CHAPTER II 

 THE CONDITIONS OF EXISTENCE FOR ANIMALS 



Owing to their rapid reproduction, living organisms are found 

 everywhere on the earth's surface in places to which they have access 

 and in which they find the necessary conditions for existence. Al- 

 though animal life is so widely distributed, its abundance and diversity 

 vary greatly from place to place and from season to season according 

 to environmental conditions. There are, in fact, few localities aside 

 from the craters of active volcanoes and recent lava flows where there 

 is no animal life, where life is probably impossible. The Dead Sea 

 merits its name, for no animal is able to exist in its waters, with their 

 great concentration of dissolved salts and the high concentration of 

 chlorides and bromides of magnesium. By contrast, Great Salt Lake, 

 with a salt content almost as great, contains diatoms, blue-green 

 algae, protozoa, and large numbers of the brine shrimp, Artemia 

 fertilis, and of the brine fly, Ephydra. The depths of the Black Sea, 

 which contain much hydrogen sulphide, are devoid of life. One seeks 

 in vain for animal organisms in springs rich in carbon dioxide, or in 

 fumaroles, and they are exceedingly rare in the ice wastes near the 

 poles (see Chapter XIX) , and in the depths of moving sand. 



A varying supply of matter and energy is necessary for the exist- 

 ence of life. The substances required for the growth and reproduction 

 of an organism, for the maintenance of its energy and for the pro- 

 duction of secretions, are designated as foods. For animals these are 

 primarily organic matter of animal or vegetable origin since, unlike 

 green plants, animals are unable to manufacture food from inorganic 

 substances. Special salts supply needed elements such as potassium, 

 sodium, and calcium, among others, and some of these may be imbibed 

 independently of the animal food. Organic foods supply large amounts 

 of chemical energy, which are released as work and heat in the body 

 of the animal. This energy is developed to best advantage in the 

 presence of oxygen, which is important for its own wealth of chemical 

 energy. The release of such energy occurs by means of chemical trans- 

 formations, and, as these can take place only in solutions, water, as a 

 solvent, is an indispensable requirement for life. Some energy is also 

 directly available to animals in the form of light and heat. 



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