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, the ferns, club-mosses, 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 liv- 
erworts are an offshoot from the main ancestral trunk which ter- 
minated in our flowering plants, a distinct section of it, or a back- 
step 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 some- 
what in composition and texture from the one beneath it. But 
earthquakes, volcanic activity, and other alterating forces have 
distorted, upheavea, and otherwise played havoc with the regu- 
larity 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, primi- 
tive 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 thicknesses 
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 or longer than 
the two succeeding ones combined. Here, over midway through 
this immense deposit, lie imbedded the remains of countless for- 
ests— forests of a far different aspect and relationship than the 
hardwood and conifer forests of today. 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-like seed-plants which 
ages after gave us the cycads, modern conifers, and flowering- 
plants. But preceding this period which we know as the carbon- 
iferous, or age of coal, is a stretch of time and a deposit 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 momen- 
tous in all plant hjstory. For along its low coasts, with broad 
marshes, over which the sea tides came and went for long dis- 
tances, arose perhaps the first land plants. Here, through the cen- 
*One should not think of these divisions of geological time as more sharply 
separated than historical periods. 
2 
