any locality note the advent and progress of the seasons more
accurately than does the calendar. Plants and seeds which have lain
asleep during the winter are awakened not by the birth of a month, but
by the return of heat and moisture in proper proportions. This may be
early one year and late another, but, no matter what the calendar
says, the plants respond to the call and give evidence of spring,
summer, or autumn as the case may be. The surface of the earth is not
flat. We have valleys and we have mountains; we have torrid and we
have temperate zones. The plant life of the world has been adjusted to
these varied conditions, and as a result we have plants with certain
characteristics growing in the tropics at sea-level, but a very
different class of plants with {118} different habits and
characteristics inhabiting the elevated regions of this same zone. It
must be remembered that even under the tropics some of the highest
mountains carry a perpetual snow-cap. There is therefore all possible
gradations of climate from sea-level to the top of such mountains,
even at the equator, and plant life is as a result as varied as is
climate. Each zone, whether determined by latitude or by altitude,
possesses a distinctive flora.

But altitude and latitude are not the only factors which have been
instrumental in determining the plants found in any particular
locality. This old earth of ours has not always been as we see her
to-day. The nature we know and observe is quite different from that
which existed in earlier ages of the earth's history. The plants, the
trees, and the flowers that existed upon the earth during the age when
our coal was being deposited were very different from those we now
have. There has been a change, but, strange as it may seem, there are
in some places upon the earth to-day some of the same species of
plants which were abundant during the coal-forming periods. These are
among the oldest representatives of the plant world now extant. Then
we are told that there was a period when the north temperate zone was
covered with a great ice field which crowded down as far as southern
Pennsylvania and central Ohio. This naturally brought about a profound
change in the location and character of the plants of this region.
There are in the Black Hills of Dakota species of plants which have no
relatives anywhere in the prairie region, and no means is known by
which these representatives of a Rocky Mountain family cou