MODIFICATION OF THEORY EXAMINED. 105 



Referring now to Mr. Wallace's argument: When 

 glacial conditions in the North Atlantic attained their 

 maximum development, "can we suppose," he asks, 

 " that the mere change from the distant sun in winter, 

 and near sun in summer to the reverse, could bring- 

 about any important alteration — the physical and geo- 

 graphical causes of glaciation remaining unchanged ?" 

 Here, to begin with, we have an impossible state of 

 things assumed. It is assumed in this question that 

 it is possible for the winter solstice to pass from 

 aphelion to perihelion, and the physical causes to 

 remain unchanged. It is assumed as possible that 

 the astronomical conditions might be reversed without 

 a reversal of the physical conditions. 



When the winter solstice is in aphelion, it sets in 

 operation many physical causes, the tendency of which 

 is to produce an accumulation of snow and ice ; but 

 when the solstice-point moves round to the perihelion, 

 the tendency of these causes is reversed, and they then 

 undo what they had previously done — melt the snow 

 and ice which they had just produced. Now, what 

 Mr. Wallace asks is this : When, owing to the winter 

 solstice being in aphelion during a high state of eccen- 

 tricity, a glacial condition of things is produced, will 

 the fact of the solstice-point being moved round to 

 perihelion remove the glacial condition, if the physical 

 causes remain unchanged in their mode of operation? 

 My reply is, it certainly would not. Here it is assumed 

 that the physical causes are working in opposition to 

 the astronomical ; that when the solstice is in peri- 

 helion the action of the physical causes, instead of 

 being reversed, as it should be according to theory, 

 still continues to produce and maintain a glacial state 

 of things, the same as it did when the solstice-point was 

 in aphelion ; and he asks, Will the astronomical causes 



