THE STORY TOLD BY FOSSIL PLANTS 



Each successive group of plants that appeared upon the 

 scene illustrates a second great principle, which is called 

 "adaptive radiation" — that is, from time to time, by pro- 

 gressive modifications, certain groups became dominant, such 

 as the club mosses, horsetails, and seed ferns of the Carbonif- 



FiG, 1. — Diagram showing the successive numerical dominance 

 of progressively more complex plants through the geologic eras 

 and the progressive increase in the complexity of the more familiar 

 floras. 



The space assigned to the several eras (Archeozoic, Proterozoic, etc.) 

 corresponds roughly with the length of time included in them. The 

 Archeozoic is the earliest era, the Cenozoic the latest. The heavier lines 

 show the increase or decrease in the number of the different kinds of plants 

 through geologic time. The flowering plants are now (in late Cenozoic 

 time) by far the most numerous, and their rise to dominance has been 

 rapid; yet representatives of the- older forms still persist, though they are 

 less abundant than the flowering plants. 



erous period, the cycads of the Mesozoic era, or the flower- 

 ing plants of the Cenozoic era. The members of these 

 groups became adapted to a great variety of environment 

 and tended to occupy all the available places on the land, 

 and some of them, such as the water ferns or the higher 



[157] 



