394 TROPICAL NATURE v 
zones ; while, estimated potentially, in proportion to their life- 
sustaining power, the lands which enjoy a practically tropical 
climate (extending as they do considerably beyond the geo- 
graphical tropics) are very much larger than the temperate 
regions of the earth. 
Combining the effects of all these various causes, we are 
quite able to understand the superiority of the tropical parts 
of the globe, not only in the abundance and variety of their 
forms of life, but also as regards the ornamental appendages 
and vivid coloration which these forms present. 
