394 



GENERAL BIOLOGY 



by its effect upon climate, and the latter by its effect on the changing 

 strata which go to make up the earth's surface. In fact, it is the 

 changing climatic conditions which give us the terms "ages" or "peri- 

 ods," such as the carboniferous age and the glacial period. 



Fig. 245. 



Composite Palaeontological Chart, compiled from many authors, showing geo- 

 logical strata and fossil-forms found in each. It will be noted that the number of 

 years assigned each stratum varies from any given amount to ten and even a 

 hundred times that number. The student must therefore realize that all such 

 estimates are only guesses. What he must know is the relative percentage of 

 time and the relative percentage of depth of each layer and speak only in terms 

 of "eras" and depths. 



Professor Osborn has just described (Natural History, for November-Decem- 

 ber, 1921) a Tertiary man living long before the ice-age. 



