Sir Charles Lyell and Professor Haughton respectively estimated the expiration of time from the commencement of the Cambrian at two hundred and forty and two hundred millions of years, basing their calculations on the rate of modification of the species of mollusca, in the one case, and on the rate of formation of rocks and their maximum thickness, in the other. This, moreover, is irrespective of the vast periods during which life must have existed, which on the development theory necessarily preceded the Cambrian, and, according to Mr. Darwin, should not be less than in the proportion of five to two. In fine, one school of geologists and zoologists demand the maximum periods quoted above, to account for the amount of sedimentary deposit, and the specific developments which have occurred; the other considers the periods claimed as requisite for these actions to be unnecessary, and to be in excess of the limits which, according to their views, the physical elements of the case permit. Mr. Wallace, in reviewing the question, dwells on the probability of the rate of geological changes having been greater in very remote times than it is at present, and thus opens a way to the reconciliation of the opposing views so far as one half the question is concerned. Having thus adverted to the principles upon which various theorists have in part based their attacks on the problem of the estimation of the duration of geological ages, I may now make a few more detailed observations upon those later periods during which man is, now, generally admitted to have existed, and refer lightly to the earlier times which some, but not all, geologists consider to have furnished evidences of his presence. I omit discussing the doubtful assertions of the extreme antiquity of man, which come to us from American observers, such as are based on supposed footprints in rocks of secondary age, figured in a semi-scientific and exceedingly valuable popular journal. There are other theories which I omit, both because they need further confirmation by scientific investigators, and because they deal with periods so remote as to be totally devoid of significance for the argument of this work. Nor, up to the present time, are the evidences of the existence of man during Miocene and Pliocene times admitted as conclusive. Professor Capellini has discovered, in deposits recognised by Italian geologists as of Pliocene age, cetacean bones, which are marked with incisions such as only a sharp instrument could have produced, and which, in his opinion, must be ascribed to human agency. To this view it is objected that the incisions might have been made by the teeth of fishes, and further evidence is waited for.