92 Dr. J. Croll on the Physical Theory 



effects resulting from those physical agencies called into 

 operation by an increase of eccentricity. To have speculated 

 on hypothetical geographical conditions different from those 

 which now obtain, and on the influence which these may have 

 had in bringing about the glacial epoch, would have been on 

 my part perfectly absurd, as I knew we had no evidence of 

 the existence of any such conditions. Besides, my aim was 

 to account for that epoch from known and established facts 

 and principles without the introduction of hypothetical causes. 

 I fear that the fact of my making little or no allusion to 

 geographical conditions in my explanations may have unfor- 

 tunately led Mr. Wallace and others to conclude that I alto- 

 gether ignore, or, at least, undervalue their importance, which 

 is certainly not the case. 



Although Mr. Wallace so frequently alludes to the impor- 

 tance of geographical conditions, I am not sure if he believes 

 that during the glacial epoch those conditions differed mate- 

 rially from what they are at present, or that glaciation could 

 have been greatly influenced by any difference which did 

 exist. 



7. Mr. Wallace alludes to one or two geographical condi- 

 tions which, if they had existed during the glacial epoch, 

 would have greatly aided glaciation ; as, for example, if a 

 land-barrier had extended from the British Isles, across the 

 Faroe Islands and Iceland to Greenland, cutting off from 

 Northern Europe the warm waters of the Atlantic, including 

 the Gulf-Stream. " The result," he says, " would almost 

 certainly be that snow would accumulate on the high moun- 

 tains of Scandinavia till they became glaciated to as great an 

 extent as Greenland." 



It would be easy to multiply cases of this kind where a 

 distribution of land and water different from the present might 

 have been more favourable to glaciation than the present ; 

 but the question is, Did any such difference favouring glaci- 

 ation actually exist during the glacial epoch ? I have never 

 been able to find any evidence that it did. Many a change 

 in geographical conditions has taken place during Tertiary 

 times, some of which were doubtless favourable to glaciation ; 

 but have we any evidence that during the glacial epoch the 

 geographical conditions were more favourable than they 

 are at present ? Unless this can be shown to be the case, 

 there is no necessity for referring to a difference in geogra- 

 phical conditions during that epoch as a cause of glaciation. 

 This being so, it does not follow, because in my explanation 

 of the cause of the glacial epoch I may not, like Sir Charles 

 Lyell and others, have speculated on the effects which might 



