Many important biological consequences might be drawn from this study of mild geological climates, but this book is not the place for them. In the first chapter we saw that one of the most remarkable features of the climate of the earth is its wonderful uniformity through hundreds of millions of years. As we come down through the vista of years the mild geological periods appear to represent a return as nearly as possible to this standard condition of uniformity. Certain changes of the earth itself, as we shall see in the next chapter, may in the long run tend slightly to change the exact conditions of this climatic standard, as we might perhaps call it. Yet they act so slowly that their effect during hundreds of millions of years is still open to question. At most they seem merely to have produced a slight increase in diversity from season to season and from zone to zone. The normal climate appears still to be of a milder type than that which happens to prevail at present. Some solar condition, whose possible nature will be discussed later, seems even now to cause the number of cyclonic storms to be greater than normal. Hence the earth's climate still shows something of the great diversity of seasons and of zones which is so marked a characteristic of glacial epochs. FOOTNOTES: [Footnote 58: Rollin T. Chamberlin: Personal Communication.] [Footnote 59: F. H. Knowlton: Evolution of Geologic Climates; Bull. Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.] [Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of Geological Climates, in Am. Jour. Sci., 1921.] [Footnote 61: G. R. Wieland: Distribution and Relationships of the Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.] [Footnote 62: D. T. MacDougal: Botanical Features of North American Deserts; Carnegie Instit. of Wash., No. 99, 1908.] [Footnote 63: _Loc. cit._] [Footnote 64: H. H. Clayton: Variation in Solar Radiation and the Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.] [Footnote 65: B. Helland Hansen and F. Nansen: Temperature Variations in the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths. Inst., Vol. 70, No. 4, Washington, 1920.] [Footnote 66: The climatic significance of ocean currents is well discussed in Croll's Climate and Time, 1875, and his Climate and Cosmogony, 1889.] [Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.] [Footnote 68: W. W. Garner and H. A. Allard: Flowering and Fruition of Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp. 377-400.] [Footnote 69: Report of Committee on Sedimentation, National Research Council, April, 1922.] CHAPTER XI TERRESTRIAL CAUSES OF CLIMATIC CHANGES