22 PRIMITIVE ANIMALS 



it obtains its nitrogen from the mineral nitrates in 

 the soil, its hydrogen from water, its oxygen from 

 the atmosphere and from water, and its phosphorus 

 and sulphur from salts in the soil. The green plant, 

 therefore, is the means by which the elements 

 necessary for life, after they have, broken away from 

 the living bodies of animals and plants, are brought 

 back again and put into circulation in the stream of 

 life ; and if it were not for the existence of green 

 plants all life on the earth would very soon come 

 to an end. 



Animals are therefore absolutely dependent in 

 the end on the existence of green plants, but animals 

 in their turn, although not necessary for the life of 

 plants, form a very large part of the raw material 

 from which plants draw their sustenance. Thus 

 animals as the result of their activity breathe out 

 C0 2 into the atmosphere, which supplies the plants 

 with their carbon. The excreta and dead bodies of 

 animals manure the ground for plants, i.e. decompose 

 into substances which ultimately yield the nitrates, 

 phosphates and sulphates from which the plant 

 builds up its body. 



In this manner there is a continual circulation 

 of the chemical elements necessary for life from 

 plant to animal and animal to plant ; but although 

 the animal ultimately yields up its substance to the 

 plant, yet the existence of animals is in no way so 



