TORTOISES AND TURTLES. 29 



dispersal of the members thereof, compelling 

 them to seek a livelihood in an environment 

 quite different to that of the centre from which 

 they started. Thus we get an exchange con- 

 stantly taking place between the inhabitants of 

 the sea and those of the land. We find the crab 

 deserting his natural element to climb palm-trees 

 for cocoa nuts, and mammals which have adopted 

 the life of fishes. Such an exchange, however, 

 can only take place under certain conditions the 

 emigrants must adapt themselves to the require- 

 ments of their environment ; and this brings 

 about a more or less complete transformation of 

 the body. 



Among the Chelonia we have many instances 

 of this. Originally terrestrial, some have adopted 

 a fresh water habitat, others have taken to the 

 sea. The modification which these aquatic forms 

 have undergone are sufficiently well marked to- 

 render them easily distinguishable from their 

 stay-at-home relatives. Hence we get Land Tor- 

 toises, Water Tortoises, and Turtles. 



What the ancestral Tortoise may have been 

 like we do not know, but its descendants do not- 

 appear to have found any great necessity to 

 change their form after once the general archi- 

 tecture of the body had been determined on. 

 This much is to be gathered from the fact that 

 the fossil remains of these creatures, which occur 

 in remote geological formations the earliest 

 known Chelonian occurring in the Upper Keuper 

 of Wiirtemberg -differ but little from the same 

 parts of the skeleton of its nearest modern repre- 

 sentative. It is only in minor characters, divid- 



