THE ORIGIN OF VERTEBRATE!) ANIMALS 177 



interest is, that every Amphibian repeats this 

 history in its own development. Intermediate 

 stages are not merely possible, but are known to 

 actually occur at the present day, a most important 

 point. The Axolotl is one of these, and is provided 

 with large gills and also lungs ; the Siren is similar, 

 and is found in the swamps of the Southern States 

 of North America ; the mud-fish Lepidosiren, of 

 Brazil, Protopterus of Africa, and Ceratodus of 

 Queensland are other examples. 



The origin of terrestrial Vertebrates is probably 

 revealed to us in this way, and it must be considered 

 that the fresh- water forms are descended from marine 

 ancestors and the terrestrial forms from the fresh 

 water. 



Reptilia. — Reptiles are an exceedingly abundant 

 group, both absolutely and relatively, in the second- 

 ary period : a great number of families and orders 

 are extinct. Although their actual origin is uncertain, 

 their descent from gill-breathing ancestors is proved 

 by their passing through the gill-cleft stage in their 

 development. No reptiles, however, have gills. 



It is uncertain whether reptiles are derived 

 from Amphibia, or descended from gill-breathing 

 ancestors directly. The gigantic Ichthyosaurus 

 (Fig. 30) and Plesiosaurus (Fig. 31), which have 

 more than five digits in the hand and foot, have 

 been supposed to indicate an independent origin 

 from fishes, but are more probably a reversion to 

 the paddle-like form in an aquatic group. More- 

 over, the oldest reptiles yet obtained as fossils are 

 pentadactyl, and not provided with paddles. Of 



