of Secular Changes of Climate. 97 



for its effect on climate, if it had any, was in opposition to 

 that of the astronomical and physical agencies. It would 

 prove a hindrance, not a help. 



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 geographical 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. 



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 previ- 

 ously done — melt the snow and ice which they had just pro- 

 duced. Now, what Mr. Wallace asks is this : When, owing 

 to the winter solstice being in aphelion during a high state 

 of eccentricity, 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 un- 

 changed 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 perihelion 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 in this 

 struggle manage to overpower the physical and produce a 

 melting of the ice? I unhesitatingly reply, no ; for the 

 physical causes are far more powerful than the astronomical. 

 The astronomical causes, as we have seen, are perfectly unable 

 to produce a glacial state of things without the aid of the 

 physical. How, then, could we expect that they could re- 

 move this glacial state if the physical causes were actually 

 working against them ? 



In thus setting the physical causes against the astronomical, 

 Mr. Wallace is basing his argument for the nondisappearance 

 of the snow and ice on a state of things which cannot possibly 

 under the circumstances exist. His question, to have con- 



Phil. Mag. S. 5. Vol. 17. No. 104. Feb. 1884. H 



