REPLY TO CRITICS. 19 



In a question so complex and difficult as that of the 

 cause of the Glacial Epoch, depending as it does on 

 the consideration of so many different elements, some 

 of which are but little understood, logical analysis 

 rather than mathematics will require to be our 

 instrument in the meantime. The question must first 

 assume a clear, definite, and logical form before 

 mathematics can possibly be applied to it. 



It is objected that my language is wanting in quan- 

 titative precision — that I use such terms as "great," 

 " very great," " small," " comparatively small," and so 

 forth without any statement of the units of comparison 

 relatively to which these expressions are employed. No 

 one reasoning on the combined influence of a multitude 

 of physical causes could well avoid the almost continual 

 use of such terms. Besides, my critic forgets that in 

 almost every case in which I use these terms numerical 

 exactness is not attainable ; and even if it were, it 

 would, as a rule, be of little service, seeing that the 

 conclusion generally depends on the simple fact that 

 one quantity is less or greater than another ; not on 

 how much less or how much greater the one may be 

 than the other. Although my arguments are logical, 

 few writers, I venture to say, have done more than 

 myself to introduce definite quantitative exactness 

 into the questions I have discussed. 



Temperature of Space. — One of the most important 

 factors in the theory of geological climate resulting 

 from changes in the eccentricity of the earth's orbit is 

 obviously the temperature of stellar space. Unless we 

 have, at least, some rough idea of the proportion which 

 the heat derived from the stars bears to that derived 

 from the sun, we cannot form any estimate of how much 

 the temperature of our earth would be lowered or raised 

 by a given decrease or increase of the sun's distance. 



