196 ISLAIs'D LIFE 



cation between the tropical and Arctic oceans was occasion- 

 ally interrupted in one or other direction ; but if we look 

 at a globe instead of a Mercator's chart of the world, we shall 

 see that the disproportion between the extent of the polar 

 and tropical seas is so enormous that a single wide opening, 

 with an adequate impulse to carry in a considerable stream 

 of warm water, would be amply sufficient for the complete 

 abolition of polar snow and ice, when aided by the absence 

 of any great areas of high land within the polar circle, such 

 high land being, as we have seen, essential to the production 

 of perpetual snow even at the present time. 



Those who wish to understand the effect of oceanic cur- 

 rents in conveying heat to the north temperate and polar 

 regions, should study the papers of Dr. Croll already re- 

 ferred to. But the same thing is equally well shown by 

 the facts of the actual distribution of heat due to the Gulf 

 Stream. The difference between the mean annual tem- 

 peratures of the opposite coasts of Europe and America is 

 well known and has been already quoted, but the diiference 

 of their mean vjintcr temperature is still more striking, and 

 it is this which concerns us as more especially aflecting the 

 distribution of vegetable and animal life. Our mean 

 winter temperature in the west of England is the same as 

 that of the Southern United States, as well as that of 

 Shanghai in China, both about twenty degrees of latitude 

 further south ; and as we go northward the difference in- 

 creases, so that the winter climate of Nova Scotia in Lat. 

 45° is found within the Arctic circle on the coast of Norway ; 

 and if the latter countr}^ did not consist almost wholly of 

 precipitous snow-clad mountains, it would be capable of 

 supporting most of the vegetable products of the American 

 coast in the latitude of Bordeaux.^ 



of the t-wo continents during the Tertiary epoch, see my Geographical 

 Distribution of Animals, Vol. I. pp. 140-156, 



^ Professor Haughton has made an elaborate calculation of the differ- 

 ence between existing climates and those of Miocene times, for all the 

 places where a Miocene flora has been discovered, by means of the actual 

 range of corresponding species and genera of plants. Although this 

 method is open to the objection that the ranges of plants and animals are 

 not determined by temperature only, yet the results may be approxi- 

 mately correct, and are very interesting. The following table which 



