262 THE DOCTRINE OF DESCENT. 



differentiation of structure is to be observed, as in the 

 Lacertilia, or Crocodilia, it goes no further than a modi- 

 fication of the form of the articular surfaces of the verte- 

 brae, or of the degree to which the internal nasal aper- 

 tures are surrounded by bone. The osteological differ- 

 ences, which alone are exhibited by fossil remains, have 

 doubtless been accompanied by many changes in the 

 organization of the destructible parts of the body ; but 

 everything tends to show that the amount of change in 

 the organization of reptiles since their first known ap- 

 pearance upon the earth, is not great in itself; and is 

 wholly insignificant, if we take into consideration the 

 lapse of time, and the changes of the surface of the 

 globe, which are represented by the Mesozoic and Ter- 

 tiary formations. 



" From the point of view of the evolution hypothesis, 

 it is necessary to suppose that the Reptilia have all 

 sprung from a common stock, and I see no justification 

 for the supposition that the rapidity of their divergence 

 from this stock was greater before the epoch of the 

 Trias than it has been since. Consequently, seeing 

 that the approximation of the oldest known representa- 

 tives of the different orders is so slight, reptiles must 

 have lived before the Trias for a length of time, com- 

 pared with which that which has elapsed from the 

 Triassic epoch until now is small — in other words, the 

 commencement of the existence of reptiles must be 

 sought in a remote palaeozoic epoch." 



Comparison thus points us back to ages which afford 

 no record of the actual derivation of this class. Even 

 the Ichthyosauria and Plesiosauria, so frequently men- 

 tioned in conjunction, deviate widely from one another 



