The first effect of the Ice-Age on this primitive humanity would be to lead to a beginning of the development of racial characters. The pigment under the skin of the negro is a protection against the actinic rays of the tropical sun; the white man, with his fair hair and eyes, is a bleached product of the northern regions; and the yellow or brown skin seems to be the outcome of living in dry regions with great extremes of temperature. As the northern hemisphere divided into climatic zones these physical characters were bound to develop. The men who went southward developed, especially when fully exposed to the sun on open plains, the layer of black pigment which marks the negroid type. There is good reason, as we shall see to think that man did not yet wear clothing, though he had a fairly conspicuous, if dwindling, coat of hair. On the other hand the men who lingered further north, in South-western Asia and North Africa, would lose what pigment they had, and develop the lighter characters of the northerner. It has been noticed that even a year in the arctic circle has a tendency to make the eyes of explorers light blue. We may look for the genesis of the vigorous, light-complexioned races along the fringe of the great ice-sheet. It must be remembered that when the limit of the ice-sheet was in Central Germany and Belgium, the climate even of North Africa would be very much more temperate than it is to-day. As the ice-sheet melted, the men who were adapted to living in the temperate zone to the south of it penetrated into Europe, and the long story of the Old Stone Age opened. It must not, of course, be supposed that this stage of human culture only began with the invasion of Europe. Men would bring their rough art of fashioning implements with them, but the southern regions are too little explored to inform us of the earlier stage. But as man enters Europe he begins to drop his flints on a soil that we have constant occasion to probe--although the floor on which he trod is now sometimes forty or fifty feet below the surface--and we obtain a surer glimpse of the fortunes of our race. Most European geologists count four distinct extensions of the ice-sheet, with three interglacial periods. It is now generally believed that man came north in the third interglacial period; though some high authorities think that he came in the second. As far as England is concerned, it has been determined, under the auspices of the British Association, that our oldest implements (apart from the Eoliths) are later than the great ice-sheet, but there is some evidence that they precede the last extension of the ice.