of Secular Changes of Climate. 87 



produce a severe glacial epoch, the changing phases of precession 

 have very little, if any, effect on the character of the climate, as 

 mild or glacial, though it may modify the seasons ; but when the 

 eccentricity becomes moderate and the resulting climate less severe, 

 then the changing phases of precession bring about a considerable 

 alteration and even a partial reversal of the climate." — P. 153. 



Again, — " It follows that towards the equatorial limits of a glaci- 

 ated country alternations of climate may occur during a period of 

 high eccentricity, while near the pole, where the whole country is 

 completely ice-clad, no amelioration may take place. Exactly the 

 same thing will occur inversely with mild Arctic climates." — 

 P. 154. 



I have, on the contrary, maintained that the more severe 

 the glacial condition of the one hemisphere, the warmer and 

 the more equable would necessarily be that of the other; for 

 the very same combination of causes which would tend to 

 cool the one hemisphere would necessarily tend to warm the 

 other. The process to a large extent consists of a transfer- 

 ence of heat from the one hemisphere to the other. Conse- 

 quently the one hemisphere could not be heated without the 

 other being cooled, or the one cooled without the other being 

 heated. The hotter the one, the colder the other, and the 

 colder the one, the hotter the other. It therefore follows 

 that the more severe the glacial conditions, the warmer and 

 more equable must be the interglacial warm periods. But, 

 according to Mr. Wallace, there could be no warm inter- 

 glacial periods, either in temperate or polar regions, except 

 during the commencement and towards the close of a glacial 

 epoch. 



Before, however, proceeding to examine in detail the steps 

 by which he arrives at this modification of my theory, it will 

 be as well that the reader should have a clear and distinct 

 knowledge of what that theory really is, and w r hat it professes 

 to explain. These I shall now briefly state in the most 

 general terms, for misapprehension in regard to the main 

 features of the theory lie at the root of most of the objections 

 which have been urged against it. 



General Statement of the Theory. — 1st. It is not professed 

 that the theory will account for the condition of climate during 

 all past geological ages. It treats mainly of the cause of 

 Glacial Epochs; and one of its essential elements is that these 

 epochs consist of alternate changes, to a greater or less extent, 

 of cold and warm periods; or, in other words, that glacial 

 epochs must consist of alternate glacial and interglacial 

 periods. The chief, though not the sole, aim of the theory is 

 to account for geological climate in so far as such epochs are 



