DISTRIBUTION AND EVOLUTION 107 



plants, especially mosses and hepaticse, that cover the whole 

 surfaces of the leaves of forest trees with an exquisite tracery, 

 thus obtaining the perpetual moisture they require from con- 

 densation on the cool surfaces of the leaves. 



in great river valleys, where by the annual rising of the 

 stream miles of alluvial plains are regularly under water for 

 several months, both trees and shrubs have become adapted to 

 these strange conditions, and the greater part, if not all, the 

 species are quite distinct from those which grow on the un- 

 flooded land. 



All these, and many other characteristic features of tropical 

 vegetation, can be explained by the general constancy of the 

 inorganic conditions, especially the climatic ones, which have 

 undoubtedly prevailed there during whole geological periods, 

 subject only to those very slow changes due to elevation, de- 

 pression, and denudation of the land itself. These latter have 

 been so extremely gradual as to act as a gentle stimulus to the 

 various agencies continually bringing about modification of 

 specific form? ; and as the climatal conditions throughout all 

 these changes have continued to be highly favourable to the 

 support of vegetation and of animal life, there has been a con- 

 stant tendency to produce and maintain an almost exact 

 equilibrium between the various species in the same area. 

 None being better adapted to the environment than a great 

 many others, none are able to monopolise large areas to the 

 exclusion of others, as is the case in the more changeable tem- 

 perate or cold regions. Whether we consider the differences 

 between day and night temperatures, the variations of tem- 

 perature from month to month or from year to year, or those 

 extreme variations which we experience once perhaps in a gen- 

 eration or in a century, such as excessively cold winters, ex- 

 cessive droughts or excessive rains in summer, or long periods 

 of dry and cold winds — all alike are unkno\\Ti in the equa- 

 torial regions, save in a few^ limited and quite exceptional areas. 

 In these more favoured portions of our earth there prevails 

 such a general approach to uniformity of conditions (without 



