But water is even scantier in the desert than food; and against want of water, therefore, the camel has had to provide himself, functionally at least, if not structurally, quite as much as against want of herbage. His stomach has accordingly acquired the power of acting as an internal reservoir, and he can take in as much water at the Bahrs or Wadys, where he rests for a while on his toilsome march, as will supply his needs for four or five days together. There are some differences in this respect, however, between the two chief varieties of the camel. The African kind is most abstemious, and best adapted to sandy deserts; the Bactrian, a product of more varied and better-watered country, is larger and stronger, but less patient of hunger and thirst, while at the same time it can manage to subsist and to make its way into somewhat rockier and more rugged country.
One other adaptive peculiarity the camel possesses which marks it out essentially as a desert-born animal, modified for generations by the baking expanse of Sahara or the Arabian sand-wastes. On those scorching surfaces a horse could not lie down to rest in the eye of the sun without scalding or excoriating his tender skin. But all the parts of the camel's body which touch the sweltering sand in his ordinary patient kneeling position are provided with callosities of thickened hide, which enable him to rest on the burning surface without danger or discomfort. The only other desert-haunting ruminant of similar habits, the giraffe, has analogous callosities for the same purpose on his breast and knees. Such adaptive characters, however, are never a key to real relationship; they necessarily result from mere exposure to the same circumstances; and hence we get the seemingly paradoxical principle, so well enunciated by Mr. A. R. Wallace, that the more useful any organ or point of structure is to its possessor, the less is its value as a test of systematic position. Unseen little bones and internal organs, which fail to strike the imagination of the outside observer, are rightly used as the keys to underlying relationship by the systematic biologist. The real affinities of the camel are closest, indeed, not with the giraffe which so strongly resembles it, but with the llama and alpaca, so remotely connected in outer seeming, and so widely separated from it in space by an entire hemisphere.
Camels, llamas, and alpacas alike, then—to sum up the conclusion to which we have all along been tending—represent a very simple and early ruminant type, unmarked by any of those higher features induced in the ruminants of the open plains of the great continents by the necessity for protection from the advanced carnivores. They recall for us in their main points of structure, as well as in their low and undeveloped grade of intelligence, the general characteristics of the ruminant race at the dawn of its