158 ESSAY ON CLASSIFICATION. 



of our mountain systems have been thus illustrated, palae- 

 ontologists, extending their comparisons between the fossils 

 of different formations more carefully to all the successive 

 beds of each great era, have observed more and more 

 marked differences between them, and satisfied themselves 

 that faunae also have been more frequently renovated than 

 was formerly supposed ; so that the general results of 

 geology proper and of palaeontology concur in the main to 

 prove, that, while the globe has been at repeated intervals, 

 and indeed frequently, though after immensely long periods, 

 altered and altered again, until it has assumed its present 

 condition, so also have animals and plants, living upon its 

 surface, been again and again extinguished and replaced 

 by others, until those now living were called into existence, 

 with man at their head. The investigation is not in every 

 case sufficiently complete to show everywhere a coinci- 

 dence between this renovation of animals and plants and 

 the great physical revolutions which have altered the 

 general aspect of the globe, but it is already extensive 

 enough to exhibit a frequent synchronism and correlation, 

 and to warrant the expectation that it will, in the end, 

 lead to a complete demonstration of their mutual depend- 

 ence, not as cause and effect, but as steps in the same pro- 

 gressive development of a plan which embraces the physical 

 as well as the organic world. 



In order not to misapprehend the facts, and perhaps to 

 fall back upon the idea that these changes may have been 

 the cause of the differences observed between the fossils of 

 different periods, it must be well understood, that, while 

 organized beings exhibit, through all geological formations, 

 a regular order of succession, the character of which will 

 be more fully illustrated hereafter, this succession has 

 been from time to time violently interrupted by physical 



