304 COSMIC PHILOSOPHY. [pt. 11. 



and epochs of strongly contrasted seasons, through which 

 alternately each hemisphere passes, must grow more and 

 more different in the degrees of their contrast : and con- 

 trariwise during decrease of the eccentricity. 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 21,000 years to complete; and that involved 

 by the variation of the orbit's eccentricity, gone through in 

 millions of years." * 



The astronomic rhythms here enumerated are peculiarly 

 interesting from the fact that, owing to their comparatively 

 simple character, they are susceptible of mathematical treat- 

 ment, so that their direct dependence on the principle of the 

 persistence of force can be quantitatively demonstrated. In 

 ascending to the order of phenomena next above them in 

 point of complexity — the geologic phenomena occurring on 

 the earth's surface — we enter a region where such quantita- 

 tive proof, save of a very crude sort, cannot be obtained. 

 The great complexity of geologic as contrasted with astro- 

 nomic rhythms is shown by the fact that whereas on the 

 one hand, we can readily calculate the variations of eccentri- 

 city in the earth's orbit which have taken place during 

 millions of years gone by or which are sure to take place 

 during millions of years to come, on the other hand we are 

 not yet able to assign an approximate date for the most 

 recent epoch at which our northern hemisphere was covered 

 with glaciers. According to Mr. Wallace this epoch may 

 have occurred no more than seventy thousand years ago, 

 while others would assign to it an antiquity of at least two 

 hundred thousand years, and there are yet others who urge 

 strong arguments in behalf of the opinion that a million of 

 years is barely enough to have produced the changes which 

 1 First Principles, pp. 256, 257. 



