It is interesting to note, too, how much alike in build and gait are these three thoroughbred desert roamers, the giraffe, the ostrich, and the camel or dromedary. In their long legs, their stalking march, their tall necks, and their ungainly appearance they all betoken their common adaptation to the needs and demands of a special environment. Since food is scarce and shelter rare, they have to run about much over large spaces in search of a livelihood or to escape their enemies. Then the burning nature of the sand as well as the need for speed compels them to have long legs which in turn necessitate equally long necks, if they are to reach the ground or the trees overhead for food and drink. Their feet have to be soft and padded to enable them to run over the sand with ease; and hard horny patches must protect their knees and all other portions of the body liable to touch the sweltering surface when they lie down to rest themselves. Finally, they can all endure thirst for long periods together; and the camel, the most inveterate desert-haunter of the trio, is even provided with a special stomach to take in water for several days at a stretch, besides having a peculiarly tough skin in which perspiration is reduced to a minimum. He carries his own water-supply internally, and wastes as little of it by the way as possible. What the camel is among animals that is the cactus among plants--the most confirmed and specialised of desert-haunting organisms. It has been wholly developed in, by, and for the desert. I don't mean merely to say that cactuses resemble camels because they are clumsy, ungainly, awkward, and paradoxical; that would be a point of view almost as far beneath the dignity of science (which in spite of occasional lapses into the sin of levity I endeavour as a rule piously to uphold) as the old and fallacious reason 'because there's a B in both.' But cactuses, like camels, take in their water supply whenever they can get it, and never waste any of it on the way by needless evaporation. As they form the perfect central type of desert vegetation, and are also familiar plants to everyone, they may be taken as a good illustrative example of the effect that desert conditions inevitably produce upon vegetable evolution.