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 fact that they persist through a series of deposits which, geology informs us, have taken a long while to make. If the geological clock is wrong, all the naturalist will have to do is to modify his notions of the rapidity of change accordingly. And I venture to point out that, when we are told that the limitation of the period during which living beings have inhabited this planet to one, two, or three hundred million years requires a complete revolution in geological speculation, the _onus probandi_ rests on the maker of the assertion, who brings forward not a shadow of evidence in its support.