APPARITION OF SPECIES 95 



the development of animals and plants, this would 

 appear not as a smooth and continuous stream, but 

 as a series of great waves, each rising abruptly, and 

 then descending and flowing on at a lower level along 

 with the remains of those preceding it. This will be 

 explained more in detail in the following pages, in 

 which it may be necessary to mention briefly some of 

 the leading facts ascertained by geology. 



Geological investigation has disclosed a great 

 series of stratified rocks composing the crust of the 

 earth, and formed at successive times, chiefly by the 

 agency of water. These can be arranged in chrono 

 logical order ; and, so arranged, they constitute the 

 physical monuments of the earth s history. We must 

 here take for granted, on the testimony of geology, 

 that the accumulation of this series of deposits has 

 extended over a vast lapse of time, and that the suc 

 cessive formations contain remains of animals and 

 plants, from which we can learn much as to the order 

 of introduction of life on the earth. Without entering 

 into geological details, it may be sufficient to present 

 in the condensed table on the opposite page this 

 grand series of formations, with the general history of 

 life as ascertained from them. 



In the oldest rocks known to geologists those of 

 the Eozoic time some indications of the presence of 

 life are found. Great beds of limestone are contained 

 in these formations, vast quantities of carbon in the 

 form of graphite, and thick beds of iron-ore. All 



