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." 1 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. 
