ation, unlike any to be
found in any climate to-day.

We will look more closely into its nature presently. First let us see
how these primitive types of plants come to form the immense forests
which are recorded in our coal-beds. Dr. Russel Wallace has lately
represented these forests, which have, we shall see, had a most
important influence on the development of life, as somewhat mysterious
in their origin. If, however, we again consult the geologist as to the
changes which were taking place in the distribution of land and water,
we find a quite natural explanation. Indeed, there are now distinguished
geologists (e.g. Professor Chamberlin) who doubt if the Coal-forests
were so exceptionally luxuriant as is generally believed. They think
that the vegetation may not have been more dense than in some other
ages, but that there may have been exceptionally good conditions for
preserving the dead trees. We shall see that there were; but, on the
whole, it seems probable that during some hundreds of thousands of
years remarkably dense forests covered enormous stretches of the earth's
surface, from the Arctic to the Antarctic.

The Devonian period had opened with a rise of the land, but the sea eat
steadily into it once more, and, with some inconsiderable oscillations
of the land, regained its territory. The latter part of the Devonian
and earlier part of the Carboniferous were remarkable for their great
expanses of shallow water and low-lying land. Except the recent chain
of hills in Scotland we know of no mountains. Professor Chamberlin
calculates that 20,000,000, or 30,000,000 square miles of the present
continental surface of Europe and America were covered with a
shallow sea. In the deeper and clearer of these waters the earliest
Carboniferous rocks, of limestone, were deposited. The "millstone grit,"
which succeeds the "limestone," indicates shallower water, which is
being rapidly filled up with the debris of the land. In a word, all the
indications suggest the early and middle Carboniferous as an age of vast
swamps, of enormous stretches of land just above or below the sea-level,
and changing repeatedly from one to the other. Further, the climate
was at the time--we will consider the general question of climate
later--moist and warm all over the earth, on account of the great
proportion of sea-surface and the absence of high land (not to speak of
more disputable causes).

These were ideal conditions for the primitive