THE CARBONIFEROUS PERIODS 489 



(Fig. 462) ; (2) the bones of their skulls are similar in position and 

 arrangement; (3) some primitive amphibians have rings of bony 

 plates (sclerotic plates) about the eyes ; (4) the structure of the fin 

 of the fringe-finned ganoids was such that a leg might have been 

 formed from it by modification. 



Rise of Amphibians. — The rise of amphibians was a momentous 

 step in the evolution of life, but it was not a surprising one. Fishes 

 were the predominant race of the Devonian, and it was to be expected 

 that the structure of some of them would in time become modified 

 to take advantage of the realm of the lands where food was either 

 more easily obtainable or of a more nutritious quality than in the seas, 

 or where the competition was less keen. The extensive swamps of the 

 Devonian and Carboniferous and the shiftings of the seas have been 

 assigned as the immediate causes of the rise of the amphibians from 

 fishes. It seems more probable, however, that during every period 

 of the Paleozoic, swamps and shallow water were present, in which 

 amphibians would have been evolved had fishes been present which 

 were so constructed that by slight modifications they could become 

 adapted to land conditions. How this was accomplished is shown in 

 the development of the individual amphibian, which in the tadpole 

 stage is physiologically a fish, but which later breathes by means of a 

 simple sack-like lung instead of gills. It is important to note that 

 the Carboniferous amphibians, more than their modern relatives, 

 possessed characters closely allying them to the fishes, and that their 

 fishlike characters were more like those of Devonian than of modern 

 fishes. 



Some doubtful amphibian tracks have been found in the Devonian, 

 but no bones earlier than the Carboniferous have been discovered. 

 However, the well-developed limbs of the earliest species indicate a 

 line of ancestors that lived in the Devonian. Amphibians attained 

 their greatest importance in the Carboniferous (Pennsylvanian and 

 Permian) and have taken a very subordinate place since the Triassic. 

 Even before the close of the Pennsylvanian they had begun to give 

 place to the reptiles. 



Reptiles. — The reptiles of the Permian were even more varied than 

 the amphibians and have been placed in three groups. The first group 

 (cotylosaurs) includes the most primitive reptiles known, its mem- 

 bers differing from other reptiles, among other particulars, in having 

 a roofed-over skull. All the members of this class that are known 

 had very short necks: short, stout limbs; rather heavy bodies, and 



