Page:Popular Science Monthly Volume 38.djvu/259

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THE surface of the earth, with its division of land and water its diversities of climate, and its various elevations, offers to the world of plants as well as to animals a complexity of life-conditions to which their organisms are compelled to adapt themselves if they would even exist.

Few regions exhibit to so large an extent such even, uniform, and original character, as that vast desert expanse which stretches through southern Arabia and northern Africa from the Persian Gulf to the Atlantic Ocean. This uniformity is the result of the correspondence of the desert tract with the same degrees of latitude, and of its never departing from the subtropical regions. Since, also, the elevation of the land seldom greatly exceeds 3,000 feet, the temperature conditions, however much they may vary in single places in the course of a day, are as a whole more uniform than they would be in a similar tract running north and south, and marked by important elevations. The midday heat in the desert rises to over 120° Fahr., while at night the cold, in consequence of the rapid radiation, sometimes makes itself very unpleasantly felt, and in winter descends below the freezing-point. More unfavorable to the development of animal life than the temperature is the want of water, both running and standing, as well as the absence of rain and dew. Sufficient water and a thin surface soil are found only in the oases, which exercise an influence over the distribution of life like that of the presence of the numerous islands in the great ocean. Even including the oases, vegetation is very scanty; the immense territory of the Sahara, with an area of upward of 2,500,000 square miles, harbors only 560 species of plants; while the Japanese Islands, having only one seventeenth the area, 150,000 square miles, support not less than 2,745 species. Most of the desert vegetation is deficient in quality as well as quantity; the plants are sparse, generally small, with inconspicuous gray leaves, and often covered with sand. Many plants that are usually annual develop, under the influence of life in the desert, long roots reaching down to the ground water, and become perennial. Monocotyledonous plants are represented only by dry, tough grasses, like the esparto, and by a few palms in the oases. Woods, the chief resorts of animal life, are wanting.

Most of the scanty fauna is concentrated in the oases. The oasis of Bachariel, according to the French entomologist Lefevre, swarms with insects at certain seasons, which would yield a rich harvest to the collector if he would stay there long enough to