Herbert Spencer points out similar truths in that part of his philosophy concerning the rhythm of motion: "Every planet, during a certain long period, presents more of its northern than of its southern hemisphere to the sun at the time of its nearest approach to him; and then, again, during a like period, presents more of its southern hemisphere than of its northern—a recurring coincidence which, though causing in some planets no sensible alterations of climate, involves in the case of the earth an epoch of twenty-one thousand years, during which each hemisphere goes through a cycle of temperate seasons, and seasons that are extreme in their heat and cold. Nor is this all. There is even a variation of this variation. For the summers and winters of the whole earth become more or less strongly contrasted, as the eccentricity of its orbit increases and decreases. . . . So that in the quantity of light and heat which any portion of the earth receives from the sun, there goes on a quadruple rhythm, that of day and night; that of summer and winter; that due to the changing position of the axis at perihelion and aphelion, taking twenty-one thousand years to complete; and that involved by the variation of the orbit's eccentricity, gone through in millions of years."
These phenomena illustrate the regularity of averages on an immense scale. The differences in temperature between unusually hot or cold seasons in a given year all offset one another when reduced to an average of a decade or of a century, just as we assume that the great differences between glacial and tropical temperatures manifest approximate uniformity in the long period above considered. It is thus clear that circumstances or the motions of events lead to sustained average results in spite of seeming irregularities. The slowness with which some great changes take place is equivalent to the establishment of permanent conditions as far as the short duration of our individual consciousness is concerned. The glacial period, whether due to the precession of the equinoxes or some other cause, involves a lapse of time far longer than is covered by the historical record of the earliest races, along down the line of mingled civilization and barbarity to the present time.
In deference to those who are too cautious to accept any doctrine of averages in nature, it is well to give full weight to an opinion in a letter from Professor C. H. Hitchcock, regarding the glacial period. He thinks that every agency must be considered, including "obliquity of orbit, precession of the equinoxes, axial variation, and elevated planes at the north." He adds, "If you can prove that in an ice age at the north the climate about the south pole was ameliorated, then the fact that it is somewhat colder there now may be of service." Beside the variation in ocean-level, we may consider it probable that, when the earth cooled from its primeval molten state, it was left with
- "First Principles," pp. 256, 257.