1875.] 333 [Staler. 



North Atlantic, at various stages of the earth's history, from the time 

 of the carboniferous beds down to the later tertiaries, we have the 

 most unquestionable evidence of the occurrence, from time to time, 

 of periods of sufficient heat to permit the development of extensive 

 forests or of marine life that attests a similar warmth. 



When facts of this general nature were first observed, it was the 

 fashion to account for them by the hypothesis that the polar axis was 

 subject to great changes of position ; thereby quite altering the rela- 

 tions of the regions which lie within the Arctic Circle. The impossi- 

 bility of this view having been sufficiently demonstrated on physical 

 grounds, and the hypothesis definitely abandoned by naturalists, it 

 remains for us to consider what other changes can take place cal- 

 culated in their nature to bring up the circumpolar temperature. 



A simple hypothesis would be that the temperature of the earth's 

 surface varying with the temperature of the sun, it might be possible 

 that its whole surface should be so elevated that the temperature of 

 the circumpolar regions would be of the degree of warmth we now find 

 only in tropical or sub-tropical regions. This, however, is unsatis- 

 factory, inasmuch as it would require an increase of the heat of the 

 regions near the equator to about double its present average. Any 

 such temperature would necessarily bring about the destruction of 

 all tropical life. This is by no means an improbable objection to this 

 view. There is, however, reason to believe that the evaporation con- 

 sequent on any great elevation of temperature in tropical regions would 

 bring about an enormous deposition of moisture about the poles, and 

 lead to glacial conditions by producing a vast precipitation of snow. 



There is another and far more satisfactory hypothesis which does 

 less violence to the order of nature, and at the same time gives us the 

 required result. We may suppose that the ocean currents, those 

 mightiest agents in the surface machinery of the earth, may from 

 various causes carry at one time far greater quantities of heat toward 

 the poles than at other times. Mr. James Croll has pointed out the 

 very great value of the Gulf Stream in carrying heat to within the 

 Arctic Circle. If we could double the amount of this heat brought 

 within the Arctic Circle by the ocean currents, we would, beyond 

 doubt, produce a change of climate great enough to account for all 

 the facts. This it seems to me we may do by a very simple and 

 natural process. Of the two great northern branches of the equato- 

 rial current, the Gulf Stream and the Japan current, but one now 

 carries its full effect to the region within the Arctic Circle; the 



