[from the text] Climate as an element of physical environment is so well recognized that there is no need to demonstrate its importance. By common consent it is held to be a primary factor not only in the life of plants, animals, and man as they exist today, but in their entire evolution. Moreover, among the main elements of physical environment it alone is subject to pronounced changes in comparatively brief periods. The form of the lands, the location of mountains, and the composition of the atmosphere are doubtless all subject to great changes, but these are too slow to have much effect upon a single generation of living beings or even upon all the generations that have existed during the period since man emerged from barbarism. Small climatic changes, however, such as those from one year to the next, or from one decade to another, are constantly in progress, and their far-reaching results are a matter of every-day experience. Moreover, it is quite possible that larger changes have taken place during the past 2,000 or 3,000 years, and if this is the case their effects must have been correspondingly important. The investigation of this possibility is the purpose of this volume.