As we have made the theoretical part of this chapter already too long, we must hurry on. We can only say that this view is founded on the fact that when a country was covered with snow and ice, it had so to speak, a great amount of cold stored up in it, so much, in fact, that it would not be removed by the sun of a new geological Summer. This ought to be acceptable to such geologists as are willing to admit the advance and retreat of the great glacier, but yet doubt the fact of the interglacial mild climate. But now to return to the question of time about two hundred and twenty thousand years ago. Then the Northern Hemisphere, according to this theory, was in the grasp of a Glacial Age. According to Mr. Wallace, as long as the eccentricity remained high, there could be no great amelioration of climate, except along the southern border of the ice sheet, which might, for causes named, vary some distance during the Great Year. Two hundred thousand years ago the eccentricity, then very high, reached a turning point. It then steadily, though gradually, diminished for fifty thousand years; at that time the eccentricity was so small, though considerably larger than at present, that it is doubtful if it was of any service in producing a change of climate.31 At that time, also, the Northern Hemisphere was passing through the Summer season of the Great Year. We ought, therefore, to have had a mild interglacial season. Except in high northern latitudes the ice should have disappeared. This change we would expect to find more marked in Europe than in America. We need only recall how strong are the evidences on this point. Nearly all European writers admit at least one such mild interval, and though not wanting evidence of such a period in America, our geologists are much less confident of its occurrence. But from that point the eccentricity again increased. So when the long flight of years again brought secular Winter to the Northern Hemisphere, the glaciers would speedily appear, and as eccentricity was again high, they would again hold the country in their grasp. Fifty thousand years later, or one hundred thousand years ago, it passed its turning point again; eighty thousand years ago, it became so small that it probably ceased to effect the climate. Since then it has not been very large. Twenty-five thousand years ago it was less than it is now, but it is again growing smaller. According to this theory, then, the Glacial Age commenced about two hundred and twenty thousand years ago. It continued, with one interruption of mild climate, for one hundred and forty thousand years, and finally passed away eighty thousand years ago.