98 Dr. J. Croll on the Physical Theory 



sistency, should be this : — When glacial conditions were at 

 their height &c, " can we suppose that the mere change from 

 the distant sun in winter and the near sun in summer, to the 

 reverse, could bring about any important alteration — the geo- 

 graphical causes of glaciation remaining unchanged ?" If the 

 question is put thus, and it is the only form in which it can be 

 put to be consistent with the theory which Mr. Wallace him- 

 self advocates, then my reply is, that the change from the 

 distant sun in winter and near sun in summer to the near 

 sun in winter and distant sun in summer, aided by the change 

 in the physical causes which this would necessarily bring 

 about, would certainly be sufficient to cause the snow and 

 ice to disappear without any change in the geographical 

 condition of things. The combined influence of the astro- 

 nomical and physical causes, when the winter solstice is in 

 perihelion, is perfectly sufficient to undo all that they had 

 previously done when the solstice was in aphelion. When the 

 action of the causes is reversed, the effects will be reversed. 



Had the glacial epoch been produced by geographical 

 causes, then it is probable thai: the ice would not have dis- 

 appeared 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 probably 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, however, that the same geographical condition 

 of things which favours the accumulation of ice when the 

 winter solstice is in aphelion, favours its disappearance 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 re- 

 sult 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 disappearance of the ice on 

 the hemisphere with the perihelion winter, whether that hemi- 

 sphere be the northern or the southern. 



The tendency of the combined influence of all the causes — 



