Sir Charles Lyell has urged that geographical changes (elevations and subsidences) would of themselves be sufficient to bring about a glacial period, which (he says) would be the result of a great continent being formed round the North Pole while oceanic conditions prevailed at the Equator. Another theory is that the heat given out by the sun is not always equal, being sometimes more (when even polar countries enjoy a warm climate) and sometimes less (when only the equatorial regions are habitable). The objection to this theory is, of course, that we have no proof that our sun is of greatly variable heat. Whatever may have been the cause of the glacial period, we know as a proved fact that a long time ago (as measured by years, although the event itself is among the latest of the many changes recorded in the geological history of the earth) the climate of the British Isles was so intensely cold that the greater part of this country was covered with ice and snow, and we know also that this intense cold was sufficient to change in many respects the habits and appearance of the animals and vegetation of the earth. How much this was the case can be gathered from the fact that in the period which preceded it animals which now live in the tropics roamed in the Arctic circle, and figs and magnolias grew in Greenland. The last word we shall have to say on the climatic conditions of this period is that the Ice Age had its sub-periods and divisions like all other epochs and in them the ice sometimes retreated, and consequently in parts of the earth where there had been snow and ice, and where there were to be ice and snow again, the wintry conditions retreated (for centuries, perhaps, at a time), and the valleys and plains basked during these intervals in sun and rain and warmth. These epochs are called "inter-glacial epochs." The life of the regions not much affected by the rigours of snow and ice is gradually being ascertained by geologists now. One of its most marked features was the retreat of the northern and Asiatic animals before the advancing ice towards the warmer tropics and Equator; these animals journeyed back northward again whenever the retreating ice would let them. The great _Proboscideans_, the Mastodon and the Mammoth were members of this group, and so were the bear, the bison, the musk ox. With these mingled towards the south several types (which were gradually becoming extinct in North America) such as the horse, tapir, llama, and the sabre-tooth cat. A second prominent feature was a southern group in the western hemisphere, consisting of gigantic sloths, armadillos, and water-hogs; and now for the first time the interest of animal life shifts to South America.