a key? And if a key at all, it will be an accurate key, and enable us to
calculate back with some precision to the date of the glacial epoch;
and again to the time when a tropical flora flourished in what is now
northern Europe, _i.e._ to the date of the Carboniferous era.

This aspect of the subject has recently been taught with vigour and
success by Dr. Croll in his book "Climate and Time."

A brief and partial explanation of the matter may be given, because
it is a point of some interest and is also one of fair simplicity.

Every one knows that the climatic conditions of winter and summer
are inverted in the two hemispheres, and that at present the sun is
nearest to us in our (northern) winter. In other words, the earth's
axis is inclined so as to tilt its north pole away from the sun at
perihelion, or when the earth is at the part of its elliptic orbit
nearest the sun's focus; and to tilt it towards the sun at
aphelion. The result of this present state of things is to diminish
the intensity of the average northern winter and of the average
northern summer, and on the other hand to aggravate the extremes of
temperature in the southern hemisphere; all other things being
equal. Of course other things are not equal, and the distribution
of land and sea is a still more powerful climatic agent than is the
three million miles or so extra nearness of the sun. But it is
supposed that the Antarctic ice-cap is larger than the northern,
and increased summer radiation with increased winter cold would
account for this.

But the present state of things did not always obtain. The conical
movement of the earth's axis (now known by a curious perversion of
phrase as "precession") will in the course of 13,000 years or so
cause the tilt to be precisely opposite, and then we shall have the
more extreme winters and summers instead of the southern
hemisphere.

If the change were to occur now, it might not be overpowering,
because now the excentricity is moderate. But if it happened some
time back, when the excentricity was much greater, a decidedly
different arrangement of climate may have resulted. There is no
need to say _if_ it happened some time back: it did happen, and
accordingly an agent for affecting the distribution of mean
temperature on the earth is to hand; though wheth