WHAT EVOLUTION IS 43 



their significance was reached; they 

 are the remains of once living organ- 

 isms, and they disclose a continuous and 

 real history of plant and animal life. 



When this history is looked into, 

 it is found to have, as might be ex- 

 pected, a profound bearing on evolu- 

 tionary matters. It is by no means 

 easy to determine hov^ long living 

 things have existed on the earth. 

 Estimates vary from a hundred-mil- 

 lion to two thousand-million years. 

 But from an evolutionary standpoint 

 such enormous periods, and even 

 such differences in the estimates, are 

 not so significant as the kinds of or- 

 ganisms that are shown to be present 

 at different periods in the earth's 

 history and the sequences that this 

 history discloses. Sketched very 

 broadly, it may be said that during 

 about the first two-thirds of the period 

 in which life has been on the globe 



