10 CLASSIFICATION OP MONOCARDIAN ANIMALS. 



bat, while the hinder ones were clearly intended for 

 walking : the jaws are enormonsly prolonged, analogous 

 in their length to those of a woodcock : the whole 

 structure, in short, is such an extraordinary compound 

 of a reptile and a bird, that no doubt can remain on 

 the affinity between the two classes ; for although the 

 passage is not marked by existing animals so clearly as 

 that between quadrupeds and fishes, it is quite evident 

 that the Pterodactyli are more aUied to birds than to any 

 other vertebrated animals out of the class of reptiles. 



(8.) By thus tracing the natural series of the verte- 

 brated animals according to their affinities, we find they 

 form one great circle. Commencing with quadrupeds, we 

 pass on to fishes ; to these succeed the amphibians and 

 the reptiles : these latter are followed by birds ; and 

 birds, as already explained, bring us back again, by a 

 different route, to quadrupeds. 



(9-) ^^'^e are now to investigate, however, the truth of 

 another proposition formerly stated regarding natural 

 groups j which was, that the aberrant divisions of every 

 circle formed a distinct circle by themselves, quite in- 

 dependent of their union with the two typical circles. 

 Now, the aberrant divisions of the Vertebrata are the 

 fishes, amphibians, and reptiles. Two questions there- 

 fore arise : first. Is there not a greater similarity between 

 these three, than there is between them and quadrupeds 

 and birds ? and secondly, Is this similarity so strong as 

 to favour the belief that they actually do unite into a 

 circle of their own ? The first question must, of course, 

 be answered in the affirmative; for although an ordinary 

 observer might easily mistake an eel for a serpent, a 

 salamander for a lizard, a young frog for a fish, or 

 even a Chironectes for a frog, no one is likely to confound 

 any one of these animals vdth a quadruped or a bird. 

 As to the second question, we have the opinion of 

 Cuvier, — an opinion adopted by others, that many of the 

 cartilaginous fishes evince a decided affinity to the 

 reptiles ; and this is the very point where the two 

 extremes of the monocardian animals would meet, if 



