o-operation of other causes, have
produced the observed diversities of climate, is that which seems best
to meet the conditions presented. It is based on the known properties
of land and water as to the absorption, radiation, and convection of
heat, and on the remarkable diversities of climate in similar
latitudes arising from this cause at present. Farther, it accords with
the known fact that very great changes of level have occurred in
connection with the glacial period. This theory undoubtedly embraces a
true cause, admitted by all geologists, and it dispenses with the
necessity of believing in the recurrence of glacial periods at regular
intervals. It farther accords best with the evidence afforded by
fossils, and especially by fossil plants. It has also the merit of
directing due attention to the diversities of geographical conditions
at different periods, and of dealing with causes of change operating
within the earth itself. The only doubt with respect to it is its
sufficiency to explain the changes which have occurred, and the view
entertained of this will depend very much on the interpretation of the
facts as to the intensity of the last glacial period. If moderate
views can be taken of this, and if means can be found, by a less
obliquity of the ecliptic or otherwise, to furnish a continuous supply
of light in the arctic regions, the difficulties which have been
alleged against it would disappear.

(8.) In connection with former periods of cold and warmth, and with
the existence of temperate and tropical vegetation in polar latitudes,
we should not forget that view which takes into account the probable
effects of different conditions of the atmosphere, and the greater
quantity of carbonic acid present in it, in early geological periods.
This would, of course, best apply to the palaeozoic floras, in so far
as our present knowledge extends; but there may have been similar
conditions in later periods. Dr. Sterry Hunt thus states this
hypothesis:

"The agency of plants in purifying the primitive atmosphere was long
since pointed out by Brongniart, and our great stores of fossil fuel
have been derived from the decomposition, by the ancient vegetation,
of the excess of carbonic acid of the early atmosphere, which through
this agency was exchanged for oxygen gas. In this connection the
vegetation of former periods presents the curious phenomenon of plants
allied to those now growing beneath the tropics flourishing w