Vol. 57.] CLIMATE OP THE PLEISTOCEXE EPOCH. 421 



south-westerly winds, ^ associated with the presence of lower 

 pressure to the north. The Pleistocene savage of that region, 

 sheltering himself behind a rock or a sandhill from the tropical 

 rain, with his back to the west, must always have had, to use 

 the formula quoted on p. 405, the higher barometric pressure on 

 his right, and the lower on his left hand. 



It is not difficult to understand how this state of things might 

 have arisen. At present, when a high-pressure system lies over the 

 North of Europe, the barometer is often low in the Mediterranean 

 area, cyclones and anticyclones being necessarily complementary, one 

 to the other, as are the troughs and crests of the waves of the sea. 

 During the Pleistocene Epoch also, high-pressure conditions, more 

 or less permanent, over the European ice-sheet, would have been 

 attended by lower pressure over the warmer regions to the south of 

 it, but the cyclonic centres would have sometimes reached farther 

 south than at present. Moreover, I think, for reasons assigned later 

 on, that during the period of maximum glaciation in Europe," the 

 Gulf Stream must have been excluded from the Arctic Ocean, 

 possibly by the elevation of the submarine ridge which exists 

 between Greenland and Scandinavia, or by its being blocked with 

 ice, causing the Polar Sea to the north of it to be more or less 

 permanently frozen over, at any rate in winter. If this were so, 

 high-pressure conditions might have been then more or less prevalent 

 from Davis Strait to the Ural Mountains. The influence of an 

 anticyclone of such importance in the north must have tended to 

 drive southward the low-pressure area of the !Xorth Atlantic, and 

 with it the anticyclone now lying at all seasons of the year off the 

 western coast of Morocco, the position of which prevents winds from 

 the ocean from blowing over the Sahara (figs. 4 & 5, pp. 414 & 415). 

 I have given hypothetically in fig. 21 (p. 458) the meteorological 

 conditions under which moist winds from the ocean might have 

 caused a humid climate in the Saharan region. 



It is not necessary to suppose that during the Pleistocene Epoch 

 the low-pressure system of the Mediterranean region always 

 occupied the same actual position. It would not only have shifted 

 from day to day as cyclones do at present, but seasonally also, now 

 to the north, and again to the south, in accordance with changes 

 in the situation of the anticyclone of the European ice-sheet, but 

 the latter, I think, must have been prevalently an area of higher 

 pressure than that of the warmer regions immediately south 

 or west of it. At present the Greenlandic anticyclone sometimes 

 extends far southward, and at other times it recedes, changing 

 for the better or the worse the temporary climate of the Atlantic 

 and of the British Isles. Similar changes must have occurred 



^ No moisture reaches the Sahara at present from the Mediterranean, as it 

 is intercepted by the Atlas Mountains, and this must also have been the case 

 during the Pleistocene Epoch. 



^ I shall state my reasons farther on (p. 435) for thinking that the maxim una 

 glaciation of North America may not have been coincident Avith that of 

 Europe. 



