42 CAUSES WHICH FAVOUR OR [pt. i 



ing less than 10 inches of rain at Batticaloa on the east coast. 

 The change at the summit-level is so sudden that one may some- 

 times find a wet climate at one end, and a dry and sunny one at 

 the other end, of the short summit tunnel on the railway. The 

 position is largely reversed during the other monsoon, so that 

 very many species can grow on both sides, though usually with 

 different periodicity, Para rubber, for example, ripening its 

 seeds on one side of the mountains in February, on the other 

 in August. 



In South India the chain of the Western Ghats causes a heavy 

 fall of rain in the south-west monsoon on the western side, while 

 the north-east monsoon is comparatively dry, so that there is 

 a great difference in the climate of the two sides, and many 

 species are confined to one or the other. This contrast in climate 

 and vegetation between lee and weather*sides is also well shown 

 in the trade belts in the tropical Pacific islands, large and small, 

 and is very marked in the Andes, in the section from 10° to SO"" 

 south of the equator. The wind striking them is usually the 

 easterly trade wind, and their western side is almost completely 

 dry. Farther south the eastern side is comparatively dry, be- 

 cause of the westerly winds from the Pacific Ocean. Chains 

 that run north and south are of greater importance in this con- 

 nection than chains that run cast and west, regarded simply 

 as mountain chains causing differences in rainfall, for the question 

 is less complicated Avith change of temperature following lati- 

 tude. But from the general historical point of view of geo- 

 graphical distribution, the east and west chains, by forming 

 barriers to the plants spreading south or north with the ad- 

 vancing or retreating cold of a glacial period, have been, in all 

 probability, of enormously greater importance than the chains 

 that run north and south. An immense number of species, and 

 even genera, have probably perished against the chain of moun- 

 tains that runs east and west with few gaps from Spain to 

 eastern Asia. 



The effect of the drier climate on one side of a chain of moun- 

 tains M'ill generally be to encoiu-age a more herbaceous type of 

 vegetation. So long as there is a reasonable amount of rainfall, 

 not too much concentrated into one period of the year, the usual 

 type of covering of the soil, in countries that have not been dis- 

 turbed by ice periods, or by man, is forest. But below a certain 

 amount of rain, forest does not seem readily to survive, nor to 

 occupy new ground, even if it survive upon ground that was 



