30 THE STORY OF REPTILE LIFE. 



ing the larger or smaller groups of species one 

 from another, that modifications occur among the 

 living Chelonia. 



This conservatism is really very remarkable 

 when we reflect that they were already an 

 ancient group long before we have any record of 

 the advent of the birds upon the earth. It 

 seems difficult to realise that the conditions of 

 life through such enormous periods of time can 

 have affected them so little. 



Fig. 4. The big-headed tortoise. This creature is remarkable for the 

 large size of the head and the extraordinary flatness of the shell. 



In the history of the past, many animals have 

 sought protection for the body by encasing it in 

 armour-plate. It would seem, however, that such 

 protection either ends in bringing about the 

 extermination of the species by which it has 

 been adopted, or in being discarded by later 

 generations. The Chelonia afford an exception 

 to this rule. Judging by the conditions of life 

 which obtain to-day, it is not easy to see why 

 this should be so, inasmuch as the Tortoises and 

 Turtles do not appear to be beset by any more 

 formidable enemies than their unprotected rep- 

 tilian neighbours. The key to the mystery may 



