CHAPTER VI 
THE PRIMARY CONDITIONS OF 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 
106 
