96 DISCUSSIONS IN CLIMATOLOGY. 



3. Mr. Wallace agrees with me in regard to the 

 mutual reactions of the physical agents. He maintains 

 with me that these physical agencies not only all lead 

 to one result — the accumulation of snow and ice — but 

 that their efficiency in bringing about this result is 

 strengthened by their mutual reactions on one 

 another. At pp. 137-139 he gives a variety of 

 examples of these mutual reactions, and says that 

 they "produce a maximum of effect which, without 

 their aid, would be altogether unattainable." 



4. As has already been shown, we both agree as to 

 the necessity of certain geographical conditions for 

 the production of the glacial epoch. For although 

 that epoch was mainly brought about by the physical 

 agencies, yet these agencies could not have produced 

 the required effect unless the necessary geographical 

 conditions had been supplied, these being necessary for 

 their effective operation. 



5. Mr. Wallace admits, of course, that the necessary 

 geographical conditions existed during the glacial 

 epoch; for, unless this had been the case, no glacial 

 epoch could have occurred. Therefore, all that was 

 required to produce glaciation was an amount of 

 eccentricity sufficient to set the physical agencies into 

 operation. Be it observed, it did not require, in addi- 

 tion to the physical agencies, some changes in the 

 geographical conditions, or some new conditions; for 

 the geographical conditions being existent, all that was 

 then required to bring about the glacial epoch was the 

 operation of the physical agencies. The overlooking 

 of this fact has led to much confusion. For example, 

 210,000 years ago, with winter in aphelion, " the pro- 

 blem to be solved," says Mr. Wallace, "is, whether the 

 snow that fell in winter would accumulate to such an 

 extent that it would not be melted in summer, and so 



