44 CAUSES WHICH FAVOUR OR [pt. i 



one finds a place with only 40 inches of rainfall (5, 4, 5; 4, 4, 2; 

 2, 2, 2; 3, 4, 5) which shows as much of the character of rain- 

 forest and a wet climate as does Kandy with 82 inches. Evi- 

 dently the distribution of the rainfall, and of the humidity of 

 the air, which largely goes with it, is of much greater importance 

 than the actual total. Rio, with 40 inches, is better suited to 

 plants needing a moist climate than Cherrapunji with 470; its 

 svm is not so hot as that of tropical Asia, and its season of less 

 rainfall coincides with the weaker sim of June-September. 



Change of distribution of rainfall, if at all sudden, usually 

 coincides with the presence of a mountain chain. The presence 

 of the mountains may alter the periodicity of the rain, as in 

 Ceylon (abo^•e), when the only plants that can cross the boundary 

 will be those that can alter their periodicity; or it may completely 

 alter the rainfall, as in the case of the Andes, where the flora is 

 very markedly different on the two sides. Gradual change, on 

 the other hand, will usually accompany gradual change of rain- 

 fall. Change of dampness of air, again, if permanent between 

 one place and another, will involve differences in the plants in 

 their reactions to moistiu-e, and some will be more drought- 

 resistant than others. 



Change of temperature is usually of a more permanent, or 

 regularly recurring nature, especially in the tropics. At Colombo 

 in Ceylon, for example, the maximum is usually about 88° F., 

 the minimum about 75°, all the year round, except for a small 

 increase from PVbruary to May. At Rio, on the edge of the 

 tropics, there is more range, from say 98° absolute maximum 

 in summer to 52° absolute minimum in winter, and at Nuwara 

 Eliya (ele^'ation 6000 feet) in Ceylon the absolute maxima and 

 minima are about 81° and 28°, with much greater daily ranges 

 in dry than in wet weather. The farther one goes from the 

 equator, or the higher in the moimtains, the greater the range 

 on the whole, whether annual or diurnal, and the range is 

 also greater the farther one goes inland from the sea. The 

 extreme variation of all is reached b}- going both north and 

 inland, to the centre of northern Siberia, where it may touch 

 80° in summer, and — 60° in winter. 



More rapid change of temperature is experienced in ascending 

 a mountain, the mean falling about 3-4° F. for every 1000 feet 

 of ascent. Correlated with this is the rapid change of the com- 

 position of the flora, as compared with the change experienced 

 in going north or south at the same level and under the same 



