vi DISTRIBUTION AND EVOLUTION 99 



or the small quantity of decaying leaves or moss that collects 

 there such plants belong to many natural orders and are 

 very numerous. Then there are the climbers, far more 

 abundant than in any temperate forests, which either root 

 in the ground and then, by various means, climb up to 

 the summits of the loftiest trees, or which begin life by 

 rooting in a lofty fork of a great tree, and then send down 

 roots to the ground and branches into the air, sometimes 

 remaining as a small bush or tree, at others growing so 

 rapidly above, and clinging around the supporting tree so 

 closely with its roots, as finally to kill its foster-parent, when 

 its clinging roots unite and grow into a trunk, with hardly 

 anything to show that one tree has replaced another. Then 

 again there are numerous small trees of from 20 to 30 

 feet high, which live entirely in the shade beneath the great 

 forest trees. Many of these have bright-coloured or con- 

 spicuous flowers growing directly out of the trunk, while 

 there are none at all among the crown of leaves at the top. 

 This appears to be an adaptation to bring the flowers within 

 sight of the butterflies, bees, and other insects which fly near 

 the ground, and thus secure for them the advantages of being 

 cross-fertilised. Then again there are many delicate creeping 

 plants, especially mosses and hepaticae, 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 

 unflooded 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, depression, 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 



