most ancient to the later ages, indications were found of
a climate arrangement more and more distinct--in the high latitudes,
from the ultratropical through the tropical, the temperate, down to the
present frigid state; in lower latitudes the declining process stopping
short at an earlier point. It therefore appeared that there has been a
production of climates both in an order of time and, in an order of
locality, the greatest change having occurred in the frigid zone, which
has passed through all mean temperatures, an intermediate change in the
temperate, and a minimum in the torrid zone. The general effect has thus
been to present a succession of surfaces on the same planet adapted to a
varied organization, and offering a more magnificent spectacle than if
we were permitted to inspect many different planets; for in them there
might be no necessary connexion of their forms of life, but in this
there is, so that, were our knowledge of Comparative Physiology more
perfect, we might amuse ourselves with intercalating among the plant and
animal organisms familiar to us hypothetical forms that would make the
series complete, and verify our principles by their subsequent discovery
in the deep strata of the earth.

Does not this progression of life in our planet suggest a like
progression for the solar system, which in its aggregate is passing in
myriads of years through all organic phases? May we not also, from our
solar system, rise to a similar conception for the universe?

There are two very important considerations, on which we must dwell for
the complete understanding of the consequences of these changes: 1st.
The mechanism of the declining temperature; 2d. Its effect in the
organic world.

[Sidenote: The nature of terrestrial declining temperature.] 1st. A
uniformly high temperature could never be manifested all over the
surface of our planet through any heating influence of the sun. A high
and uniform temperature unerringly points to an internal cause; and the
gradual appearance of climates, manifesting a relatively increasing
power of the sun, indicates the slow diminution of that internal heat.
But this is precisely the conclusion which was come to from a
contemplation of the earth from a purely physical point of view. So long
as its intrinsic heat overpowered that derived from the sun, it was not
possible that any thing answering to climates could be established; and,
until a certain degree of cooling by radiat