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.29 About two hundred and fifty thousand years ago the earth’s path around 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 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.30