the lands of high polar areas and both Europe and America, since we know that an abundant vegetation spread from there, as a center, to both these countries. But at that epoch circumstances were different. The great North Temperate lands were in a "comparatively fragmentary and insular condition." There were great inland seas in both Europe and Asia, through which powerful currents would have flowed from the Indian Ocean to Arctic regions. Somewhat similar conditions prevailed in North America. The western part was in an insular condition. A great sea extended over this part of the country, joining the Arctic probably on the north, through which heated water would pour into the polar sea. And so, instead of a Glacial Age, we find evidence of a mild and genial climate, with an abundant vegetation. We thus see that there are two theories as to the cause of the Glacial Age presented for our consideration. Both of them have received the sanction of scholars eminent for their scientific attainments. On inspection we see they are not antagonistic theories. They may both be true for that matter, and all would admit that whatever effect they would produce singly would be greatly enhanced if acting together. Indeed, there are very good reasons for supposing both must have acted in unison. There seem to be very good reasons for not believing that the eccentricity of the earth's orbit, acting alone, produced the glacial 158 cold. If that were the case, then whenever the eccentricity was great we should have a Glacial Age. Now, at some period of time during the long-extended Tertiary Age we are certain the eccentricity of the earth's orbit became very great, much more so, in fact, than that which is supposed to have produced the cold of the Quaternary Age. But we are equally certain there was no glacial epoch during this age. What other explanation can we give for its non-appearance except that geographical conditions were not favorable? But, on the other hand, there are certain features connected with the phenomena of the Glacial Age that seem very difficult of explanation, if we suppose that geographical changes alone produced them. We must remember that evidences of the former presence of glaciers are found widely scattered over the earth. We shall, therefore, have to assume an elevation not only for America and Europe, but extend it over into Asia, and take in the Lebanon Mountains, for they also show distinct traces of glaciers. And this movement of elevation must also have affected the Southern Hemisphere, the evidence being equally plain that at the same comparatively late date glaciers crushed over Southern Africa and South America. This is seen to prove too much. Again, how can we explain the fact that some time during the Glacial Age we had a submergence, the land standing several hundred feet lower than now, but still remained covered with ice, and over the submerged part there sailed icebergs and ice-rafts, freighted with their usual
debris? That such was the state of things in Europe we are assured by some very good authorities. Neither do geographical causes afford an adequate explanation of those changes of temperature that surely took place during the Glacial Age. These last considerations show us how difficult it is to believe that 159 geographical causes could have produced the Glacial Age. We are assured that all through the geological ages the continents had been increasing in size and compactness, and that just at the close of the Tertiary Age they received a considerable addition of land to the north. The astronomer also informs us that at a comparatively recent epoch the eccentricity of the earth's orbit became very great. The conditions being favorable, it is not strange that a Glacial Age supervened. We have been to considerable length in thus explaining the position of the scientific world in regard to the cause of the Glacial Age. Our reason for so doing is that this age is, we think, so connected with the Paleolithic Age of man, that it seems advisable to have a clear understanding in regard to it. What we have to say is neither new nor original. It is simply an earnest endeavor to represent clearly the conclusions of some of our best scholars on this subject, and we have tried to give to each theory its due weight. Our conclusions may be wrong, but, if so, we have the consolation of erring in very good company. We have now gone over the ground and are ready to see what dates can be given. Though the numbers we use seem to be very large indeed, they are so only in comparison with our brief span of life. They are insignificant as compared with the extent of time that has surely rolled by since life appeared on the globe. Let us, therefore, not be dismayed at the figures the astronomer sets before us. About two hundred and fifty thousand years ago the earth's path around 160 the sun was much the same as that of the present. No great changes in climate were liable to take place at that time. During the next fifty thousand years the eccentricity steadily increased. Towards the end of that time all that was necessary to produce a glacial epoch in the Northern Hemisphere was favorable geographical causes, and that our earth should reach its point nearest the sun in Summer. This it must have done when about half that time had elapsed. We can in imagination see what a slow deterioration of climate took place. Thousands of years would come and go before the change would be decisive. But a time must have at length arrived when the vegetation covering the ground was such as was suited only for high northern latitudes. The animals suited for warm and temperate regions must have wandered farther south; others from the north had arrived to take their place. We can see how well this agrees with the changes of climate at the close of the Pliocene Age. The snows of the commencing Glacial Age would soon begin to fall, finally the sun would not melt them off of the high lands, and mountain peaks, and so a Glacial Age would be ushered in. We have referred to the fact that the earth reaches its perihelion point a little earlier each year, and, as a consequence, we would have periods of mild climate alternating the cold. This extended period of time, equal to twenty-one thousand of our ordinary years, has been named the Great Year of our globe. Mr. Wallace has pointed out some very good reasons for thinking Mr. Croll's theory must be modified on this point. He thinks that when once a Glacial Age was fairly fastened on a hemisphere, it would retain its grasp as long as the eccentricity remained high, but whenever the Summer of the Great Year came to that 161 hemisphere, it would melt back the glacial ice for some distance, but this area would be recovered by the ice when the Winter of the Great Year supervened. These effects would be different when the eccentricity itself became low. Then we would expect the glacial conditions to vanish entirely when the Summer of a Great Year comes on. 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. 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 162 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. What shall we say to these results? If true, what a wonderful antiquity is here unfolded for the human race, and what a wonderful lapse of time is included in what is known as the Paleolithic Age! How strikingly does it impress upon our minds the slow
