Page:Popular Science Monthly Volume 60.djvu/569

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THE EVOLUTION OF FISHES.

The few living lung fishes resemble the salamanders almost as closely as they do fishes, and they may well be ranged as a class by themselves midway between the primitive sharks and the amphibians. The few living forms show these intermediate characters in the development of lungs and the primitive character of the pectoral and ventral limbs. Those now extant give but little idea of the great variety of extinct dipnoans, but the obvious suggestion that with the lung fish is the place of divergence of the higher vertebrates from the fish series may be the correct one. The living genera are three in number, Neoceratodus in Australian rivers, Lepidosiren in the Amazon and Protopterus in the Nile. These are all mud fishes, some of them living through most of the dry season encased in a cocoon of dried mud. Of these forms Neoceratodus is certainly the nearest to the ancient forms, but its embryology, owing to the shortening of its growth stages due to its environment, has thrown little light on the question of its ancestry.

From some branch of the dipnoans the ancestry of the amphibians and through them that of the reptiles, birds and mammals may be traced, although some reason exists for regarding the primitive Crossopterygium as the point of divergence. It may be that the Crossopterygian gave rise to Amphibian and Dipnoan alike.

In the process of development we next reach the characteristic fish mouth in which the upper jaw is formed of maxillary and premaxillary elements distinct from the skull. The upper jaw of the shark is part of the palate, the palate being fused with the quadrate bone which supports the lower jaw. That of the dipnoan is much the same. The development of a typical fish mouth is the next step in evolution and with its appearance we note the decline of the air-bladder in size and function.

The next great offshoot is the group of crossopterygians, fishes which still retain the old-fashioned the of pectoral and ventral fin, the archipterygium. In the archaic tail, enameled scales and cartilaginous skeleton the crossopterygian shows its affinity with its dipnoan ancestry. Thus these fishes unite in themselves traits of the shark, lung-fish and Ganoid. The few living crossopterygians, Polypterus, and Erpetoichthys are not very different from those which prevailed in Devonian times. The larvæ possess external gills with firm base and fringelike rays, suggesting a resemblance to the pectoral fin itself which develops from the shoulder-girdle just below it and would seem to give some force to Kerr's contention that the archipterygium is only a modified external gill. In Polypterus the archipterygium has become short and fan-shaped, its axis made of two diverging bones with flat cartilage between. From this type it is thought that the arm of the higher forms has been developed. The bony basis may be the humerus,