598 GEOLOGICAL HISTORY. 



numbers and variety of genera, in the Mesozoic ; and now they are a 

 nearly extinct group. 



(5.) Amphibians, beginning in the Subcarboniferous, were, in the 

 Carboniferous age and the Triassic period, the most prominent kind 

 of Reptilian life, and of formidable size, with scaly armor, and teeth ; 

 after that, they dwindled ; and now the tribe is represented only by 

 little, inferior, naked-skinned Frogs and Salamanders. 



(6.) True Reptiles, which began in the Carboniferous, had posses- 

 sion of the waters, the land, and the air, in the later Mesozoic, and 

 far exceeded, in size, in variety, and vastly also in numbers, the Reptiles 

 of the present era ; great, swimming, snake-like Mosasaurs, having a 

 length of seventy five feet ; swimming Enaliosaurs, of twenty to fifty 

 feet ; Dinosaurs, sometimes walking like bipeds, fifteen feet high ; and 

 Pterosaurs, flying bat-like, with, in some cases, a spread of wings of 

 twenty to twenty-five feet. 



(7.) Mollushs of the highest class — that of the Cephalopods — 

 began in the Silurian, in kinds having straight, chambered shells ; 

 coiled forms followed ; and then, in the Mesozoic, a wonderful variety 

 of the most complex and largest kinds, with and without shells, existed ; 

 but nearly every genus with chambered external shells disappeared at 

 the close of the Mesozoic ; and now the only species are three or four 

 of the Silurian and all-time genus, Nautilus. 



These examples are enough to prove that the culmination of types, 

 and then a dwindling in numbers, size, and grade, have always been 

 involved in the system of progress. At the same time, many tribes, 

 on the same principle, have their era of culmination now. This is 

 true of Gasteropods, among Mollusks, of Birds, of the higher Insects, 

 of Teliost Fishes, probably of Crustaceans. Mammals culminate now 

 in Man, while brute Mammals reached their climax in the Champlain 

 period of the Quaternary. Other examples of the condition of some 

 of the more prominent tribes through time are presented in the tables 

 on pages 386, 589. 



7. (1.) The earliest species under a type are not necessarily the low- 

 est. — If we may trust the records, Echinoderms, or the highest type 

 of Radiates, were represented by species (Cystids and Crinids), long 

 before the inferior type of Polyps existed ; this can hardly be ac- 

 counted for satisfactorily on the supposition that the earliest Polyps 

 made no calcareous secretions, seeing that the ocean's waters were then 

 eminently calcareous. (2.) The highest group of Cryptogams, the 

 Ground Pines, were a prevailing form of terrestial vegetation, long 

 before there were Mosses. (3.) There were huge Crocodilians in the 

 world, long before there were limbless Snakes, like those of the present 

 world. The great Labyrinthodonts were vastly superior in every re- 



