AQUATIC REPTILES 



20I 



Even to-day there are tendencies toward marine invasion 

 observed among several of the surviving families of Hzards 

 and crocodiles of seashore frequenting habits. 



ADAPTIVE RADIATION OF AQUATIC REPTILES 



Fig. 77. Independent Reversed Adaptation to the Aquatic Zones in Twelve 

 Orders of Reptiles, Originating on Land and Entering the Seas. 



Diagram showing the manner in which twelve of the eighteen orders of reptiles descend 

 from the terrestrial (land-living) zone into the paludal (swamp-frequenting) zone, thence 

 into the littoral-fluviatile (fresh-water and brackish-water) zone, thence into the littoral- 

 marine (salt-water) zone, and finally into the pelagic zones of the high seas. This final 

 marine pelagic phase of evolution is attained in only six orders, namely, the plesiosaurs, 

 Chelonia (sea- tortoises), ichthyosaurs, mosasaurs (marine lizards), crocodiles, and 

 certain ophidians (true sea-snakes found far out at sea in the Indian Ocean). Nine of 

 the reptilian orders give off not only one but from two to five independent branches 

 seeking ac^uatic life, of which si.\ independently reach the full pelagic high-sea phase. 



Still more remarkable than the law of reversed adaptation 

 is that of alternate adaptation, which has been brilliantly 



