The role of climate in the life of today suggests its importance in the past and in the future. No human being can escape from the fact that his food, clothing, shelter, recreation, occupation, health, and energy are all profoundly influenced by his climatic surroundings. A change of season brings in its train some alteration in practically every phase of human activity. Animals are influenced by climate even more than man, for they have not developed artificial means of protecting themselves. Even so hardy a creature as the dog becomes notably different with a change of climate. The thick-haired "husky" of the Eskimos has outwardly little in common with the small and almost hairless canines that grovel under foot in Mexico. Plants are even more sensitive than animals and men. Scarcely a single species can flourish permanently in regions which differ more than 20 deg.C. in average yearly temperature, and for most the limit of successful growth is 10 deg..[1] So far as we yet know every living species of plant and animal, including man, thrives best under definite and limited conditions of temperature, humidity, and sunshine, and of the composition and movement of the atmosphere or water in which it lives. Any departure beyond the limits means lessened efficiency, and in the long run a lower rate of reproduction and a tendency toward changes in specific characteristics. Any great departure means suffering or death for the individual and destruction for the species. Since climate has so profound an influence on life today, it has presumably been equally potent at other times. Therefore few scientific questions are more important than how and why the earth's climate has varied in the past, and what changes it is likely to undergo in the future. This book sets forth what appear to be the chief reasons for climatic variations during historic and geologic times. It assumes that causes which can now be observed in operation, as explained in a companion volume entitled _Earth and Sun_, and in such books as Humphreys' _Physics of the Air_, should be carefully studied before less obvious causes are appealed to. It also assumes that these same causes will continue to operate, and are the basis of all valid predictions as to the weather or climate of the future.