Throughout geological history the slow and almost imperceptible differentiation of the earth's surface has been one of the most noteworthy of all changes. It has been opposed by the extraordinary conservatism of the universe which causes the average temperature today to be so like that of hundreds of millions of years ago that many types of life are almost identical. Nevertheless, the differentiation has gone on. Often, to be sure, it has presumably been completely masked by the disturbances of the solar atmosphere which appear to have been the cause of the sharper, shorter climatic pulsations. But regardless of cosmic conservatism and of solar impulses toward change, the slow differentiation of the earth's surface has apparently given to the world of today much of the geographical complexity which is so stimulating a factor in organic evolution. Such complexity--such diversity from place to place--appears to be largely accounted for by purely terrestrial causes. It may be regarded as the great terrestrial contribution to the climatic environment which guides the development of life. FOOTNOTES: [Footnote 97: Encyclopaedia Britannica, 11th edition: article "Ocean."] [Footnote 98: C. E. P. Brooks: The Meteorological Conditions of an Ice sheet and Their Bearing on the Desiccation of the Globe; Quart. Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70.] [Footnote 99: Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U. S. Geol. Survey.] [Footnote 100: Quoted by Schuchert in The Evolution of the Earth.] [Footnote 101: Smithsonian Physical Tables, Sixth Revision, 1914, p. 142.] [Footnote 102: Chamberlin, in a very suggestive article "On a possible reversal of oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373, 1906), discusses the probable climatic consequences of a reversal in the direction of deep-sea circulation. It is not wholly beyond the bounds of possibility that, in the course of ages the increasing drainage of salt from the lands not only by nature but by man's activities in agriculture and drainage, may ultimately cause such a reversal by increasing the ocean's salinity until the more saline tropical portion is heavier than the cooler but fresher subpolar waters. If that should happen, Greenland, Antarctica, and the northern shores of America and Asia would be warmed by the tropical heat which had been transferred poleward beneath the surface of the ocean, without loss _en route_. Subpolar regions, under such a condition of reversed deep-sea circulation, might have a mild climate. Indeed, they might be among the world's most favorable regions climatically.] [Footnote 103: Encyclopaedia Britannica: article "Ocean."] [Footnote 104: Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132, 1906; and T. C. Chamberlin: The Origin of the Earth, 1916.] [Footnote 105: Personal communication.] [Footnote 106: R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of Wash., No. 106, 1908.]