LIMITATION OF SPECIES IN TIME. 157 



their remains in successive geological formations, is cir- 

 cumscribed, step by step, witliin narrower, more definite, 

 and also more equable periods. Species are truly limited 

 in time, as they are limited in space, upon the surface of 

 the globe. The facts do not exhibit a gradual disappear- 

 ance of a limited number of species, and an equally gradual 

 introduction of an equally limited number of new ones ; 

 but, on the contrary, the simultaneous creation and the 

 simultaneous destruction of entire faunae, and a coinci- 

 dence between these changes in the organic world and 

 the great physical changes our earth has undergone. Yet, 

 it would be premature to attempt to determine the extent 

 of the geographical range of these changes, and still more 

 questionable to assert their synchronism upon the whole 

 surface of the globe, in the ocean and upon dry land. 



To form adequate ideas of the great physical changes 

 which the surface of our globe has undergone, and the fre- 

 quency of these modifications of the character of the earth's 

 surface, and of their coincidence with the changes observed 

 among the organized beings, it is necessary to study at- 

 tentively the works of Elie de Beaumont. 1 He, for the 

 first time, attempted to determine the relative age of the 

 different systems of mountains, and first showed also, that 

 the physical disturbances occasioned by their upheaval 

 coincided with the successive disappearance of entire 

 faunae, and the reappearance of new ones. In his earlier 

 papers he recognized seven, then twelve, afterwards fifteen, 

 such great convulsions of the globe ; and now he has traced, 

 more or less fully and conclusively, the evidence that the 

 number of these disturbances has been at least sixty, per- 

 haps one hundred. But, while the genesis and genealogy 



4 ELIE DE BEAUMONT, Notice sur (LEOP. v.,) Ueber die geognotischen 

 les systemes dc Montagnes ; Paris, Systeme von Deutschland,Leonhard's 

 1852, 3 vols. 12mo. ; see, also, BUCH, Taschenb., 1824, II, p. 501. 



