THE CLASSIFICATION OF FOSSIL PLANTS 
amount of dryness or aridity over large areas of the 
globe was exceptional. Besides the striking revolutions 
in climate, such as the Permian glaciation in the southern 
hemisphere and the Pleistocene glaciation in the northern 
hemisphere, there were a number of minor vacillations in 
climatic conditions; each one greatly influencing the exist¬ 
ing floras. 
If the climate gradually gets colder or drier, the animal 
life may adjust itself by migration or adaptation. Some 
animals move south or north, or uphill or downhill. 
They cover themselves with heavy hair or lose it. A 
plant’s only resource is adaptation by certain protective 
measures, for which there is but a limited opportunity. 
Those plant types which can not adjust themselves, die 
in the unfavorable regions, though they may continue to 
exist in more favorable portions of the globe. When con¬ 
ditions change again and become favorable once more 
in a given locality, such plants frequently move back, if 
no insuperable geographic barrier exists. This migration 
of plants, however, is slow. It depends upon the scattering 
of seeds by wind, water, and animals; it usually takes a 
great many years for a migrating plant to cover any 
considerable distance. 
This is what happened in Europe and North America 
after the glacial epoch. Plant life in the Tertiary period 
47 
