X.] THE COMMON FROG. 151 



has been said earlier, the Crocodilia amongst reptiles, 

 of which they may be deemed as the prophetic pre- 

 cursors, so to speak, though certainly not the direct 

 ancestors. 



Thus the class Batrachia, as a whole, presents a 

 very interesting analogy and parallelism with the 

 class Reptilia. 



It is a parallelism, moreover, which reminds us of 

 that which exists between the various orders of 

 Placental mammals and the great subdivisions of the 

 pouched or Marsupial order of mammals. We have 

 carnivorous, insectivorous, arboreal, aquatic, herbivor- 

 ous, marsupial beasts, as we have carnivorous, insecti- 

 vorous, arboreal, aquatic and herbivorous placental 

 beasts. The harmonious variations of the placental 

 and marsupial groups thus present us with excellent 

 instances of affinities independently evolved and not 

 due to hereditary influence. 



In a similar way it seems probable that the sub- 

 divisions (orders) of the class Batrachia, mim.ic, as it 

 were, quite independently the subdivisions (orders) of 

 the class Reptilia. 



The Frogs' class, as a whole, shows as many strik- 

 ing affinities to some or other fishes. It does so in 

 the possession of gills and of a branchial apparatus 

 during one time of life at the least ; a large para- 

 sphenoid in the skull ; the often persistently unseg- 

 mented terminal part of the notochord ; the single ven- 

 tricular cavity of the heart ; the presence of a biilbiis 

 aortcB ; the development of a nerviis lateralis ; the com- 

 munication between the urinary canal and the oviduct, 

 and certain other characters of less importance. 



