162 THE GEOGRAPHICAL DISTRIBUTION 



diles, tortoises, and serpents frequent the rivers, marshes, 

 and moist woods. The seas teem with crustaceans and 

 every order of molluscous animals. The shores are covered 

 with their shells, which, in these sunny regions, acquire the 

 most rich and variegated hues. The insects are as brilliant 

 as they are numerous. There can be no doubt whatever 

 that all the rich coloring which is spread over aniniality as 

 well as vegetation in tropical countries, is to be attributed to 

 the brightness of the sun's rays. Tropical birds, for ex- 

 ample, reared under an artificial temperature in cold coun- 

 tries, never acquire that brilliancy of plumage which dis- 

 tinguishes them in their native haunts. 



As we pass from tropical into temperate climates, the 

 heat decreases, the rays of the sun become more oblique, 

 and consequently less vivid ; in a word, all the exciting- 

 causes of vegetation gradually diminish in intensity. The 

 tall and graceful palm tree, the plantain and the banana, 

 the cotton-tree and the sugar-cane are no longer visible. 

 Vegetation is despoiled of its magnificence and variety, and 

 takes a humbler and simpler form. Accordingly we find 

 that plants with ligneous and persistent stems are fewer in 

 number, and that there is a greater predominance of such 

 as are herbaceous, and which therefore perish annually. 

 * Plants with herbaceous stems have precisely the same 

 growth, as far as it goes, as those which are ligneous and 

 persistent. Any one can speedily convince himself of this. 

 There is visible on the cross section the same concentrical 

 disposition of the matter of the stem into pith, wood and 

 bark, and the same development of branches in the axils of 

 the leaves. But the heat is not spread through a sufficient 

 number of months, and the period is too short for the plant 

 to run through all the phases of its development. The whole 

 process is therefore stopped in its first stages, and the stem 



