ISO CLASSIFICATION OF AMPHIBIANS. 



among the tortoises^ or chelonian reptiles (one of the 

 best marked groups m nature), we have precisely the 

 same variation. The true tortoises walk only upon dry 

 land^ while the marine turtles are entirely aquatic^ and 

 have their feet converted into fins. The turtles are aber- 

 rant in their own family, so also are the flying lizards 

 in theirs ; nevertheless the difference, after all, between a 

 Pterodactylus and an Ichthyosaurus is so great that 

 many other forms must have intervened between them. 

 (129.) We have hitherto discussed this question 

 without any reference to analogies, or to that theory of 

 representation which we have so often employed to 

 illustrate a uniformity of plan in the creation Let us 

 therefore now see how far the foregoing views on the 

 situation of the Pterodactyli will bear analogical tests. 

 "We have already shown that the group of reptiles 

 in which we have placed the flying lizards, is that 

 which leads to the fish, thereby forming one circu- 

 lar group of the aberrant vertebrated animals. But, 

 as by this disposition of the Enalosauri, they will also 

 occupy that part of the reptile circle which comes 

 nearest to birds, we should have been totally at a loss 

 to conceive how this affinity could be established, had 

 it not been for the discovery of these winged reptiles. 

 The dragon lizard, it is true, from being provided with 

 dilated processes in the shape of wings, might be sup- 

 posed to constitute one link in the chain between 

 reptiles and birds ; but in that animal the resemblance 

 is merely analogical ; for all its four feet are perfectly 

 formed, like the rest of the lizards, and these wing-like 

 processes, although evidently intended to assist the 

 animal as it springs from one branch to another in 

 search of insects, are yet totally incapable of carrying 

 it through the air. No one would think of fixing 

 upon the flying squirrels of America, or the flying fish 

 of the Atlantic, as animals intervening between their 

 own classes and that of bhds. The dragon-lizards 

 {Draco), of which we are now speaking, are precisely 

 in the same relationship, and they, no doubt^ represent 



