rise again. Seas have receded in many places and encroached 
on the land in others. Lakes have been cut out and rivers 
formed to carry off their surplus waters. Great oceans of ice 
have swept down from the north, levelling hills, cutting valleys, 
and moving thousands of tons of rock and soil. It is hardly 
necessary to state what would become of fragile plant remains 
amid such turmoil. 
Hence, the rock-written story of the evolution of plants is 
largely concerned with those having woody stems — in other 
words, horsetails, conifers, and flowering plants. As to the 
primitive ancestors of these already complex types, we can only 
speculate. As to whether the mosses and liverworts are an off- 
shoot from the main ancestral trunk which terminated in our 
flowering plants, a distinct section of it, or a backstep from some 
fern type, can be more definitely discussed only when the rocks 
have given up more of their facts— -when the plant graves have 
given up more of their dead. 
If the rock strata of the earth’s crust had undergone no vol- 
canic or other serious disturbance during the time it was laid 
down, one might visualize a section through it as a slice cut 
from a huge cake composed of many layers, each layer differing 
somewhat in composition and texture from the one beneath it. 
But earthquakes, volcanic activity, and other alterating forces 
have distorted, upheaved, and otherwise played havoc with the 
regularity of these layers until even a geologist has his troubles 
in correctly identifying their different outcroppings. These layers 
being laid down in sequence, the bottom one is naturally the 
most ancient, as well as the hardest to investigate. In order to 
make them easier to study, many layers are grouped together, 
and the time in which they were laid down is called an era. These 
eras are again subdivided into periods, etc. The three main divi- 
sions, or eras, are generally known as the Paleozoic, Mesozoic, 
and Cenozoic, the latter taking in the present time. 
Certain kinds of plants were most plentiful and formed the 
predominant flora of each of these divisions. Hence they are 
often known respectively as the age of pond-scums, seaweeds, 
primitive seed-plants, ferns, club-mosses, and horsetails; the age 
of conifers, cycads, and ginkgos; the age of flowering plants* 
(our own age). Beneath the Paleozoic rocks lie immense thick- 
nesses of other rocks, the pre-Cambrian, almost if not entirely 
devoid of fossil remains, hence as yet contributing nothing to the 
story of plant life. 
In fact, the first beginnings of the imperfectly connected his- 
tory of the world’s flora, past and present, is found high up in 
the rocks of the Paleozoic era, an era perhaps as long as or longer 
than the two succeeding ones combined. Here, over midway 
through this immense deposit, lie imbedded the remains of 
countless forests — forests of a far different aspect and relation- 
ship than the hardwood and conifer forests of to-day. Here lie 
the known remains of at least two or three great branches of the 
ancestral tree of our plant world — the ferns; the club-mosses, 
horsetails and their relatives, and the groups of fern-Hke seed- 
plants which ages after gave us the cycads, modern conifers, and 
flowering plants. But preceding this period which we know as 
the carboniferous, or age of coal, is a stretch of time and a de- 
posit of rock, of which as yet little is known, but of which that 
little makes us believe it to be probably the most interesting and 
most momentous in all plant history. For along its low coasts, 
*One should not think of these divisions ot geological time as more sharply 
separated than historical periods. 
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