REPLY TO CRITICS. 55 



act and react upon each other ; but the reaction of the 

 effect tends to weaken the cause. Those physical 

 agents to which I have referred, no doubt, in their 

 mutual actions and reactions, obey the same law ; but 

 in reference to one particular result, viz. the accumu- 

 lation and conservation of snow, those mutual reactions 

 strengthen one another. This is not reasoning in a 

 circle, as Mr. Searles Wood supposes ; for the reaction 

 of an effect may on the whole weaken the cause, and 

 yet in regard to a particular result it may strengthen 

 it. In the case under consideration the agents not 

 only act in one direction, but their efficiency in acting 

 in that one direction is strengthened by their mutual 

 reactions. This curious circumstance throws a flood 

 of light on the causes which tended to bring about the 

 glacial epoch. 



To begin with, we have a high state of eccen- 

 tricity. This leads to long and cold winters. The 

 cold leads to snow; and although heat is given out in 

 the formation of the snow, yet the final result is that 

 the snow intensifies the cold: it cools the air and leads 

 to still more snow. The cold and snow bring a third 

 agent into play — fogs, which act still in the same 

 direction. The fogs intercept the sun's rays ; this 

 interception of the rays diminishes the melting-power 

 of the sun, and so increases the accumulation. As the 

 snow and ice continue to accumulate, more and more 

 of the rays are cut off; and, on the other hand, as the 

 rays continue to be cut off, the rate of accumulation 

 increases, because the quantity of snow and ice melted 

 becomes thus annually less and less. In addition, the 

 loss of the rays cut off by the fogs lowers the tempera- 

 ture of the air and leads to more snow being formed, 

 while, again, the snow thus formed chills the air 

 still more and increases the fogs. Again, during the 



