at the
energy radiated from year to year was supplied from year to year--a
doctrine which would have suited Hutton perfectly. But the fact that so
eminent a physical philosopher has, thus recently, held views opposite to
those which he now entertains, and that he confesses his own estimates to
be "very vague," justly entitles us to disregard those estimates, if any
distinct facts on our side go against them. However, I am not aware that
such facts exist. As I have already said, for anything I know, one, two,
or three hundred millions of years may serve the needs of geologists
perfectly well.

III. The third line of argument is based upon the temperature of the
interior of the earth. Sir W. Thomson refers to certain investigations
which prove that the present thermal condition of the interior of the
earth implies either a heating of the earth within the last 20,000 years
of as much as 100 deg. F., or a greater heating all over the surface at some
time further back than 20,000 years, and then proceeds thus:--

"Now, are geologists prepared to admit that, at some time within the last
20,000 years, there has been all over the earth so high a temperature as
that? I presume not; no geologist--no _modern_ geologist--would for a
moment admit the hypothesis that the present state of underground heat is
due to a heating of the surface at so late a period as 20,000 years ago.
If that is not admitted we are driven to a greater heat at some time more
than 20,000 years ago. A greater heating all over the surface than 100 deg.
Fahrenheit would kill nearly all existing plants and animals, I may
safely say. Are modern geologists prepared to say that all life was
killed off the earth 50,000, 100,000, or 200,000 years ago? For the
uniformity theory, the further back the time of high surface-temperature
is put the better; but the further back the time of heating, the hotter
it must have been. The best for those who draw most largely on time is
that which puts it furthest back; and that is the theory that the heating
was enough to melt the whole. But even if it was enough to melt the
whole, we must still admit some limit, such as fifty million years, one
hundred million years, or two or three hundred million years ago. Beyond
that we cannot go."[22]

[Footnote 22: _Loc. cit._ p. 24.]

It will be observed that the "limit" is once again of the vaguest,
ranging from 50,000,000 years to 300,000,000. And the reply is, once
more, that, for an