THE CLASSIFICATION OF FOSSIL PLANTS 
plant fossils from this age, except for some doubtful re¬ 
mains of algae, but there are sponges, corals, and worm 
trails. 
The Archeozoic and the Proterozoic may be considered 
merely the introductory chapters of the earth’s history. 
They correspond to the prehistoric era in the life of man 
on earth. The history of organic life for which we have 
ample documentary evidence in fossils is divided into 
periods corresponding to antiquity, the Middle Ages, and 
modern times. We have the Paleozoic, or Old Age of 
animal life, the Mesozoic, or Middle Age of animal life, 
and the Cenozoic, or New Age of animal life. These sub¬ 
divisions are based primarily on the main stages of animal 
development, rather than upon those of plant develop¬ 
ment. The development of plant life is always a step 
ahead of the animal devlopment; for, as we have pointed 
out, plants are the basis of animal life, inasmuch as they 
provide the food which makes any other form of life 
possible. 
The Paleozoic era may be roughly divided into an 
earlier and a later period. The earlier Paleozoic contains 
three subdivisions: the Cambrian, the Ordovician, and 
the Silurian periods. There are algal remains from all 
three periods and also many fossils of sea animals; fish 
were the most prominent form of animal life all through 
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