content of carbonic acid have been the direct cause of variations of climate. It is necessary, therefore, to assign agencies adequate to bring about such alternations of poverty and wealth of carbonic acid. The agencies must operate to produce great cycles of climatic change which are recognized through study of the geologic record. Among others, these comprise an ancient event of extensive glaciation in India, Australia and South Africa, closely following the period of mild climate during which the Coal Measure flora flourished. The agencies must further promote subsidiary action by which minor oscillations of climate may be explained, since within the latest, the Pleistocene, period of glaciation, at least five, and probably more, advances of the ice occurred in alternation with intervals of comparative mildness, during which the ice retreated notably. Depletion and enrichment of the atmosphere must furthermore occur within reasonable limits of geologic periods. And cause must be shown why the atmospheric changes promoted glaciation about peculiarly local centers. In searching the sources of carbonic acid, Chamberlin has been led to reconsider the original constitution of the atmosphere, and thus also theories of the origin of the earth, including the nebular hypothesis. Thither this review may not follow him, but it will be of interest to advert to his views as to the conditions affecting biologic evolution, which are also causally related to variations of the carbonic acid contained in the air.
Carbonic acid, or as it is more accurately called, carbon dioxide, CO2, occurs in many relations and plays many parts in the economy of the world. In some of these activities it enters into permanent combinations and is lost to the atmosphere. In others it passes through cycles of combination and release by which it is temporarily withdrawn from and subsequently returned to the air. If the atmosphere's resources in CO2 be compared with a bank account, we may suppose that the balance follows one or the other of two familiar cases. In the one example there may have been originally a definite though possibly large deposit, which has not since been added to, but upon which many drafts have been and are being drawn. Under this assumption, however rich the atmosphere once was, it is now by comparison poverty stricken. On the other hand, there is reason to believe that the original capital in CO2 was not materially greater than it is now, but that losses have been nearly balanced by gains. The first example represents a view held by geologists who believe that the atmosphere was exceedingly dense, moist and charged with carbon dioxide in early ages of the earth's history; the second illustrates the conceptions based on modern advances of biology and geology, and its acceptance is essential to the hypothesis of glaciation here discussed.
The sources from which fresh contributions may be made to the atmosphere are suggested by the occurrence of carbon dioxide among
