes, the trigger-fishes, the catfishes, the eels, the morays, the butterfly-fishes, the porgies, the perch, the bass, the pipefishes, the trumpet-fishes, the mackerels, and the John-dories, with the sculpins, the anglers, the flounders, the blennies, and the cods. That all these groups, generalized and specialized, arose at once is impossible, although all seem to date from the Eocene times. Doubtless each of them had its origin at an earlier period, and the simultaneous appearance is related to the fact of the thorough study of the Eocene shales, which have in numerous localities (London, Monte Bolca, Licata, Mount Lebanon, Green River) been especially favorable for the preservation of these forms. Practically fossil fishes have been thoroughly studied as yet only in a very few parts of the earth. The rocks of Scotland, England, Germany, Italy, Switzerland, Syria, Ohio, and Wyoming have furnished the great bulk of all the fish remains in existence. In some regions perhaps collections will be made which will give us a more just conception of the origin of the different groups of bony fishes. We can now only say with certainty that the modern families were largely existent in the Eocene, that they sprang from ganoid stock found in the Triassic and Jurassic, that several of them were represented in the Cretaceous also, that the Berycoids were earliest of the spiny-rayed fishes, and forms allied to herring the earliest of the soft-rayed forms. Few modern families arose before the Cretaceous. Few of the modern genera go back to the Eocene, many of them arose in the Miocene, and few species have come down to us from rocks older than the end of the Pliocene. The general modern type of the fish-faunas being determined in the latter Eocene and the Miocene, the changes which bring us to recent times have largely concerned the abundance and variety of the individual species. From geological distribution we have arising the varied problems of geographical distribution and the still more complex conditions on which depend the extinction of species and of types.
Fig. 253.—Decurrent Flounder, Pleuronichthys decurrens Jordan & Gilbert. San Francisco.
Factors of Extinction.—These factors of extinction have been recently formulated as follows by Professor Herbert Osborn. He considers the process of extinction as of five different types: