CLIMATIC ZOXES 25 



as Eucalyptus and Grcvillea (Proteaceae) could not adapt them- 

 selves to the rigors of a northern winter, even should they succeed 

 in reaching these regions. 1 



In both northern and southern hemispheres there is a more 

 or less extensive mingling of warm temperate and tropical 

 species near the limits of the zones, and sometimes extensive 

 migrations from one zone to the other, due to climatic or topo- 

 graphical reasons. Thus in New Zealand tropical genera may 

 extend to 50°. 



Vegetation reaches its maximum development within the 

 tropics, where optimum conditions of temperature and moisture 

 prevail. Portions of the equatorial belt, such as the great islands 

 of the Malay Archipelago, Java, Borneo, New Guinea, the rich 

 lowlands of the Amazon and the West Coast of Africa, exhibit 

 the most exuberant vegetation to be found anywhere. Uni- 

 form high temperature, heavy rainfall and rich soils, combine 

 to produce a maximum luxuriance of plant-growth, shown in 

 dense jungles of giant trees of many species, loaded down with 

 rampant creepers and epiphytic growths. Sometimes the shade 

 is too dense for much undergrowth, but wherever light pene- 

 trates, there quickly develops an inpenetrable thicket of rank 

 vegetation, and the ground is carpeted with ferns and other 

 shade-loving plants. 



It must be remembered, however, that not everywhere in 

 the tropics does one encounter such luxuriance of growth. Re- 

 gions of low rainfall, or poor soil, may consist of dreary arid 

 grass-lands, or deserts of the most pronounced type, as in the 

 Sahara and parts of Australia. 



On the other hand, in exceptional cases due to specially favor- 

 able conditions of temperature and moisture, vegetation of 

 decidedly tropical character may be found far beyond the actual 

 tropics. This is particularly the case in parts of the southern 

 hemisphere, where owing to unusually heavy rainfall, and absence 

 of extreme cold, many tropical types have extended their range 

 far beyond tropical latitudes, and the forests are very different 

 from those in corresponding latitudes in the northern hemisphere. 



Unlike the temperate regions of the northern hemisphere, 



1 For a fuller discussion of this question see Wallace, A. \\\, Island Lif> \ pp. 

 486-487. 



