114 THE ADVANCE OF SCIENCE 



tions of the same region have been widely 

 different, and seeks the explanation of 

 this important truth from the sister sci- 

 ences. The facts that, in the middle of 

 the Tertiary epoch, evergreen trees 

 abounded within the arctic circle ; and 

 that, in the long subsequent Quaternary 

 epoch, an arctic climate, with its accom- 

 paniment of gigantic glaciers, obtained 

 in the northern hemisphere, as far south 

 as Switzerland and Central France, are 

 as well established as any truths of sci- 

 ence. But, whether the explanation of 

 these extreme variations in the mean tem- 

 perature of a great part of the northern 

 hemisphere is to be sought in the con- 

 comitant changes in the distribution of 

 land and water surfaces of which geology 

 afll^ds evidence, or in astronomical con- 

 ditions, such as those to which I have 

 referred, is a question which must await 

 its answer from the science of the fu- 

 ture. 



