If we are to reason from the records of the past it seems highly probable that at least the middle latitudes of the Northern Hemisphere are slowly passing out of a dry period that has prevailed for the past two hundred years or more. For several hundred years all the great glaciers have receded, but we should not expect such recession to continue indefinitely. Geology furnishes abundant evidence that great changes took place in the climate of the earth during the prehistoric ages; that there were several glacial periods, the last occurring during pleistocene times, somewhere between twenty and fifty thousand years ago, and that there were intervals between the culminations of the Ice Ages of probably fifty thousand to one hundred thousand years. Between these long winters, that have meant death and desolation to much of what are now the most civilized portions of the earth, there have been warm periods of thousands of years’ duration. Fossil remains show that regions far north, now covered with perpetual ice, once supported a luxuriant flora and fauna, and many regions in the temperate and equatorial zones that are now deserts were once overgrown with forests and teeming with animal life. The fundamental thing of the cosmos is change—birth, growth, maturity; then decline, senility, death, decay, disintegration; and always a renaissance, or new birth. Energy and life seem to be eternal, but ever undergoing change. The Great Ice Cap may again cover New England, the region of the Great Lakes, and flow southward to the Ohio River, but the change will be so gradual, if it does come, that there will be no great cities to be ground beneath the feet of the boreal monster; cold that will precede the ice cap will destroy them and they will be buried beneath the dust of accumulating ages before their icy tombstone is erected. Then the healthful and invigorating climate of the north part of our country will be transferred to the region of the Gulf of Mexico. Civilization will and must migrate with the shifting of the climatic belts. Because these changes cannot possibly concern us personally, we have almost neglected the study of the great forces that silently yet most persistently are at work altering the conditions under which future man must live and work out the destiny of coming generations.