THE EVOLUTION OF PLANTS 241 



recently as 20 million years ago. The lateness of their 

 appearance and the rapidity with which they have spread, 

 until they are now the dominant element in the flora 

 of the land, indicate how well they are adapted to their 

 environment. "Nothing is more extraordinary in the 

 history of the vegetable kingdom," wrote Darwin to 

 Hooker, "than the apparently very sudden or abrupt 

 development of the higher plants." 



' The construction of a pedigree of the Vegetable King- 

 dom is a pious desire, which will certainly not be realized 

 in our time; all we can hope to do is to make some very 

 small contributions to the work. Yet we may at least 

 gather up some fragments from past chapters in the history 

 of plants, and extend our view beyond the narrow limits 

 of the present epoch, for the flora now living is after all 

 nothing but one particular stage in the evolution of the 

 Vegetable Kingdom." 1 



154. The Element of Geological Time. How many 

 years has it taken for the evolution of the higher Angio- 

 sperms that is, from the dawn of the fossil record in the 

 Silurian period to the present? No one knows. From a 

 study of the thickness of rock strata, and a knowledge of 

 the probable time required for the depositing of those 

 strata as sediment on the floor of the ancient oceans, and 

 their elevation and denudation to their present condition 

 by weathering and erosion, geologists have been able to 

 suggest relative measures of geologic time. Paleozoic 

 time is long, twice as long as Mesozoic time, and Meso- 

 zoic time must be at least twice as long as Cenozoic time. 

 The actual age of the earth is, however, a problem which 

 engages the attention of physicists as well as geologists. 



1 Scott, D. H. "Studies in Fossil Botany," p. 3. 

 16 



