XI PALEONTOLOGY AND EVOLUTION 365 



which Eeptiles passed into Birds must have pre- 

 ceded them. In fact there is, even at present, 

 considerable ground for suspecting the existence 

 of Dinosauria in the Permian formations ; but, in 

 that case, lizards must be of still earlier date. 

 And if the very small differences which are 

 observable between the Crocodilia of the older 

 Mesozoic formations and those of the present day 

 furnish any sort of approximation towards an 

 estimate of the average rate of change^ among the 

 Sauropsida, it is almost appalling to reflect how far 

 back in Palaeozoic times we must go, before we 

 can hope to arrive at that common stock from 

 which the Crocodilia, Lacertilia, Ornitkoscelida, 

 and Plesiosauria, which had attained so great a 

 development in the Triassic epoch, must have 

 been derived. 



The Amphibia and Pisces tell the same story. 

 There is not a single class of vertebrated animals 

 which, when it first appears, is represented by 

 analogues of the lowest known members of the 

 same class. Therefore, if there is any truth in 

 the doctrine of evolution, every class must be vastly 

 older than the first record of its appearance upon 

 the surface of the globe. But if considerations of 

 this kind compel us to place the origin of ver- 

 tebrated animals at a period sufficiently distant 

 from the Upper Silurian, in which the first Elas- 

 mobranchs and Ganoids occur, to allow of the 

 evolution of such fishes as these from a Vertebrate 



