MODIFICATION OF THEORY EXAMINED. 107 



causes, then it is probable that the ice would not have 

 disappeared till these causes were changed. Had the 

 ice, for example, been simply due to an elevation of 

 the land, as some have argued, then it would not pro- 

 bably have disappeared till the land became lowered. 

 But it was the result of no such cause. It was due, 

 not to an elevation of the land, but to a number of 

 physical causes, brought into operation by a high 

 state of eccentricity. This Mr. Wallace fully admits 

 and maintains. A certain geographical state of things 

 was, of course, necessary to enable the astronomical 

 and physical causes to produce the required effect ; 

 and this was really all that geographical conditions 

 had to do in the matter. Let this be observed, how- 

 ever, that the same geographical condition of things 

 which favours the accumulation of ice when the 

 winter solstice is in aphelion, favours its disappear- 

 ance when the solstice is in perihelion. This is 

 obvious, because the same combination of physical 

 agencies which makes the hemisphere in aphelion 

 cold, makes the one in perihelion warm. The heating 

 of the one is, to a large extent, the result of the 

 cooling of the other. It is the transference of heat by 

 ocean-currents from the hemisphere in aphelion to the 

 one in perihelion which is a main reason why the 

 former is cold and the latter warm. Hence a change 

 in geographical conditions is unnecessary for the dis- 

 appearance of the ice on the hemisphere with the 

 perihelion winter, whether that hemisphere be the 

 northern or the southern. 



The tendency of the combined influence of all the 

 causes — astronomical, physical, and geographical — is 

 to cool the one hemisphere and to warm the other, to 

 accumulate the ice on the one, and remove it from 

 the other. Consequently the same total combination 



