06 TROPIGxVL NATURE, AND OTHER ESSAYS. 



general in tlie forests of extra-tropical countries, is 

 probably dependent on tlie extreme equability and per- 

 manence of tlie climate. Atmospheric conditions are 

 mucli more important to the growth of plants than any 

 others. Their severest struggle for existence is against 

 climate. As we approach towards regions of polar cold 

 or desert aridity the variety of groups and species regu- 

 larly diminishes ; more and more are unable to sustain 

 the extreme climatal conditions, till at last we find only 

 a few specially organized forms which are able to 

 maintain their existence. In the extreme north, pine or 

 birch trees ; in the desert, a few palms and prickly shrubs 

 or aromatic herbs alone survive. In the equable equa- 

 torial zone there is no such struggle against climate. 

 Every form of vegetation has become alike adapted to 

 its genial heat and ample moisture, which has probably 

 changed little even throughout geological periods ; and 

 the never-ceasing struggle for existence between the 

 various sjjecies in the same area has resulted in a nice 

 balance of organic forces, which gives the advantage, 

 now to one, now to another, species, and prevents any 

 one type of vegetation from monopolising territory to 

 the exclusion of the rest. The same general causes have 

 led to the filling up of every place in nature with some 

 specially adapted form. Thus we find a forest of smaller 

 trees adapted to grow in the shade of greater trees. 

 Thus we find every tree supporting numerous other forms 

 of vegetation, and some so crowded with epiphytes of 

 various kinds that their forks and horizontal branches 

 are veritable gardens. Creeping ferns and arums run 

 up the smoothest trunks ; an immense variety of climbers 

 hang in tangled masses from the branches and mount over 



