432 



EVOLUTIONARY MORPHOLOGY 



all the Tetrapods is the possession of limbs ending in five 

 digits, and it has already (see p. 315) been shown that the skele- 

 ton of the pectoral fin of the Osteolepid fish Sauripterus is 

 such as to render it easy to suppose that the pentadactyl limb 

 arose from a fin like that of the Osteolepids. Osteolepis is 

 Devonian, and the earliest known amphibia are from the Lower 

 Carboniferous. It is fairly certain, therefore, that at some time 

 in the Devonian, fish living in the estuaries and fresh-water 

 basins became subjected to the desiccation which characterised 

 this period. They were able to breathe atmospheric oxygen 



Fig. 176. — A few examples of different types of Amphibia. (Not drawn 



to scale.) 



a, restoration of Stegocephalian ; b, male newt in breeding season 

 (Urodele) ; c, Amblystoma (larval form or Axolotl showing the external 

 gills) ; d, frog (Anuran) ; e, Ichthyophis (Gymnophiona) . (e after 

 Sarasin). 



by means of their nostrils and lungs, and as they floundered 

 about in the mud, the number of rows of radials in their fins 

 became reduced to five, separate from one another instead of 

 being united by the web of a fin. The persistence of the 

 lateral-line canals shows that these animals still spent much 

 of their time in the water, and their excursions on land 

 probably took the form of wandering from pond to pond. 

 In fact, the amphibia never succeeded in making themselves 

 completely independent of water, and for three reasons. In 

 the first place the eggs had to be laid in water, and the larval 



