178 



THE ORIGIN AND EVOLUTION OF LIFE 



not only a dual breathing system of gills and lungs, but a dual 

 motor equipment of limbs and of a propelling median fin in 

 the tail region. 



So far as known, the primordial Amphibia in their form were 

 chiefly of the small-headed, long-bodied, small-Hmbed, tail-pro- 



FiG 59. Descent of the Amphibia 



The Amphibia — in which the fin is transformed into a limb (Thinopus) — are believed to 

 have evolved from an ancestral ganoid fish stock of Silurian age through the fringe- 

 finned ganoids. From this group diverge the ancestors of the Reptilia and the sala- 

 mander-like Amphibia which give rise to the various salamander types, also to branches 

 of limbless and snake-like forms (Aistopoda, modern Coecilians). The other great 

 branch of the solid-skulled Amphibia, the Stegocephalia, was widespread all over the 

 northern continents in Permian and Triassic time (Cricotas, Eryops), and from this 

 stock descended the modern frogs and toads (Anura). Prepared for the author by 

 W. K. Gregory. 



pelled type of the modern salamander and newt. The large- 

 headed, short-bodied types (Amphibamus) were precocious 

 descendants of such primordial forms. In Upper Carbonifer- 



