It is not likely that the deserts of the southwest have always been as dry as they are now. As the amount of rainfall slowly lessened through thousands of years, the animals could migrate when it became too dry; but the plants, fixed in one place, had either to give up and die, or change their characters and habits to suit the demands of the changing climate. The fact that these extremely dry deserts are filled with plant life to-day is without doubt due to this ability to change. In a moist, warm climate plants are luxuriant; they take up a large amount of water through their roots and evaporate it through the leaves. If placed in a desert, such plants would immediately wither and die. To avoid too rapid evaporation the bodies of the desert plants have become smaller, and their leaves have either shrunk greatly or wholly disappeared. Strong-smelling, resinous juices exude from the remaining leaves and stems, and form a surface varnish through which water passes with difficulty. Some forms of plant life, such as the prickly-pear, are provided with fleshy stems which hold a supply of moisture to be drawn upon during the long dry season. Men and animals are sometimes saved from death by chewing the pulp of the prickly-pear or other cactuses. After a period of exceptional drought, the stems of the prickly-pear lose their bright green color and become shrunken. [Illustration: FIG. 81.--PRICKLY-PEAR, BALL CACTUS, AND SPANISH BAYONET] The development of the underground part of the plant is frequently out of all proportion to the part above the surface. The manzanita, which grows in the semi-arid climate of southern California, is a low shrub with branches that are rarely large enough for fuel. The roots, however, are large and massive, and are extensively used for firewood. The desert plants are armed, not only against the dry air, but against the wandering animals which would bite them and suck their juices. The smell of the sagebrush is such that very few animals will touch it. Other plants are protected by thorns. In fact, the drier the region, the more thorny are its plants. A little shrub called the crucifixion thorn has no leaves at all, nothing but long, sharp spines. Besides the straight thorns there are curved and also barbed ones, for every conceivable form is represented among the plants of these dry lands. As the desert plants are armed against the animals, so the animals are armed against each other. Many of the insects and reptiles are extremely poisonous; the greater the heat of their habitat, the more dangerous are their bites. The horned toad, while not poisonous, is protected by having horny spines upon its head and back. The little rattlesnake known as the "side-winder" is perhaps the most dangerous of all, although the tarantula, centipede, and scorpion are formidable foes. The Gila monster, long believed to be so dangerous, is now considered non-poisonous under ordinary conditions.