252 Plant Life and Evolution 



The Tropics. With the approach to the tropics 

 the northern types of vegetation gradually disap- 

 pear, and are replaced by quite new ones. It is true 

 that such northern types as pines and oaks may 

 invade the tropics, but these are exceptional, and for 

 the most part the plants of the hotter regions of 

 the world are members of genera, and often of 

 families, not represented at all in the colder parts 

 of the world. The more intense growth conditions 

 and the fierce struggle for existence result in a 

 great diversity of plant-types adapted to all condi- 

 tions of existence. It is in the tropics that one 

 fully appreciates the possibilities of plant adaptation. 

 Every tree in a tropical jungle is a veritable botan- 

 ical garden, its trunk and branches covered with a 

 mass of epiphytic growths, and giant creepers often 

 overtop its highest branches. With this luxuriant 

 growth there has developed an almost infinite va- 

 riety of forms adapted to quite special conditions, 

 and the differences between the plant types of the 

 tropics of the Old and New Worlds are, as we have 

 seen, far greater than is the case in the temperate 

 zones. 



It is only among the older and more conserva- 

 tive types of vegetation that the same or closely 

 allied species occur. Thus while among the algae, 

 mosses, and ferns there are very many genera, or 

 even species, that are common to the tropics of both 

 hemispheres, among the flowering plants it is ex- 

 ceptional to find any genera in common, and where 



