those conclusions so firmly based that we may not
contravene them? I reply in the negative to both these questions, and I
will give you my reasons for so doing. Sir William Thomson believes that
he is able to prove, by physical reasonings, "that the existing state of
things on the earth, life on the earth--all geological history showing
continuity of life--must be limited within some such period of time as
one hundred million years" (_loc. cit._ p. 25).

The first inquiry which arises plainly is, has it ever been denied that
this period _may_ be enough for the purposes of geology?

The discussion of this question is greatly embarrassed by the vagueness
with which the assumed limit is, I will not say defined, but indicated,--
"some such period of past time as one hundred million years." Now does
this mean that it may have been two, or three, or four hundred million
years? Because this really makes all the difference.[15]

[Footnote 15: Sir William Thomson implies (_loc. cit_. p. 16) that the
precise time is of no consequence: "the principle is the same"; but, as
the principle is admitted, the whole discussion turns on its practical
results.]

I presume that 100,000 feet may be taken as a full allowance for the
total thickness of stratified rocks containing traces of life; 100,000
divided by 100,000,000 = 0.001. Consequently, the deposit of 100,000 feet
of stratified rock in 100,000,000 years means that the deposit has taken
place at the rate of 1/1000 of a foot, or, say, 1/83 of an inch, per
annum.

Well, I do not know that any one is prepared to maintain that, even
making all needful allowances, the stratified rocks may not have been
formed, on the average, at the rate of 1/83 of an inch per annum. I
suppose that if such could be shown to be the limit of world-growth, we
could put up with the allowance without feeling that our speculations had
undergone any revolution. And perhaps, after all, the qualifying phrase
"some such period" may not necessitate the assumption of more than 1/166
or 1/249 or 1/332 of an inch of deposit per year, which, of course, would
give us still more ease and comfort.

But, it may be said, that it is biology, and not geology, which asks for
so much time--that the succession of life demands vast intervals; but
this appears to me to be reasoning in a circle. Biology takes her time
from geology. The only reason we have for believing in the slow rate of
the change in living forms is the fac