354 THE CAUSES OF THE n 



been known in past times, and left remains 

 behind. 



Now, how many of those are absolutely extinct ? 

 That is to say, how many of these orders of animals 

 have lived at a former period of the world's history 

 but have at present no representatives ? That is 

 the sense in which I meant to use the word 

 " extinct." I mean that those animals did live 

 on this earth at one time, but have left no one 

 of their kind with us at the present moment. 

 So that estimating the number of extinct animals 

 is a sort of way of comparing the past creation as 

 a whole with the present as a whole. Among the 

 mammalia and birds there are none extinct ; but 

 when we come to the reptiles there is a most 

 wonderful thing : out of the eight orders, or 

 thereabouts, which you can make among reptiles, 

 one-half are extinct. These diagrams of the 

 plesiosaurus, the ichthyosaurus, the pterodactyle, 

 give you a notion of some of these extinct reptiles. 

 And here is a cast of the pterodactyle and bones 

 of the ichthyosaurus and the plesiosaurus, just as 

 fresh-looking as if it had been recently dug up in a 

 churchyard. Thus, in the reptile class, there are 

 no less than half of the orders which are absolutely 

 extinct. If we turn to the Amphibia, there was 

 one extinct order, the Labyrinthodonts, typified 

 by the large salamander-like beast shown in this 

 diagram. 



No order of fishes is known to be extinct. 



