220 TROPICAL NATURE, AND OTHER ESSAYS. 



constant, and where migration was unnecessary ; while 

 whatever direct effect may be produced by light or heat, 

 will necessarily have acted more powerfully within the 

 tropics. And lastly, all these causes have been in action 

 over an actually greater area in tropical than in tem- 

 perate zones ; while estimated potentially, in proportion 

 to its life-sustaining power, the lands which enjoy a 

 practically tropical climate (extending as they do con- 

 siderably beyond the geographical 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. 



