EVOLUTION OF THE AMPHIBIA 435 



(Anura), newts (Urodela) and Gymnophiona alive to-day. 

 An important feature in the amphibia is the outgrowth from 

 the gut to form a bladder. It is homologous with the allantois 

 of the Amniotes. 



With regard to the living amphibia, it is most important 

 to realise that they have departed far from the primitive type 

 of their Stegocephalian ancestors. This is shown by the great 

 reduction in the bones of the skull and other parts of the 

 skeleton. The Gymnophiona have evolved a worm-like 

 burrowing habit ; the frogs have become modified in connexion 

 with the habit of leaping with the hind legs, and the newts 

 have become secondarily readapted to living in water, even 

 going so far as to develop median fins which differ entirely from 

 those of fish in not possessing radials or dermal fin-rays. Now, 

 in several points, the newts of to-day resemble living Dipnoi 

 (such as Ceratodus) very closely. It is of the utmost import- 

 ance, however, to realise that these resemblances are due to 

 parallel evolution and convergence, and not to genetic affinity. 

 It is only necessary to look at the list of specialised characters 

 of Ceratodus to see that forms like it could not have given rise 

 to the Tetrapods : they are cousins and not ancestors. 

 Similarly, an examination of the specialised characters of 

 Triton and a comparison between it and the Stegocephalia 

 show that all the points in which Triton resembles Ceratodus 

 have been evolved within the amphibia, and a long time after 

 the amphibia came on land. 



Literature 



Watson, D. M. S. The Structure, Evolution and Origin of the Amphibia. 

 Philosophical Transactions of the Royal Society, Ser. B, vol. 209, 1919. 



The Evolution and Origin of the^Amphibia. Philosophical Trans- 

 actions of the Royal Society, Ser. B, vol. 214, 1926. 



