The history of fishes extends further through geological time than that of any other Vertebrates, and is perhaps more completely known to us, in consequence of the greater facilities for the preservation of their remains in aqueous deposits. If we receive Pander’s Conodonts as indicating a low type of cartilaginous fishes, these must have continued for vast ages without any elevation, and struggling for a bare existence amidst formidable Cuttle-fishes and Crustaceans, before, under more favourable conditions, they suddenly expanded into the high and perfect types of Ganoids and Sharks. If we reject the early Conodonts, then the two last-mentioned types spring together and suddenly into existence, like the armed men from the dragon’s teeth of Cadmus. They rapidly attain to numbers and grandeur unexampled in later times, and become the lords of the waters at the time when there was probably no Vertebrate life on the land. As the reptiles establish themselves on the land and in the waters, the Ganoids diminish, but the Sharks hold their own. At length the reign of reptiles is over, but the Ganoids, instead of resuming their pristine numbers, give place to the Teleosts, and become reduced to insignificance; while the Sharks, profiting by the decadence of the great marine reptiles, remain the tyrants of the seas. This history is strangely unlike a continuous evolution; but we are anticipating facts which will fall to be discussed in a subsequent chapter.
Fig. 122.—Modern Ganoids (Polypterus. Africa. Lepidosteus. America).
A Microsaurian of the Carboniferous Period (Hylonomus Lyelli).
Restored from the skeleton and dermal appendages found in an erect Sigillaria. Half natural size.