THE ERA OF THE FERNS 
were different, but the biological functions were much 
the same. Plants lived, grew, and reproduced them¬ 
selves in the same way they do now. 
An enormous time must have elapsed in the evolution 
of plant life on earth from its earliest stages to the very 
high level it had reached in the Pennsylvanian—a level 
which is very close to that of today. Yet the Penn¬ 
sylvanian period belongs to the Paleozoic, or Old Age, 
in the known history of organic life. The unknown 
history must have been infinitely longer. It must have 
extended into many hundreds of millions of years. The 
Pennsylvanian floras existed at least 300,000,000 years 
before our time, and yet they belong to our time. Plant 
evolution must be imagined as an endless procession of 
species of which we can see only the foremost groups. 
The Permian period, which follows the Pennsylvanian, 
shows a glacial epoch in the southern hemisphere, and 
the deposition of red beds, which indicate dryness. A 
sweeping climatic change seems to have come about in 
a comparatively short time. In consequence, a reduction 
of the vegetation occurred. 
While the type locality of the Permian period is in 
northeastern Russia, its best development occurs in the 
southern hemisphere, where the great glaciation of the 
era took place. Permian life is nowhere better developed 
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