CHAP, xviir. 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 two distinct and distant 

 areas; and it becomes more feasible if we admit the doctrine 

 of the coexistence of meridional belts of warmer and colder 

 climate, instead of the simultaneous prevalence of extreme 

 cold both in the eastei'n and western hemisphei-e. It also 

 seems necessary, as colder currents of water always flow to 

 lower latitudes, 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 "Physical Geo- 

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

 l^art of the polar regions by the escape of ice through Bchring'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 only 

 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 rigor of the polar cold, bat 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, haj)pen to agree singu- 



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

 duction, p. 18. 



t Herschel's Physical Geography, p. 41, 1861. 



