THE ILLUSION OF CHANCE. 211 



Herbert Spencer points out similar truths in that part of his 

 philosophy concerning the rhythm of motion : * " Every planet, dur- 

 ing 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 de- 

 creases. ... 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, tak- 

 ing 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 im- 

 mense 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 sus- 

 tained average results in spite of seeming irregularities. The slowness 

 with which some great changes take place is equivalent to the estab- 

 lishment 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 barbar- 

 ity 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. 



