This last age of the world is so intimately connected with man that it will be necessary to consider it more in detail. More particularly we may endeavour to answer, if we can, the questions of order and time involved in man's late appearance. No geologist would expect to find any remains of man or his works in the periods represented by our five earlier pictures, because in these periods the physical conditions necessary to man and the animals nearest to him in structure do not appear to have existed, and their places in nature were occupied by lower types. Nor for similar reasons would we expect to meet with man in the earlier part of that last, or cenozoic, period in which we still live; and in point of fact it is only in superficial deposits of the later part of this last great period of the earth's history that we actually meet with evidence of the existence of the human species. If there is based on this fact a question as to the actual date of man's first appearance, the physical considerations indicate about twenty millions of years for the whole duration of the earth. Setting apart, say, a fourth of this time for the early pre-geologic condition of the world, the remainder may be roughly estimated as five millions for the archæan, or eozoic, six for the palæozoic, three for the mesozoic, and one for the cenozoic.[4] Of the last, the later part, in which there is a possibility of the existence of man, will be limited to less than a quarter of a million; and within this the certainly known remains of man, whether attributed as by some to the latest inter-*glacial period, or to the post-glacial--a mere question of terms, and not of facts--cannot be older, according to the best geological estimates, than from seven thousand to ten thousand years. This, according to our present knowledge, is the maximum date of the oldest traces of man, and probably these are nearer in age to the smaller than to the larger number. [4] The absolute length of these periods is, of course, a matter of estimation; but the _relative_ lengths of the different ages may be regarded as a fair approximation, based on facts. If the reader will take the trouble to draw on paper a scale of twenty inches, each of these will represent a million of years of the earth's history, and the known duration of the human period may be indicated by a thickish line at one end of the scale. We may thus represent to the eye the recency of man's appearance, so far as at present known to science. It may be said that all this is mere assertion. It fairly represents, however, the conclusions reached on the latest geological evidence, though this evidence would demand for its full detail a larger space than the whole of this little volume. References are given below to works in which this evidence will be found.[5] [5] Lyell's _Students' Manual_; Dana's _Manual_; Prestwich's _Geology_; _The Story of the Earth_, by the author.