154 ESSAY ON CLASSIFICATION. 



the oldest geological periods had a much wider distribu- 

 tion than most recent families exhibit now, some families 

 of fishes largely represented in the Devonian system of 

 the Old World have not yet been noticed among the fossils 

 of that period in America, as, for instance, the Cephalas- 

 pids, the Dipteri, and the Acanthodi. Again, of the many 

 gigantic Eeptiles of the Triasic and Oolitic periods, none 

 are known to occur elsewhere than in Europe ; and this 

 can hardly be owing simply to the less extensive distri- 

 bution of these formations in other parts of the world, 

 since other fossils of the same formations are known from 

 other continents. It is more likely that some of them, at 

 least, were peculiar to limited areas of the surface of the 

 globe, as, even in Europe, their distribution is not ex- 

 tensive. 



Without, however, entering upon debateable ground, it 

 remains evident, that, before the establishment of the 

 present state of things, peculiar types of animals, which 

 were formerly circumscribed within definite limits, have 

 continued to occupy the same or similar grounds in the 

 present period, even though no genetic connection can be 

 assumed between them, their representatives in these dif- 

 ferent formations not belonging even to the same genera. 

 Such facts are in the most direct contradiction with any 

 assumption that physical agents could have had anything 

 to do with their origin ; for, though their occurrence within 

 similar geographical areas might at first seem to favour 

 such a view, it must be borne in mind that these beings, 

 so localized, are associated with other types, which have a 

 much wider range ; and, what is still more significant, 

 they belong to different geological periods, between which 

 great physical changes have undoubtedly taken place. 

 Thus the facts indicate precisely the reverse of what the 



