AQUATIC REPTILES 



199 



CMAMPSOSAURUS 



CRETACEOUS 



Such regeneration is attributable to the potentiahty of the 

 heredity-chromatin which still resides in the cells of the am- 

 putated surfaces. The heredity-chromatin determiners of the 

 bones of the separate digits or separate phalanges if once lost 

 in geologic time are never reacquired; on the contrary, each 

 phase of habitat adapta- 



tion is forced to commence 

 with the elements remain- 

 ing in the organism's hered- 

 ity-chromatin, which may 

 have been impoverished in 

 previous habitats. When 

 an ancient habitat zone is 

 reentered there must be 

 readaptation of the parts 

 which remain. Thus, 

 when the terrestrial rep- 

 tiles reenter the aquatic 

 zone of their amphibian 

 ancestors they cannot re- 

 sume the amphibian char- 

 acters, for these have been 

 lost by the chromatin. 

 This invariable princi- 



REPTILIA RHVTIDODON ^^,^^^,^ 



Fig. 75. Reptiles Leaving a Terrestrial 

 FOR AN Aquatic Habitat, the Beginning 

 OF Aquatic Adaptation. 



Littoral-fluviatile types independently evolve 

 in the Triassic {Rhytidodon, a phytosaur) and 

 in the Upper Cretaceous (Cliampsosaitrus). 

 These animals belong to two widely different 

 orders of reptiles, neither of which is closely 

 akin to the modern alligators and crocodiles. 

 The adaptation is convergent to that of the 

 existing gavials and crocodiles. Restorations 

 for the author by W. K. Gregory and Richard 

 Deckert. 



pie underlying reversed 

 evolution is partly illustrated (Fig. 53) in the passage from the 

 reptilian foot into the fin of the aquatic reptile and with equal 

 clearness in the passage of the wing of the flying bird into the 

 fin of the swimming bird (Fig. no). 



In no less than eleven out of the eighteen orders of reptiles 

 reversed adaptation to a renewal of aquatic life, like that of 

 the fishes and amphibians, took place in the long and slow 



