FLORISTIC REGIONS 105 



and spines for foliage, and the chevaux-de-frise of pointed 

 fleshy leaves of the Agaves of this region on the one 

 hand, and the Euphorbias and Aloes of Africa on the 

 other, is the classical example of " representation," 

 when that somewhat overweighted term is used in the 

 sense of mere parallelism of adaptation. This is, of 

 course, quite a different thing from the representation 

 of one species in one area by an allied one in another, 

 which may mean descent from a common ancestor and 

 modification following isolation. 



Between the three great meridional or continental 

 divisions of the humid Tropical Zone, the African, Indo- 

 Malayan, and American, there is little, if any, of that 

 affinity, suggestive of recent migrations, that we have 

 seen among the divisions of the Northern Zone. For 

 example, though Palms and Orchids occur in all three 

 divisions, there is no genus of Palms and there are very 

 few genera of Orchids common to the Old and New 

 Worlds. 



In no part of the zone is vegetation liable to be 

 checked by cold ; but with regard to precipitation there 

 are two main conditions : (i. ) the almost perennial rainfall 

 of the equatorial rain-forest, with heavier falls at the 

 two seasons when the sun is vertically overhead; and 

 (ii.) the existence of a dry season of three months or 

 more, producing the " savannah " and " caatinga " 

 formations, with distinctly xerophytic vegetation. This 

 latter condition occurs markedly in Eastern Equatorial 

 Africa, Western Australia, Venezuela, and Southern 

 Brazil. It is characterised by tall grasses; scattered 

 trees, some of which are deciduous, while others may 

 have swollen barrel-like stems; numerous large Euphor- 

 bias or Cacti; some Acacias, and few lianes or epiphytes. 



TROPICAL RAIN-FOREST. In the Tropical Rain-forest, 

 as we have it in Western Equatorial Africa, Southern 

 Ceylon, Southern India, the Malay Peninsula and 

 Archipelago, North Australia, and the Amazon valley, 

 the Vegetable Kingdom is seen in its greatest richness of 

 variety and luxuriance of growth and energy. A large 

 proportion of the species are arborescent, and the trees 

 forming the forests are of many species, rarely social, so 

 that no one is dominant, and have mostly straight stems, 

 reaching a height of 50-100 feet or more, with their 

 * ranches and evergreen foliage spreading in a close 



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