198 THE ORIGIN AND EVOLUTION OF LIFE 



great delta deposits of the Upper Cretaceous lying to the 

 east of the present Rocky Mountain range. The more re- 

 stricted deposition areas of drying pools and lagoons, such as 

 those observed in the Permian and Triassic shales and sand- 

 stones of Texas, entomb many forms of terrestrial life. Vistas 

 of the contemporaneous evolution of fluviatile, aquatic, and 

 marine life are afforded by the animals which perish at the 

 surface and sink to the calcareous bottom oozes of the conti- 

 nental seas of Triassic, Jurassic, and Cretaceous time. It is 

 only in the Tertiary of the Rocky Mountain region of North 

 America that we obtain a nearly continuous and uninterrupted 

 story of the successive forms of continental life, among the 

 mammals entombed in the ancient flood-plains, in the volcanic 

 ash-beds, in the lagoons, and more rarely in the littoral deposits. 



Aquatic Adaptation of the Reptilia, Direct and 



Reversed 



From the distinctively terrestrial radiations of Permian 

 time we turn to the development of aquatic habitat phases 

 among the reptiles which lived along the borders of the great 

 interior rivers and continental seas of Permian, Triassic, and 

 Jurassic time. 



This reversal of adaptation from terrestrial into aquatic 

 life is, as we might theoretically anticipate, a reversal of func- 

 tion rather than of structure, because, as above stated (p. 159), 

 it is a universal law of form evolution that ancient adaptive 

 characters once lost by the heredity-chromatin are never 

 reacquired. In geologic race evolution there is no process 

 analogous to the wonderful phenomena of individual regenera- 

 tion or regrowth, such as is seen among amphibians and other 

 primitive vertebrates, whereby the original limb may be com- 

 pletely restored from the mutilated remnant of an amputation. 



