CHAPTER VI 



THE PRIMARY CONDITIONS OP ANIMAL LIFE 



62. Primary conditions and special conditions. Certain 

 primary conditions are necessary for the existence of all 

 animals. We know that fishes can not live very long out 

 of water, and that birds can not live in water. These, 

 however, are special conditions which depend on the spe- 

 cial structure and habits of these two particular kinds of 

 backboned animals. But the necessity of a constant and 

 sufficient supply of air is a necessity common to both ; it is 

 one of the primary conditions of their life. All animals 

 must have air. Similarly both fishes and birds, and all 

 other animals as well, must have food. This is another one 

 of the primary conditions of animal life. That backboned 

 animals must find somehow a supply of salts or compounds 

 of lime to form into bones is a special condition peculiar 

 to these animals. Other animals having shells or teeth 

 composed of carbonate or phosphate of lime are subject to 

 the same special demand, but many animals have no hard 

 parts, and therefore need no lime. 



63. Food. All the higher plants, those that are green 

 (chlorophyll-bearing), can make their living substance out 

 of inorganic matter alone that is, use inorganic substances 

 as food. But animals can not do this. They must have 

 already formed organic matter for food. This organic mat- 

 ter may be the living or dead tissues of plants, or the living 

 or dead tissues of animals. For the life of animals it is 

 necessary that other organisms live, or have lived, it is 

 this need which primarily distinguishes an animal from a 



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