CROCODILIA 209 



cannot be the least doubt regarding their ancestry. None of the 

 crocodiles which we have considered, whether ancient or modern, 

 can truthfully be called purely aquatic. They never ceased to use 

 their limbs for locomotion on land, more or less of the time, or for 

 the support of the body; and many of them have subsisted, in 

 part at least, on land animals. How easy it may have been for 

 some of them to become almost wholly emancipated from land 

 habits we may easily conjecture. The gavials, as we have seen, 

 are more at home in the water than upon land; their food is chiefly 

 found in the water; but, so long as their habits restrict them chiefly 

 to fresh water, or to the vicinity of the shores, their limbs continue 

 to be used as much for crawling as for swimming. Were the gavials 

 to be driven out to sea by the stress of fresh-water conditions or 

 attracted thereto by a greater abundance of more easily obtainable 

 or better food, so far from land that they no longer would have much 

 use for their legs for the support or propulsion of their bodies, 

 in the course of time they would doubtless change to a more purely 

 aquatic type. And in that change there would be material modi- 

 fications of their structure: their limbs would become better 

 adapted to movements in the water; the skin would become 

 smoother, and the bony and horny scales would be lost, since they 

 would be not only useless in the water, but actually detrimental to 

 the well-being of the animals; and the tail would develop into a more 

 powerful organ of propulsion, as a means of increasing their speed 

 in obtaining food or in escaping their enemies, such as the sharks. 

 In fact, we can only imagine that in the transformation precisely 

 those modifications would occur which we actually find in these 

 old sea-crocodiles, the Thalattosuchia. And they are of especial 

 interest to us here because nowhere do we find a better example of 

 Nature in the act of transforming a terrestrial or subterrestrial 

 animal into a truly aquatic one. 



The group comprises only a few forms, so far as known. All 

 were of modest size among extinct reptiles, from ten to twenty feet 

 in length, and all are from the Upper Jurassic deposits of Europe. 

 They did not exist very long, probably because they found it im- 

 possible to discard old habits and old structures entirely and 

 become absolutely emancipated from the land ; their breeding habits 



