VI.] 



THE COMMON FROG. 



83 



independent origin of structures, so similar that at 

 first sight their similarity might well have been 

 deemed a conclusive evidence of afi[inity. 



Fig. 47. — Lateral view of skull of L&phiomys, showing bony lamellae behind the 



orbit. 



Here, also, we have a memorable caution against 

 hasty inferences from structural similarities. If this 

 resemblance and that of the dorsal shields are, when 

 taken together, no signs whatever of special fgenetic 

 affinity — it is difficult to say what structural likenesses 

 are to be deemed unquestionable evidences of a 

 common ancestry. 



Passing now to the skeleton of the limbs, we come 

 to a character of great significance, as it is one which 

 serves to distinguish all the limbed species of the frog's 

 class from lower vertebrates. The character is very 

 significant, because all Batrachians, in spite of their 

 numerous and important fish affinities, differ from all 

 fishes, and agree with all higher classes in that they— 

 ii they have limbs at all— have them divided into those 

 very typical segments which exist in man ; namely, 

 shoulder-bones, arm-bones, wrist-bones, and hand- 

 bones ; and into haunch-bones, leg-bones, ankle-bones, 

 and foot-bones respectively. It is difficult, then, to 

 avoid the belief that in the Batrachian class we come 



G 2 



