ii8 TYPES OF ANIMAL LIFE 



may have represented its crocodiles, ichthyosauri, dino- 

 saurs, &c., during the vast epoch which continued while 

 the carboniferous rocks were being deposited. 



This, however, is as yet a mere speculation. What is 

 certain is that the batrachians stand (as before said) 

 between reptiles and fishes. To the fishes they are 

 manifestly allied through their possession of gills and 

 the aquatic respiration they almost all practise during 

 the earlier stages of their existence. There are also 

 other anatomical points of resemblance, which it would 

 be tedious here to describe. On the other side they 

 exhibit a striking difference from all fishes and a close 

 resemblance to reptiles and higher vertebrates in the 

 structure of their limbs. In ourselves we have an 

 upper and lower limb-segment to both arm and leg, 

 we have a cluster of small bones in the root of both 

 hand and foot, and we have fingers and toes (both 

 called in zoology digits) proceeding therefrom. With 

 varying degrees of defect, the same essential structure of 

 limb exists in all reptiles and higher vertebrates, but no 

 such structure exists in any kind of fish. Reptiles have, 

 fishes have not, this " typical differentiation," as it is 

 called, of the limbs. Thus, batrachians let us down 

 gently, as it were, to the class of fishes, while retaining a 

 firm grasp with their typically differentiated limbs on 

 the class of reptiles. They do so in different degrees, 

 there being but a temporary affinity to piscine respiration 

 in the frogs, though a permanent one in the proteus and 

 the siren. 



While differing insignificantly in structure among 

 themselves, we have seen how very widely frogs all 

 differ from the other orders of their class that is, from 

 the efts (Urodela), the worm-like group (Ophiomorpha), 

 and the primitive batrachian group (Labyrinthodonta). 



