CH. V] HINDER THE DISPERSAL OF SPECIES 43 



forest in days of greater moisture. The general tendency, there- 

 tore, of the change of climate brought about by a chain of 

 mountains transverse to the prevailing damp ^nnd, is to en- 

 courage the growth upon the lee side of herbaceous and shrubby 

 plants which can stand greater extremes of drought and to 

 make It very difficult, if not impossible, for the forest species 

 whether trees or undergro^vth-herbs and shrubs, to travel into 

 the drier country. A complete barrier may thus be offered to 

 the passage of some species, while others, that would have been 

 quite unable to pass the level forest, may be enabled to pass 

 easily by the development of a mountain chain at a later period 

 like the development of the Andes in Cretaceous times. 



Distribution of rainfall and moisture of the air is of even 

 greater importance to a plant than total rainfall. The largest 

 rainfall in the world is at Waialeale, in the mountains of "the 

 Hawaiian Islands; it is also well distributed throughout the 

 year so that the place is always wet, with no dry season at all 

 As the result, it has a flora of a very moisture-loving kind 

 Uierrapunji, in Assam, which has almost as great a rainfall, but 

 badly distributed through the year (April 29 inches, Mav 50 

 inches, June 110 inches, July 120 inches, August 78 inches 

 September 57 inches, October 13 inches, and the other five 

 months only 14 inches amongst them), does not show this, but 

 has a vegetation which almost suggests a dry climate. 



Kandy in Ceylon has a very steady mean temperature just 

 over 75° F., and a rainfall Avell distributed through the year (the 

 twelve months have approximately 5, 2, 3; 7, 6. 9; 7, 6, 6; 11 

 10, and 9 inches, total about 82), and though there is a ''dry 

 season" in February and March, the flora is distinctly forest of 

 the ordinary rain-forest type. In the dry zone of northern and 

 eastern Ceylon lies Anuradhapura, with a total rainfall of 

 55 inches, distributed mainly in the north-east monsoon from 

 October to April (3, 1, 2; 7, 3, 1; 1, 2, 3; 8, 10, 9). In a hot 

 climate like Ceylon, a fall of less than 4 inches in a month is 

 practically negligible, so that there is really a long drought from 

 January to September, broken only by the April rains, and the 

 flora is of the dry-forest type, with comparati^'ely few species 

 in common with Kandy, only about 90 miles away. Calcutta, 

 on the edge of the tropics, with a hot sun, and a rainfall of 

 66 inches, is equally a "dry" chmate (rain 0-4, 1, 1-3; 2-3, oQ, 

 11-8; 13, 13-9, 10; 5-4, 0-6, 0-3). Going to the other and damper 

 hemisphere, at Rio de Janeiro, also on the edge of the tropics. 



