d only
two or three of them are deserving of much discussion. It is always
interesting to know what people think, however, even if we do not agree
with them.

The first theory named is that the glacial period is due to the decrease
of the original heat in our climate. This theory can be dismissed by
saying that the planet was cooling at the time and has been cooling ever
since, and that the reasons for an ice age are greater now than then, on
that theory. Another theory assumes that at some former period there was
a greater amount of moisture in the atmosphere; while this of course
would be the occasion for greater precipitation of snow, it does not
account for the changing conditions that would produce the ice effect.
That there was a preglacial period there is abundant evidence, in buried
forests, the filling up and changing of river beds, and other evidences
that will be referred to further on. This theory, unmodified and stated
broadly, is not satisfactory. Another way of accounting for the glacial
period is the change in the distribution of land and water, which is
supposed to affect the distribution of heat over the earth's surface.
There is much in this theory that commends itself as plausible. Another
theory supposes that the land in northern Europe and America was
elevated to a higher level at that time than it is now. Others attribute
it to variation of temperature in space and of the amount of heat
radiated by the sun. The final theory for accounting for the ice age is
attributed to what is termed the precession of the equinoxes. In short,
the precession of the equinoxes means that the division between summer
and winter is changing gradually, so that during a period of 10,500
years the summers are growing longer in the northern hemisphere and the
winters shorter. We are now in the period of long summers, but in
another 10,000 years we shall be in the period of short summers and long
winters. This difference of time between the winters and the summers is
supposed to be sufficient to change the thermal conditions sufficiently
to produce an ice age.

It is true that the conditions now are very evenly balanced, so much so
that in Switzerland the glaciers will increase for some years together,
when the conditions will change, causing them to gradually recede.
Several of the theories that have been advanced present evidences that
are entitled to careful consideration, but none of them can be said to
be entirely s