94 Dr. J. Croll on the Physical Theory 



I. Physics in relation to Mr. Wallace- } s Modification of the 



Theory. 



The grand modification, that during the height of the glacial 

 epoch the snow and ice would not disappear when precession 

 brought the winter solstice round to perihelion, I have already 

 given in Mr. Wallace's own words. As the reasons which he 

 assigns for this modification are very briefly stated by him, I 

 may here give them also in his words. 



After describing the state of North-eastern America and the 

 North Atlantic, to which I have already alluded, he says : — 



" But when such was the state of the North Atlantic (and, how- 

 ever caused, such must have been its state during the height of the 

 glacial epoch), can we suppose that the mere change from the 

 distant sun in winter and near sun in summer, to the reverse, 

 could bring about any important alteration — the physical and geo- 

 graphical 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 whenever extreme glaci- 

 ation has been brought about by high eccentricity combined with 

 favourable geographical and physical causes (and without this com- 

 bination 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 cur- 

 rents, which are the most important agents in increasing or dimi- 

 nishing 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/'— P. 150. 



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 cannot be thus changed 



