TORTOISES AND TUETLES. 43 



From the pond-tortoises we pass, by a very 

 natural transition, to the Marine Turtles. In 

 them we see the final results of adaptation to 

 jin aquatic life. Whilst the general form of 

 juhe body has undergone little or no change, 

 phe limbs have become completely transformed 

 iinto swimming paddles. In the fore-limbs the 

 J3xtent of the changes is extremely marked, 

 ^either in the fore nor the hind limbs are 

 iligits any longer to be distinguished; in ac- 

 bordance with their new functions in both limbs 

 ohey are enclosed within a common skin, so 

 fthat the once walking limbs have now become 

 < paddles," superficially bearing the strongest 

 resemblance to the paddles of the old fish 

 [lizards the Plesiosaurs and Ichthyosaurs, to 

 )e described later the Penguins among the 

 Ipirds; or the Whale tribe among the Mam- 

 |nals. In all these cases, just as in the turtles, 

 bhe paddles have been evolved by the modifica- 

 tion of limbs originally used in quite other ways. 

 [Another point of interest with regard to the 

 paddles of the turtles is, that whilst those of 

 Ihe fore-limbs were of great length, the hinder 

 |)air were extremely short. The explanation of 

 [his, of course, is obvious the long paddles were 

 lised as propellers, the short ones as rudders. 

 IChe same applies also to the ancient Ichthyo- 

 laurs, and to the modern whales. In the latter, 

 Indeed, the hind-paddles have disappeared al- 

 |ogether, the work of steering being undertaken 

 >y the tail. In some other cases, to be discussed 

 'S we proceed, we shall find this arrangement 

 xactly reversed the hind-limbs developing at 



