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 enterprise. 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 encyclopædia the description 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 prominent features of the physiognomy of the Desert World, and not to reproduce its more minute details. My embarrassment, then, arises less from the multitude and infinite variety of the objects we have to examine, than from the difficulty of harmonizing the study with the divisions of this work. How, in fact, can I establish a positive distinction between the animals of the Prairies or the Savannahs and those of the Forests, between those of the latter and the animals proper to the Mountains? For such a purpose it is needful that each of these forms of the Desert World should possess its peculiar fauna; which is true only within very narrow limits. In reality, most animals inhabit or frequent, according to circumstances, sometimes one district, sometimes another, without its being possible to assign with any amount of precision their habitual, or simply their occasional, abode.