26 



thereby to cause vapour which had been evaporated near the 

 equator to be condensed about the poles. 



We have still to consider the possible effect on climate of 

 changes in the distribution of land and ocean. Sir Charles 

 Lyell appeared to think that a warm climate over the whole 

 earth might be due to a preponderance of land near the equator 

 and of sea about the poles ; and that the reverse arrangement 

 would account for a glacial climate. It is quite true that a 

 polar continent, surrounded by a vast ocean, would be emi- 

 nently favorable to glaciation. We see this in the Antarctic 

 regions now. But it does not appear to be true that the 

 substitution of land for sea about the equator would have any 

 tendency to raise the polar temperatures, but rather the reverse, 

 because in that case there would be less evaporation about the 

 equator, and consequently less condensation about the poles, 

 and less liberation of latent heat. 



' Mr. Wallace, in his Island Life, has lately maintained that 

 the high polar temperatures of the pre-glacial periods may be 

 explained by supposing that there was formerly a more open 

 and free communication than at present between the tropical 

 and the polar seas ; — that, as he puts it, there were several Gulf 

 Streams instead of one passing comparatively warm water into 

 the polar seas. This might be satisfactory if we had to do with 

 mean temperatures or winter temperatures ; but I have endea- 

 voured to show that glaciation is almost exclusively determined 

 by the summer temperature, and we know that the distribution 

 of living beings is mainly determined by the same ; — whereas 

 an examination of Dove's map, showing the isothermal lines 

 for January and July, will make it evident that the effect of 

 the Gulf Stream on climate, though very considerable, is chiefly 

 confined to the winter. The late Mr. Hopkins, of Cambridge, in 

 his well-known paper on geological climates,* estimated the 

 effect of the Gulf Stream on the July climate of London as null. 



It would however probably make a sensible improvement in 

 the summer climate of the Polar basin if the slope of the Asiatic 



* Journal of the Geological Society, 185 1. 



