* An apology is due here in some measure. The work which I quote as of Professor Chamberlin ("Geology," 1903) is really by two authors, Professors Chamberlin and Salisbury. I merely quote Professor Chamberlin for shortness, and because the particular ideas I refer to are expounded by him in separate papers. The work is the finest manual in modern geological literature. I have used it much, in conjunction with the latest editions of Geikie, Le Conte, and Lupparent, and such recent manuals as Walther, De Launay, Suess, etc., and the geological magazines. The exception which Professor Chamberlin has in mind when he says "most of the data" is that we find deposits of salt and gypsum in the Silurian and Lower Carboniferous, and these seem to point to the evaporation of lakes in a dry climate. He admits that these indicate, at the most, local areas or periods of dryness in an overwhelmingly moist and warm earth. It is thus not disputed that the climate of the earth was, during a period of at least fifteen million years (from the Cambrian to the Carboniferous), singularly uniform, genial, and moist. During that vast period there is no evidence whatever that the earth was divided into climatic zones, or that the year was divided into seasons. To such an earth was the prolific life of the Coal-forest adapted. It is, further, not questioned that the temperature of the earth fell in the latter part of the Carboniferous age, and that the cold reached its climax in the Permian. As we turn over the pages of the geological chronicle, an extraordinary change comes over the vegetation of the earth. The great Lepidodendra gradually disappear before the close of the Permian period; the Sigillariae dwindle into a meagre and expiring race; the giant Horsetails (Calamites) shrink, and betray the adverse conditions in their thin, impoverished leaves. New, stunted, hardy trees make their appearance: the Walchia, a tree something like the low Araucarian conifers in the texture of its wood, and the Voltzia, the reputed ancestor of the cypresses. Their narrow, stunted leaves suggest to the imagination the struggle of a handful of pines on a bleak hill-side. The rich fern-population is laid waste. The seed-ferns die out, and a new and hardy type of fern, with compact leaves, the Glossopteris, spreads victoriously over the globe; from Australia it travels northward to Russia, which it reaches in the early Permian, and westward, across the southern continent, to South America. A profoundly destructive influence has fallen on the earth, and converted its rich green forests, in which the mighty Club-mosses had reared their crowns above a sea of waving ferns, into severe and poverty-stricken deserts.