THE CLASSIFICATION OF FOSSIL PLANTS 
it rests, there must have been several millions of species 
in the time between this point of origin and the base of 
the pyramid today. Of these millions we know about 
30,000, most of them imperfectly, and all of them only 
in the upper portion of the pyramid; that is, in com¬ 
paratively recent geological times. 
There is one circumstance, however, that proves very 
helpful to the paleobotanist. The present plant kingdom 
contains not only the latest and highest types of plant 
development, but also many primitive and poorly de¬ 
veloped types. By comparing these types among them¬ 
selves, we can gain some idea of plant evolution. The 
science based on this comparison of living species we 
call the comparative anatomy of plants, or, sometimes, 
comparative morphology. This study is of great value, 
but the only real chronological evidence of the develop¬ 
ment of plant life is found in the fossil plants. 
By combining all our information from both compara¬ 
tive anatomy and fossil plants, we are led to the con¬ 
clusion that the plant kingdom is composed, in its 
evolutionary history, of six great stems, most of which 
are still represented, at least partly, in living floras. 
These stems are usually arranged in a rising order of 
development, from the simplest plant-forms to the highest 
and most complex. The first great stem is that of the 
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