A VAST FIELD OF STUDY. 283 



continent. As far as the Desert World is concerned, in both hemi- 

 spheres the legions are innumerable, and their energies commensurate 

 to the greatness of the continual work of destruction and purification 

 which they seem destined to accomplish in all tropical countries. 



It is unnecessary to carry any further the parallel between the 

 two hemispheres. We shall more clearly detect their analogies and 

 differences by pursuing the study, already opened up in the Steppes 

 and Seas of Sand, of the principal species proper to the various forms 

 of the Desert, the different regions and divisions of the Savage 

 World. 



Yet I must confess that the difficulties of the study increase with 

 the extent of the field we are called upon to explore. The Steppes and 

 Wildernesses of Sand constitute, both in Africa and Asia, regions which 

 are clearly defined, and the poverty both of their fauna and their flora 

 fixes a definite limit to the researches of the naturalist. Such is 

 not the case in the immense countries which now lie before us. 

 Instead of sighing, like Alexander, for more worlds to conquer, the 

 student of science is ever deploring the impossibility of exhausting 

 even a single division of the grand work before him. "Art is long ; 

 life is short." The most industrious among us can never rise to the 

 full height of his glorious task ; must always remain like a child on 

 the shore of the ocean of truth, and be content with the few shells his 

 nerveless hand contrives to gather. In the wide regions we are about 

 to traverse we feel at every step the colossal character of the enter- 

 prise. Every instant their aspect changes ; Nature never repeats 

 herself; their products vary with the latitude, the climate, and the 

 soil. To pass in review all the trees and plants and flowers which 

 flourish there, all the animals and peoples which dwell among them, 

 would be nothing less than to embrace in a Vast encyclopaedia the de- 

 scription and history of two organic kingdoms. But such is not the 

 design of the present volume. I have not undertaken to give an exact 

 picture of nature, which would task to the uttermost the powers of men 

 of such diverse genius as Humboldt, Owen, Lyell, Darwin, Tyndall, 

 Hooker, and Ruskin, but to sketch the bold outlines and more pro- 



