ANIMAL LIFE. 509 



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 con- 

 ceive 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 condition and, if its desert conditions were withdrawn, the probability 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, adjust- 

 ing 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 charac- 

 teristic of desert life. Burrowing squirrels, prairie dogs, snakes, liz- 

 ards, 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 ques- 

 tion. 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 differ- 

 ent. 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 suc- 

 cessful 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 ani- 

 mals found in these locations have come there by choice to secure cer- 



