28 THE THEORY OF EVOLUTION 



On the whole the higher classes of vertebrates in 

 the post-Cambrian formation follow the lower, and 

 often, within the classes, the higher forms follow the 

 lower. The latter applies also in many cases to the 

 Invertebrates. 



The vertebrate classes as now represented are, 

 beginning at the lower, the Fishes, Amphibia, Eeptilia, 

 Birds, and Mammalia. 



The Fishes appear for the first time in the Silurian 

 formation, and are divided into three different groups : 

 these are the Sharks, which rank as low grade ; the 

 other groups were armoured, clumsy forms which 

 subsequently disappeared. 1 



The Amphibia appear in the upper Devonian. 2 



The Reptiles, certainly strange-looking forms, show 

 themselves for the first time in the upper Carboniferous 

 formation (' Sauravus '). 3 In the Permian there appear 

 two orders of Reptilia which have died out except one 

 (genus Hatter ia) which now exists in New Zealand and 

 has always been regarded as a stranger in our animal 

 world. Quite recently came the discovery of three 

 great groups of ' well-developed land reptiles ' in 

 the Russian Permian formation. 1 * 



Reichenbach's Lehrbuch der Paldontologie (Naturwissenschaft und Technik 

 in Lehre und Forschung), Leipzig -Berlin, 1909, Part 1. J. Bumuller handles 

 the question on briefer lines (Die EntwicJclungstheorie und der Mensch, 

 Munich, 1907, p. 50). This excellent and inexpensive work is highly 

 to be commended. 



1 E. Kayser, p. 138. 



2 Deperet-Wegner : Die Umbildung der Tierwelt, p. 229. 



3 Ibid. 



4 Ibid. p. 230. 



