MODIFICATION OF THEORY EXAMINED. 99 



present might have been more favourable to glaciation 

 than the present; but the question is, Did any such 

 difference favouring glaciation 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 differ- 

 ence in geographical conditions during that epoch as a 

 cause of glaciation. This beino* 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 have 

 resulted had the distribution of land and water been 

 different from what it is now, that I ought on this 

 account to be charged with undervaluing the import- 

 ance of geographical conditions. 



Trusting that these preliminary considerations may 

 tend to remove the partial confusion in which this 

 somewhat complex subject has been involved, I shall 

 now proceed to examine Mr. Wallace's main argument. 

 I shall consider it, first, in relation to physical prin- 

 ciples, and, secondly, in relation to geological and 

 palseontological facts. 



