The accusation that we have been running counter to the _principles_ of natural philosophy, therefore, is devoid of foundation. The only question which can arise is whether we have, or have not, been tacitly making assumptions which are in opposition to certain conclusions which may be drawn from those principles. And this question subdivides itself into two:--the first, are we really contravening such conclusions? the second, if we are, are 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.[53] 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.