208 



THE ORIGIN AND EVOLUTION OF LIFE 



and hind paddles are homodynamic, i. e., exerting equal power; 

 they are so exactly alike that it is very difficult to distinguish 

 them, whether they are provided with four broad paddles or 

 with four long, narrow, slender paddles. The plesiosaurs 



afford the first illustration we 

 have noted of another of the 

 great laws of form evolution, 

 namely, adaptation occurs far 

 more frequently through 

 changes of existing proportions 

 than through numerical addi- 

 tion of new characters. It is 

 proportional changes which 

 separate the swift-moving 

 plesiosaurs {Trinacromerion os- 

 horni), which are invariably 

 provided with long heads, short 

 necks, and broad paddles, from 

 the slow-moving plesiosaurs 

 (Elasmosaurus) , which are pro- 

 vided with narrow paddles, 

 short bodies, extremely long 

 necks, and small heads. 



It is believed that the lizard- 

 like ancestors of the mosasaurs 

 left the land early in Cretaceous 

 time ; it is certain that through- 

 out the three or four million years of the Cretaceous epoch 

 they spread into all the oceans of the world, from the conti- 

 nental seas of northern Europe and North America to those 

 of New Zealand. In Europe these animals survived to the 

 very close of Mesozoic time since the type genus of the great 



TRINACROMERION 



CRETACEOUS 



Fig. 86. Types of Marine Pelagic 

 Plesiosaurs of the American Con- 

 tinental Cretaceous Seas. 



The slow-moving, long-necked Elasmo- 

 saurus and the swift-moving, short- 

 necked Trinacromerion. The limbs 

 are completely transformed into pad- 

 dles. The great differences in the pro- 

 portions of the neck and body repre- 

 sent adaptations to greater or less 

 speed. Restorations for the author by 

 W. K. Gregory and Richard Deckert, 

 chiefly after Williston. 



