MODIFICATION OF THEORY EXAMINED. 119 



cidences purely accidental. These changes have all 

 been so recent, geologically speaking, and so general and 

 widespread in their character, that they cannot reason- 

 ably be attributed to any known geographical changes. 

 If we admit, then, that they were the result of those 

 astronomical and physical agents to which I have 

 referred them, we must also admit that those agents 

 were as efficient in producing a warm and equable 

 climate as in producing a cold and severe one. We 

 must further admit that, with a very small amount of 

 eccentricity, widely marked differences of climatic 

 conditions are brought about on the two hemispheres ; 

 that, when the winters are in perihelion, the melting 

 of the snow and ice and the increase of the Gulf 

 Stream and other northward-flowing currents are as 

 necessary a result as were the formation of the snow 

 and ice and the decrease of the Gulf Stream and those 

 currents when the winters were in aphelion. And if 

 this holds true in reference to recent and postglacial 

 times, when the eccentricity was small, it must, for 

 reasons which will presently be stated, hold true in a 

 higher degree in reference to the glacial epoch, when 

 the eccentricity was more than three times its present 

 value. 



The Mutual Reaction of the Physical Agents in 

 Relation to the Melting of the Ice. — When the winter 

 solstice is in aphelion it sets in operation, according to 

 theory, as has been shown, a host of physical causes 

 the tendency of which is to produce an accumulation 

 of snow and ice ; but when the solstice-point moves 

 round to perihelion the tendency of these causes is 

 reversed, and they then undo what they had previously 

 done — they melt the snow and ice which they had 

 just produced. The action of the causes being 

 reversed, the effects are reversed. But it must be 



