th
that culminated in the Glacial Age; though we do not mean to say, that
periods of glacial cold come and go with the regularity of our Winter.
The changes in the seasons of the year are caused by the earth's
position in its orbit, and its annual revolution around the sun. It may
be that the cause of the Glacial Age itself is of a similar nature; in
which case it is an astronomical problem, and we ought, by calculation,
to determine, with considerable accuracy, dates for the beginning and
ending of this epoch.

Nothing is clearer than that great fluctuations of climate have occurred
in the past. Many theories have been put forth in explanation. It has
been suggested that it was caused by loss of heat from the earth itself.
That the earth was once a ball of incandescent matter, like the sun,
and has since cooled down, is of course admitted. More than that, this
process still continues; and the time must come when the earth, having
yielded up its internal heat, will cease to be an inhabitable globe. But
the climate of the surface of the earth is not dependent upon the heat
of the interior. This now depends "according to the proportion of heat
received either directly or indirectly from the sun; and so it must have
been during all the ages of which any records have come down to us."<12>
Some have supposed that the sun, traveling as it does through space,
carrying the earth and the other planets with him, might, in the course
of ages, pass through portions of space either warmer or colder than
that in which it now moves. When we come to a warm region of space, a
genial climate would prevail over the earth; but, when we struck a cold
belt, eternal Winter would mantle a large part of the globe with snow
and ice. This, of course, is simply guess-work. No less than seven
distinct causes have been urged; most of them either purely conjectural,
like the last, or manifestly incompetent to produce the great results
which we have seen must be accounted for. But, amongst these, two causes
have been advanced--the one astronomical, the other geographical; and,
to the one or the other, the majority of scholars have given their
consent.

It will be no harm to see what can be said in favor of both theories.
So, we will ask the reader's attention, as it is our earnest desire to
make as plain as possible a question that has so much to do with our
present inquiry. In the course of our investigations, we can not fail to
catch glimpses of w