Taking England as an example, all before the Roman invasion is pre-historic, and with regard to this pre-historic period the evidence that we can obtain is chiefly of a geological character. The pre-historic men are essentially fossils. We know of them merely what can be learned from their bones and implements embedded in the soil or in the earth of the caverns in which some of them sheltered themselves. For the origin and date of these deposits the antiquary must go to the geologist, and he imitates the geologist in arranging his human fossils under such names as the "Paleolithic," or period of rude stone implements; the "Neolithic" or period of polished stone implements; the Bronze Period, and the Iron Period; though inasmuch as higher and lower states of the arts seem always to have coexisted, and the time involved is comparatively short, these periods are of far less value then those of geology. In Britain the age of iron is in the main historic. That of bronze goes back to the times of early Phoenician trade with the south of England. That of stone, while locally extending far into the succeeding ages, reaches back into an unknown antiquity, and is, as we shall see in the sequel, probably divided into two by a great physical change, though not in the abrupt and arbitrary way sometimes assumed by those who base their classification solely on the rude or polished character of stone implements. We must not forget, however, that in Western Asia the ages of bronze and iron may have begun two thousand years at least earlier then in Britain, and that in some parts of America the Palaeolithic age of chipped stone implements still continues. We must also bear in mind that when the archæologist appeals to the geologist for aid, he thereby leaves that kind of investigation in which dates are settled by years, for that in which they are marked merely by successive physical and organic changes. Turning, then, to our familiar geological methods, and confining ourselves mainly to the Northern Hemisphere and to Western Europe, two pictures present themselves to us: (!) The physical changes preceding the advent of man; (2) The decadence of the land animals of the Post-pliocene age, and the appearance of those of the modern.