Page:Popular Science Monthly Volume 63.djvu/513

Rh the conditions more carefully, we shall see that we must provide not only for the survival of the individual, but for succeeding generations of individuals, and, when we take into account the special adaptations that become necessary to permit of the development of eggs and the early stages of the different forms, it will be seen that the problem becomes much more difficult. As already hinted we can scarcely conceive of life originating under such unfavorable conditions, but must think of it as having gradually extended its range from adjacent, more habitable regions. We can see, too, that the special adaptation in this direction distinctly unfits the animal for a return to a more humid condition and, if its desert conditions were withdrawn, the probability is that it would succumb to the pressure of more active forms of life. In fact we may gather that desert forms have reached such situations as an effort to escape from the more rigid contest in regions more densely habited.

Many tribes of men have thus pushed out into arid territory, adjusting themselves as well as possible to the conditions, but always with a struggle against these special conditions that can be scarcely less severe than the struggle against stronger individuals or races that have attempted their subjugation or extermination.

Aquatic forms we may expect to be absent and still some such aquatic forms as may develop very rapidly in temporary pools of water and are otherwise adapted to long periods of desiccation have solved this problem. Birds and insects may by their ready locomotion easily take to temporary quarters under desert conditions, and some of them become fixed inhabitants, but usually with some degree of subterranean habit to protect themselves from the severity of the sun's rays, thus burrowing owls and many subterranean or nocturnal insects are characteristic of desert life. Burrowing squirrels, prairie dogs, snakes, lizards, etc., all follow the same line.

Another distinct line of adaptation is shown in the animal life inhabiting caves, a fauna so characteristic and so strikingly similar in different parts of the earth, though common origin is out of the question. Such life might be looked upon as an extreme of subterranean forms living near the surface of the ground, but there are different conditions to be met, and the results are in many cases widely different. Loss of eyes would seem to be the most frequent and, indeed, almost the first effect of such adaptation, and this modification alone would practically preclude such animals from ever attaining a successful hold upon ordinary conditions. Return to conditions of light would expose them mercilessly to the attacks of animals with ordinary organs of vision. Blind fishes, blind insects, blind crustaceans, all attest to the controlling influence of environment, and whether the animals found in these locations have come there by choice to secure