CHAP. xvm. OF COLDER AND MILDER CLIMATE. 367 



zones N. and S. of the equator have been referred by Mr. 

 Darwin and Dr. Hooker to migrations, which took place along 

 mountain chains running from N. to S. during some of the 

 colder phases of the glacial epoch.* Such an hypothesis 

 enables us to dispense with the doctrine that the same species 

 ever originated independently in twodistinct and distant are as ; 

 and it becomes more feasible if we admit the doctrine of the 

 co-existence of meridional belts of warmer and colder climate, 

 instead of the simultaneous prevalence of extreme cold both 

 in the eastern and western hemisphere. It also seems neces 

 sary, as colder currents of water always flow to lower lati 

 tudes, while warmer ones are running towards polar regions, 

 that some such compensation should take place, and that 

 an increase of cold in one region must to a certain extent 

 be balanced by a mitigation of temperature elsewhere. 



Sir John F. Herschel, in his recent work on e Physical Geo 

 graphy,' when speaking of the open sea which is caused in 

 part of the polar regions by the escape of ice through Behring's 

 Straits, and the flow of warmer water northwards through the 

 same channel, observes that these straits, by which the conti 

 nents of Asia and North America are now parted, ' are ooly 

 thirty miles broad where narrowest, and only twenty-five 

 fathoms in their greatest depth.' But ' this narrow channel,' 

 he adds, 'is yet important in the economy of nature, inasmuch 

 as it allows a portion of the circulating water from a warmer 

 region to find its way into the polar basin, aiding thereby not 

 only to mitigate the extreme rigour of the polar cold, but to 

 prevent in all probability a continual accretion of ice, which 

 else might rise to a mountainous height.' f 



Behring's Straits, here alluded to, happen to agree singularly 

 in width and depth with the Straits of Dover, the difference 



* Darwin, Origin of Species, ch. xi. p. 365 ; Hooker, Flora of Australia, 

 Introduction, p. 18. 



f Herschel' s Physical Geography, p. 41, 1861. 



