INTERDEPENDENCE OF ANIMALS AND PLANTS 543 
without disaster to the life of the forming organism. An 
egg may be kept before starting to hatch; but once started, 
it must continue. There is considerable variation in the 
exact manner in which the eggs of different animals develop. 
Some have a very large amount of food stored in the egg, as 
hens’ eggs, which influences the manner of embryonic growth. 
Review the following : page 6; sections 10, 50, 63, 67„ 75, 
84, 92, 117, 124, 134, 144, 145, 167, 178, 275, 281, 286; 
page 305; and figures 66-67, 75-80, 180, 191, 193. 
413. Interdependence of Animals and Plants. —As we 
have seen, plant life must precede animal life. Plants can 
take water, carbonic acid, and a few minerals, and make 
starch, sugar, proteid, etc. Before there were any animals 
on the earth, the plants were thriving and building foods. 
These foods with no animals to eat them decayed and made 
up rich soils for further plant growth. This continued till 
the animals appeared on the earth. 
These animals ate some of the foods that the plants made 
and in time died and their bodies together with the dead 
plants went back to the soil and made it still richer and able 
to produce better and more nourishing foods. This process 
has been going on for ages, and the plants have been furnish¬ 
ing the food for the animals’ bodies. At death the animal 
body goes back into the soil, making it richer in plant food 
and able to produce more plants and better food crops. 
Thus the plants are helped by the decaying animal bodies, 
and the animals themselves must have the plants for food 
during their life time. 
Of course, some animals eat flesh, but eventually in tracing 
back we come to the animal that feeds upon the plant food. 
The house cat, for example, eats mice, but the mice eat 
grains, and so we find the cat, a flesh-eating animal, depend¬ 
ent on animals that must have plant food. 
Review the following: sections 1, 15, 21, 28, 185, 245, 
311, 314. 
