462 GEOGRAPHICAL DISTRIBUTION [Chap. VI 



Letter 352 To my sorrow my old reasons for rejecting the latter alter- 

 native seem to me sufficient, and I should very much like 

 to know what you think. Let us suppose that the cold 

 affected the two Americas either before or after the Old 

 World. Let it advance first either from north or south till 

 the Tropics became slightly cooled, and a few temperate 

 forms reached the Silla of Caracas and the mountains of 

 Brazil. You would say, I suppose, that nearly all the 

 tropical productions would be killed ; and that subsequently, 

 after the cold had moderated, tropical plants immigrated 

 from the other non-chilled parts of the world. But this is 

 impossible unless you bridge over the tropical parts of the 

 Atlantic — a doctrine which you know I cannot admit, though 

 in some respects wishing I could. Oswald Heer would make 

 nothing of such a bridge. When the Glacial period affected 

 the Old World, would it not be rather rash to suppose that 

 the meridian of India, the Malay Archipelago, and Australia 

 were refrigerated, and Africa not refrigerated ? But let us 

 grant that this was so ; let us bridge over the Red Sea 

 (though rather opposed to the former almost certain com- 

 munication between the Red Sea and the Mediterranean) ; 

 let us grant that Arabia and Persia were damp and fit for 

 the passage of tropical plants : nevertheless, just look at 

 the globe and fancy the cold slowly coming on, and the 

 plants under the tropics travelling towards the equator, and 

 it seems to me highly improbable that they could escape 

 from India to the still hot regions of Africa, for they would 

 have to go westward with a little northing round the northern 

 shores of the Indian Ocean. So if Africa were refrigerated 

 first, there would be considerable difficulty in the tropical 

 productions of Africa escaping into the still hot regions of 

 India. Here again you would have to bridge over the Indian 

 Ocean within so very recent a period, and not in the line 

 of the Laccadive Archipelago. If you suppose the cold to 

 travel from the southern pole northwards, it will not help 

 us, unless we suppose that the countries immediately north 

 of the northern tropic were at the same time warmer, so 

 as to allow free passage from India to Africa, which seems 

 to me too complex and unsupported an hypothesis to admit. 

 Therefore I cannot see that the supposition of different 

 longitudinal belts of the world being cooled at different 



