The woolly rhinoceros, on the other hand, may be viewed as a northern form, since it is met with in vast abundance in the arctic regions of Siberia, as well as in Europe, and has not been found south of the Alps and Pyrenees. The cave-bear has not been discovered either in the extreme north or in the south of Europe, and may therefore be considered of temperate range; and the Irish elk, identified by Prof. Brandt, from the caves of the Altai Mountains, had a similar range in middle Europe. The mammoth, endowed with an elastic constitution, was able to endure the severity of an arctic climate in Siberia and North America, and the temperature of the latitude of Rome and the Gulf of Mexico,[264] and consequently tells us as little of the pleistocene climate as the panther, fox, or wolf. The evidence, therefore, as to climate, offered by the extinct animals in the caves is of the same nature as that of the living. There is the same mixture of northern and southern forms, which can only be accounted for satisfactorily by seasonal migrations, according to the summer heat and winter cold, such as those which are now observed to take place in Siberia and North America. Before we consider the relation of the pleistocene animals buried in the caves and river deposits to the glacial period, it is necessary to define what is meant by the term glacial. _Two Periods of Glaciation in Britain._ At the close of the pleistocene period the climate gradually became colder, until ultimately it was arctic in severity in northern Europe. The researches of many eminent observers prove that an enormous sheet of ice, like that under which Greenland now lies buried, extended over North Britain, Wales, and Ireland, leaving its mark in the far-travelled blocks of stone, the moraines, and the grooves which pass over the surface irrespective of the minor contours. The land then, most probably, as Prof. Ramsay and Sir Charles Lyell believe, stood higher than it does now. To this succeeded a period of depression, during which the mountains of Wales were submerged to a height of at least 1,300 feet; and the waves of the sea washed out of the pre-existing glacial detritus the shingle and sand, termed the “middle drift,” which occurs also in Scotland and Ireland.[265] Then the land was re-elevated above the waves, and a second period of glaciers set in, traces of which occur abundantly in Wales, Scotland, and Ireland, in the white areas in Fig. 126. They were, however, of far less extent than those which preceded them, occupying isolated areas instead of forming one continuous icy covering to the country. The glacial phenomena may be briefly summed up as follows: 1. As the pleiocene temperature was lowered, the glaciers crept down from the tops of the mountains, until at last they united to form one continuous ice sheet, moving resistlessly over the smaller hills and valleys to the lower grounds, and the first ice or glacial period set in. 2. Then followed