PALEOBOTANY 589 



more or less different in the successive chapters, may 

 represent the forms of life, which are entombed in our 

 consecutive formations, and which falsely appear to have 

 been abruptly introduced."^ These views have received 

 added emphasis from the recent development of Paleo- 

 geography. 



513. Factors of Extinction. — The question may natu- 

 rally arise, ''Why did the species common in previous geo- 

 logical ages die out, giving place to newer forms?" The 

 answer is found in the facts of struggle for existence and 

 survival of the fittest. In the words of the great American 

 botanist, Asa Gray, species may continue only ''while 

 the external conditions of their being or well-being con- 

 tinue." The struggle may be with other organisms or 

 with the physical conditions of the environment. Among 

 the more important factors of extinction, may be men- 

 tioned the following: 



1. Struggle with other plants for adequate space. This is 

 illustrated in a simple way by the crowding out of cultivated 

 plants by weeds in a garden. By more rapid germina- 

 tion and growth, and by other "weedy" characteristics, 

 the weeds get the start of the cultivated plants, occupying 

 all available space, and choking them out. 



2. Attacks of disease-causing parasites, e.g., chestnut 

 trees by a parasitic fungus, elm trees by the elm tree beetle. 



3. Changes of environment too great or too rapid to permit 

 oj readjustment. Plants are plastic organisms, and can 

 adapt or readjust themselves to considerable environ- 

 mental change, but there are limits of speed and amount 

 of change beyond which readjustment is not possible, and 

 the plant must consequently perish. If such changes 



1 Darwin, C. '^Origin of Species," New York, 1902, vol. 2, p. 88. 



