206 THE STORY OF REPTILE LIFE. 



stocks with walking limbs, and the transforma- 

 tion of these limbs into paddles, so similar, yet 

 so different, is in itself a revelation. But the 

 marvel and the interest of this convergent 

 evolution, as it is called, becomes intensified 

 when we remember that it has been repeated 

 with variations several times in the history of 

 animals. The last to undergo these structural 

 alterations were members of the warm-blooded 

 mammals the whales and their allies, and the 

 manatees, whilst a close approximation thereto 

 is going on among the seal-tribe. The uniform 

 environment has produced a marvellously uni- 

 form result, moulding reptile, bird and mammal 

 into creatures strangely alike ; and just as they 

 have come more and more to resemble one 

 another, so they have come to differ more and 

 more from the stock which gave them birth. 

 Surveyed as separate groups, we remark that the 

 earliest known examples of each tribe are the 

 smallest, and least perfectly adapted to their 

 environment. As we follow their development, 

 we find not only an increase of specialisation, 

 which means perfection of function, taking place, 

 but also an increase in bulk. Finally, just before 

 extinction takes place, we find the most highly 

 specialised of all. Toothlessness is one of the 

 commonest of these last phases of development, 

 and bespeaks adaptation to a very special kind 

 of food. Any disturbance in the balance of 

 nature, whereby this food supply became seriously 

 lessened, would necessarily bring about extinction 

 of these highly specialised forms, for variation in 

 new directions would be impossible. The Ptero- 



