of Secular Changes of Climate. 109 



accumulation of snow and ice, and as the snow and ice accu- 

 mulate the ocean-currents decrease. On the other hand, as 

 the ocean -currents diminish, the snow r and ice still more accu- 

 mulate. Thus the two effects, in so far as the accumulation 

 of snow and ice is concerned, mutually strengthen each other." 



With all this Mr. Wallace seems fully to agree ; for at 

 pp. 137-140 (' Island Life ') he gives a very clear statement 

 of the effect of these mutual reactions in the production of 

 glaciation, and says that were it not for them it is probable the 

 astronomical and other causes would not in our latitudes have 

 been sufficient to produce glaciation. In short, he concludes 

 that these reactions "produce a maximum of effect which, 

 without their aid, would be altogether unattainable." Mr. 

 Wallace thus does full justice to these mutual reactions in so 

 far as the production of glaciation is concerned ; but I am 

 convinced that he must have underestimated their importance 

 as regards the removal of the glaciation. He, however, recog- 

 nizes the fact that these mutual reactions produce an opposite 

 effect on the warm atmosphere whose winters are in perihelion. 

 " These agencies," he says, " are at the same time acting in a 

 reverse way in the southern hemisphere, diminishing the 

 supply of the moisture carried by the anti-trades, and increa- 

 sing the temperature by means of more powerful southward 

 ocean-currents ; and all this again reacts on the northern 

 hemisphere, increasing yet further the supply of moisture by 

 the more powerful south-westerly winds, while still further 

 lowering the temperature by the southward diversion of the 

 Gulf-Stream." 



Now if, during the glaciation of the northern hemisphere, 

 these mutual reactions produce the opposite effect on the 

 southern hemisphere, it is evident that they must produce this 

 same opposite effect on the northern hemisphere when its 

 winter solstice is in perihelion. Their effect then would be to 

 increase the temperature and melt the ice. When the winter 

 solstice is moving towards the aphelion, the physical agents 

 begin to act and also to react on one another, and this action 

 and reaction go on increasing in intensity till the solstice- 

 point reaches the aphelion ; but an exactly similar thing is 

 going on in the other hemisphere, only the effects are the 

 reverse. While the actions and reactions leading to an accu- 

 mulation of ice are increasing in intensity, we shall suppose, 

 on the northern hemisphere, the same increase is taking place 

 on the southern hemisphere ; but the result is a melting, not 

 an accumulation of the ice. The same process is undoing on 

 the southern hemisphere what it is doing on the northern. 

 Similarly, of course, when the northern winter solstice begins 



