152 EVOLUTION AND GENETICS 



vegetation was Abundant. Among such Amphibia, the Stego- 

 cephalia or "solid-headed" forms, were the ancestors of the rep- 

 tiles (Fig. 84). Already well adapted in many ways to the con- 

 ditions of terrestrial life, they were forced to make use of these 

 adaptations or perish when the increasing aridity of the later 

 Carboniferous deprived them even of the seasonal opportunity to 

 return to the water. 



The Reptiles were abundant and well diversified in the Permian. 

 Some of them developed peculiar structures which had no use that 

 we can now distinguish, and all of the Permian species are now 

 extinct. Among the orders represented by Permian reptiles, how- 

 ever, are probal)ly the Chelonia or turtles, and the Rhynchoce- 

 phaha, both still in existence. Primitive crocodiles also probably 

 appeared, but the other great orders of extinct reptiles and 

 the snakes, lizards and crocodiles of the present arose much 

 later. 



Among the adaptations by which the reptiles are differentiated 

 from their amphibian ancestors are the resistant integument and 

 the foetal membranes, both associated with life away from the 

 water. The integument even of the early species was tough, and 

 provided with scales or bony exoskeletal structures. Through this 

 adaptation the animals were able to remain constantly in dry air 

 without losing more water from the body than could be replaced. 

 The foetal meml^ranes, as already noted, are apparently essential to 

 reproduction elsewhere than in water. 



Beyond these essential structures the reptiles also show a great 

 variety of adaptive possibilities which resulted in the development 

 of fish-like aquatic forms, flying forms, the dinosaurs, cynodonts, 

 and a few of less importance (Fig. 85). They attained their 

 greatest diversity in the Mesozoic, where they were represented 

 by some of the greatest animals that the world has ever seen. The 

 aquatic ichthyosaurs and plesiosaurs included highly specialized 

 animals, some enormous in size. Newman graphically describes 

 one, Trmacromerion, as "a creature with all the earmarks of an 

 aquatic speed demon, and doubtless as much of a terror to the 

 fishes as were the dinosaurs to the smaller denizens of the dry 

 land." These latter were of two fundamental types, the car- 

 nivorous and herbivorous dinosaurs. They ranged from small 

 species to creatures as large as the modern whales, and from heavy, 

 sluggish forms, protected by massive bony armor, to active preda- 



