Here we encounter a formidable objection; for it seems doubtful whether the earth, in a fit state for the habitation of living creatures, has lasted long enough.

Sir W.

Thompson concludes that the consolidation of the crust can hardly have occurred less than twenty or more than four hundred million years ago, but probably not less than ninety-eight or more than two hundred million years.

These very wide limits show how doubtful the data are; and other elements may have hereafter to be introduced into the problem.

Mr.