MODIFICATION OF THEORY EXAMINED. 91 



to a large extent consists of a transference of heat 

 from the one hemisphere to the other. Consequently 

 the one hemisphere could not be heated without the 

 other being cooled, nor 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 there- 

 fore 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 interglacial periods, either in 

 temperate or polar regions, except during the com- 

 mencement 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 what 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 the 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 concerned. Although it could be 

 satisfactorily shown, for example, and this has certainly 

 not yet been done, that during some past geological 

 age, such as the Miocene, the Eocene, or the Cretaceous, 



