THE CRUST OF THE EARTH. 



the earth during the successive periods from the Silurian to the 

 Human. The relative numbers of each class prevailing at the 

 different periods are indicated by the four following signs : 



V least. 



x more numerous. 



A ttill more numerous. 



O most numerous. 



58. It appears that the creation of fishes, reptiles, birds, and 

 mammalia, the four vertebrated classes, underwent a sudden and 

 large augmentation at the epoch of transition from the tertiary to 

 the human period, and that, to crown all, man appeared for the 

 first time in the period to which he has given his name. Among 

 the infinite variety of fossils discovered in the strata of the earth, 

 there is no instance of human remains. 



59. Although the various classes of animals and vegetables, 

 when once called into existence, have continued to prevail upon 

 the earth till the present time, the same is not true of the genera 

 constituting these classes, and still less of the species of these 

 genera, as already mentioned. Species appear in particular strata, 

 no trace of which is found in any other, superior or inferior. The 

 inference is, that such animals existed only during the period cor- 

 responding to the deposition of these particular strata. 



58 



