276 Dr. J. Croll on the Cause of 



either as to the probability or to the improbability of the 

 existence of such channels as are assumed by Mr. Wallace. 

 But, looking at the question from a physical point of view, it 

 seems to me pretty evident that if such channels as he sup- 

 poses existed, allowing of a continuous flow of equatorial 

 water into the Arctic seas, it would certainly prevent the 

 formation of permanent ice around the pole, and would doubt- 

 less confer on the arctic regions a mild and equable climate. 

 This would be more particularly the case if, as Mr. Wallace 

 supposes, owing to geographical conditions, far more of the 

 equatorial water was deflected into the Arctic than into the 

 Antarctic regions. 



But at the same time I think it is just as evident that 

 these channels would not neutralize the effects resulting from 

 a high state of eccentricity. It may be quite true that the 

 physical cause brought into operation during a high state of 

 eccentricity might not be sufficient to reduce the quantity of 

 warm water flowing into the Arctic Ocean to an extent that 

 would permit of the formation of permanent ice around 

 the pole, but it certainly would greatly diminish the flow 

 into the Arctic Ocean. Supposing that at the commence- 

 ment of the last glacial epoch the volume of the Gulf-stream 

 was double what it is at present; this condition of things 

 would not have prevented the operation of those physical 

 agents which brought about the glacial epoch, although it, 

 no doubt, would have considerably modified the severity of 

 the glaciation resulting from their operation. The very 

 same thing would hold true, though perhaps in a much 

 greater degree, in reference to the channels assumed by Mr. 

 Wallace. 



If the emissive pow T er of the sun was about the same 

 during the Tertiary period as at present, and there is no 

 good grounds for supposing it was otherwise, then the 

 extra heat possessed by the northern temperate and arctic 

 regions must have been derived either from the equatorial 

 regions or from the southern hemisphere, or, what is more 

 likely, from both. If so, then the temperature either of the 

 southern hemisphere or of ihe intertropical regions, or both, 

 must have been much lower during the Tertiary period than 

 at the present day. A lowering of the temperature of the 

 equatorial regions, resulting from this transference of heat, 

 would tend to produce a more equable and uniform condition 

 of climate over the whole of the northern hemisphere. As the 

 area of the Arctic Ocean is small in comparison to that of the 

 equatorial zone, from which the warm water was derived, the 

 fall of temperature at the equator would be much less than the 



