MODIFICATION OF THEORY EXAMINED. 101 



summer, to the reverse, could bring about any important 

 alteration — the physical and geographical causes of glaciation 

 remaining unchanged? For, certainly, the less powerful 

 sun of summer, even though lasting somewhat longer, could 

 not do more than the much more powerful sun did during 

 the phase of summer in perihelion, while during the less 

 severe winters the sun would have far less power than when 

 it was equally near and at a very much greater altitude in 

 summer. It seems to me, therefore, quite certain that when- 

 ever extreme glaciation has been brought about by high 

 eccentricity combined with favourable geographical and 

 physical causes (and without this combination it is doubtful 

 whether extreme glaciation would ever occur), then the ice 

 sheet will not be removed during the alternate phases of 

 precession, so long as these geographical and physical causes 

 remain unaltered. It is true that the warm and cold oceanic 

 currents, which are the most important agents in increasing 

 or diminishing glaciation, depend for their strength and 

 efficiency upon the comparative extents of the northern and 

 southern ice-sheets, but these ice-sheets cannot, I believe, 

 increase or diminish to any important extent unless some 

 geographical or physical change first occurs." * 



Again, — " It is quite evident that during the height of 

 the glacial epoch there was a combination of causes at work 

 which led to a large portion of North-Western Europe and 

 Eastern America being buried in ice to a greater extent 

 even than Greenland. Among these causes we must reckon 

 a diminution of the force of the Gulf Stream, or its being- 

 diverted from the north-western coasts of Europe ; and what 

 we have to consider is, whether the alteration from a long- 

 cold winter and short hot summer, to a short mild winter 

 and long cool summer would greatly affect the amount of ice 

 if the ocean-currents remained the same. The force of these 

 currents is, it is true, by our hypothesis modified by the 

 increase or diminution of the ice in the two hemispheres 

 alternately, and they then react upon climate ; but they 



* "Island Life," p. 150. 



