THE ORIGIN OF SPECIES 357 



necessary for paleontologists to invent it. What then are the 

 main facts offered by the study of the fossil remains of extinct 

 animals and plants? 



In the first place it must be made clear that geologists are 

 able to determine, with remarkable accuracy in most cases, 

 the sequence in time, or CHRONOLOGICAL SUCCESSION, of the 

 rock strata composing the Earth's surface. The main outline 

 of this scheme of geological chronology was understood long 

 before the evolution of organisms was a crucial question; so 

 that we may consider the evidence which it affords of the 

 chronological succession of the fossil remains exhibited by 

 the various strata, as impartial testimony to the order of ap- 

 pearance on the Earth of the different types of animals and 

 plants. 



The following geological time-table summarizes the 

 panoramic succession of life as it is seen by the paleontolo- 

 gist. It is useless to attempt to state the absolute duration 

 of geologic time, because we have little more than guesses to 

 depend on, though there are fairly reliable data in regard to 

 the relative length of the various eras. Perhaps the conserva- 

 tive estimate of 500,000,000 years at least half of which 

 was before the Permian period will serve to spell the 

 Earth's unfathomable past. 



