Vol. 57.] CLIMATE OF THE PLEISTOCENE EPOCH. 423 



—51° Fahr.^ It is, therefore, the winter conditions only of thiS' 

 region which now prevent the northward extension of the forest- 

 zone.^ 



The January isotherms of the Northern Hemisphere (fig. 6, p. 418)' 

 are very irregular, that of 0° Fahr. reaching in Mongolia as far south 

 as lat. 45° N. (the latitude of Turin), and towards the north, 

 extending nearly to Spitsbergen. 



As before urged (p. 420), it seems improbable that during the 

 Pleistocene Epoch the winter isotherms were more nearly in accordance 

 with the parallels of latitude than they are now, the climate of the 

 Northern Hemisphere being then milder and more uniform. It is 

 still less probable that the distribution of atmospheric pressure was 

 similar to that of the present day, but with a generally increased, 

 temperature.^ The more reasonable hypothesis seems to be that 

 comparative heat and cold were at that time local, as they are at 

 present, but that the winter climate of Northern Siberia was then 

 abnormally mild, as now it is abnormally cold. 



Although the region extending from North -eastern Siberia to 

 Alaska has been the scene of much tectonic disturbance, and great 

 changes of level have taken place there at a comparatively recent 

 period, it does not appear that Behring Strait was more open daring 

 the period in question than at present. On the contrary, remains of 

 the mammoth occur, not only on both sides of the strait, but, as 

 just stated, in the archipelagos also, and such an uprise as would 

 join the latter to the continent, would unite Asia and America, and 

 close the strait altogether. 



The comparatively mild climate of these regions during Pleistocene 

 times could not, therefore, have been due to the introduction of 

 currents of warm water from the Pacific into the Polar basin. So 

 we must fall back on the explanation which in the cases before 

 discussed presents the least difiiculty, that the arrangement of the 

 areas of high and low pressure, and the consequent direction of the 

 prevalent winds, were then difi'erent from those now obtaining. 



The severity of the climate of the Liakof Islands and the North 

 Siberian coast during winter is largely due to the southerly and 

 south-westerly winds from the frozen wastes of Central Asia which 

 then prevail there (fig. 4, p. 414). The winter climate of Alaska is 

 also severe, but from a diflPerent cause; in the latter country northerly 

 winds from the Polar Sea then prevail. The Mongolian and 

 Manchurian highlands must, I think, have been at least as cold in 

 winter during the Pleistocene Epoch as they are now, if not colder, 

 so that trees could never have flourished in Northern Siberia while 

 the winter winds blew for the most part from a southern or south- 



^ Prof. James Geikie states that a minimum temperature of — 9i°"6 Fahr. 

 has been registered in this region ; see ' Grreat Ice Age' 3rd ed. (1894) p. 703. 



^ Sir Henry Howorth has expressed the opinion that Siberia enjoyed a more 

 equable as well as a temperate climate during the Mammoth Period, Geol. Mag. 

 1881, p. 256. 



^ At present there is a difference of more than 80° Fahr. between the average 

 annual temperature of Great Britain and that of the Liakof Islands. 



