ve been very different from those which we find at this day, for
the question of climate depends upon other matters besides sunbeams.

Yet we need not abruptly stop our retrospect at any epoch, however
remote. We may go back earlier and earlier, through the long ages which
geologists claim for the deposition of the stratified rocks; and back
again still further, to those very earliest epochs when life began to
dawn on the earth. Still we can find no reason to suppose that the law
of the sun's decreasing heat is not maintained; and thus we would seem
bound by our present knowledge to suppose that the sun grows larger and
larger the further our retrospect extends. We cannot assume that the
rate of that growth is always the same. No such assumption is required;
it is sufficient for our purpose that we find the sun growing larger
and larger the further we peer back into the remote abyss of time past.
If the present order of things in our universe has lasted long enough,
then it would seem that there was a time when the sun must have been
twice as large as it is at present; it must once have been ten times as
large. How long ago that was no one can venture to say. But we cannot
stop at the stage when the sun was even ten times as large as it is at
present; the arguments will still apply in earlier ages. We see the sun
swelling and swelling, with a corresponding decrease in its density,
until at length we find, instead of our sun as we know it, a mighty
nebula filling a gigantic region of space.

Such is, in fact, the doctrine of the origin of our system which has
been advanced in that celebrated speculation known as the nebular theory
of Laplace. Nor can it be ever more than a speculation; it cannot be
established by observation, nor can it be proved by calculation. It is
merely a conjecture, more or less plausible, but perhaps in some degree
necessarily true, if our present laws of heat, as we understand them,
admit of the extreme application here required, and if also the present
order of things has reigned for sufficient time without the intervention
of any influence at present unknown to us. This nebular theory is not
confined to the history of our sun. Precisely similar reasoning may be
extended to the individual planets: the farther we look back, the hotter
and the hotter does the whole system become. It has been thought that if
we could look far enough back, we should see the earth too hot for life;
back further still