Page:Early Man in Britain and His Place in the Tertiary Period.djvu/142

114 minimum the spotted hyæna and hippopotamus and other southern animals roamed to their northern limit. Thus every part of the middle zone has been successively the frontier between the northern and southern groups, and consequently their remains are mingled together in the caverns and river deposits, under conditions which prove them to have been contemporaries in the same region. In some of the caverns, such as that of Kirkdale, the hyæna preyed upon the reindeer at one time of the year and the hippopotamus at another. In this manner the association of northern and southern animals may be explained by their migration according to the seasons, and their association over so wide an area as the middle zone by the secular changes of climate, by which each part of the zone in turn was traversed by the advancing and retreating animals.

Secular changes of climate in the Pleistocene age are clearly marked in Britain north of a line connecting the Bristol Channel with the valley of the lower Thames, and passing due eastward into Germany and Russia, by the traces of glaciation, by the erratics, or blocks of stone transported far away from the rocks from which they were torn, and by the accumulations of clay and sand known as the glacial drift. They imply the following