MODIFICATION OF THEORY EXAMINED. 83 



they are nevertheless implied in the theory. Other 

 points, again, regarded as modifications, are simply 

 facts lying altogether outside of the theory, which can 

 in no way affect it. 



Before proceeding, however, to examine in detail 

 Mr. Wallace's modifications of the theory, it may be 

 as well to consider one or two minor points on which 

 I differ from him, as this will save the necessity of 

 referring to them when we come to discuss his main 

 argument. 



Effect of Winter Solstice in Aphelion. — At page 

 126 ("Island Life") he says: — "We may therefore say 

 generally, that during our northern winter, at the time 

 of the glacial epoch, the northern hemisphere was 

 receiving so much less heat from the sun as to lower 

 its surface-temperature on an average about 35° F., 

 while during the height of summer of the same period 

 it would be receiving so much more heat as would 

 suffice to raise its mean temperature about 60° F. 

 above what it is now." In a footnote he adds that 

 " the reason of the increase of summer heat being 60° 

 while the decrease of winter cold is only 35°, is because 

 our summer is now below and our winter above the 

 average." 



There is surely a confusion of ideas here. It is of 

 course true that, as our summer at present occurs in 

 aphelion and our winter in perihelion, the tempera- 

 ture of the former is below and that of the latter 

 above the average ; but this can afford no grounds for 

 the result Mr. Wallace attributes to it unless it be 

 assumed (for which there are no astronomical grounds) 

 that our summer is 25° further below the average 

 than our winter is above it. 



On the Storage of Gold. — In a section on the Effects 

 of Snow on Climate, Mr. Wallace points out the 



