MODIFICATION OF THEORY EXAMINED. 135 



amount of eccentricity at the two periods when these 

 ice-sheets were respectively developed. In short, if 

 the last great ice-sheet can be explained without the 

 supposition of a high state of eccentricity, then there 

 does not appear to be any real necessity for any theory 

 of eccentricity in accounting for the Glacial Epoch. 



If we adopt the Physical theory of the cause of the 

 Glacial Epoch, we are compelled to maintain that the 

 last two great Ice-periods were the indirect results of 

 a high state of eccentricity, and in this case we can 

 hardly avoid the conclusion that the mild intervening 

 period was due to the same cause. The occurrence of 

 a mild interglacial period between the two ice-periods 

 is directly in opposition to Mr. Wallace's view — that 

 during a high state of eccentricity the ice would not 

 disappear but be continued. It is in perfect harmony, 

 however, with that which I advocate ; for during high 

 eccentricity a mild and equable condition of climate, 

 when the winters occur in perihelion, is as much a 

 necessary result as a cold and glacial condition when 

 they occur in aphelion. 



The facts of Geology thus to me appear, so far, to be 

 as much opposed to Mr. Wallace's modifications as are 

 the principles of Physics. 



Difficulty in detecting the Climatic Character of the 

 Earlier Interglacial Periods. — It follows according to 

 theory that, other things being equal, the greater the 

 amount of eccentricity the more equable and mild will 

 the interglacial periods be. It is probable, therefore, 

 that some of the earlier interglacial periods were 

 milder and more equable than the last. It may be 

 difficult, in the present state of our knowledge, to 

 prove this conclusion by direct geological and palason- 

 tological evidence ; but, on the other hand, it is certainly 

 impossible to disprove it by that means. The absence 



