The author exhibited a diagram of the eccentricity of the earth's orbit and the precession of the equinoxes, from which he deduced certain important views as to the climates of past geological ages and the changes of organic life. During the past three million years the eccentricity has been almost always much greater than at present, on the average twice as great, and for long periods more than three times as great. It was shown that when the eccentricity was greatest the heat received from the sun at the greatest and least distances was as 3 to 4; and, owing to the precession of the equinoxes, the winters of the northern hemisphere would be rendered intensely cold and much longer for periods of 10,500 years, while during the alternate periods the winters would be mild and short, the summers cool and long, leading to an almost perpetual spring. We thus have cold or glacial epochs for about 10,000 years, alternating with mild epochs for the same period, whenever the eccentricity was high, and this was the case for fully the half of the last three million years; and, as such alternations must have occurred during every glacial epoch, the fact of intercalated warm periods and the migrations consequent on them, which have been detected by geologists, must be looked upon as the normal condition of things. But during the last 60,000 years (probably the whole time elapsed since the close of the last glacial epoch) the eccentricity has been very small, and the alternations of climate and consequent migrations very slight; and as Mr. Darwin holds that alternations of climate are, by means of the consequent migrations, the most powerful cause of modifications of species, there must have been a comparative stability of species during that period of time, from which alone we obtain our idea of the rate of specific change. This idea will therefore be erroneous; and the rate of change during past geological ages may have been, and probably was, much more rapid than has hitherto been thought possible. During three million years before and one million after the recent epoch, no less than 130 alternations of climate occurred (each of 10,000 years' duration), when the eccentricity was more than double what it is now; and these incessant changes were thought, on Darwinian principles, to supply a vera causa for a rapid change of species, and thus enable us considerably to reduce the duration of geologic periods, which had heretofore been measured by data derived from the period of organic stability since the last global epoch.