28 COMPARATIVE ZOOLOGY. 
and gluten. When the plant is eaten by the animal, the 
ternary products, as starch, sugar, gum, and cellulose, are 
simply carried by the blood to the lungs, and decomposed 
by oxidation back to carbonic acid and water, in which 
process heat is generated. The other vegetable products, 
as albumen, etc., containing nitrogen, go to sustain the 
animal, forming blood, muscle, cartilage, ete. These, giv- 
ing way to fresh material (for there is constant renewal), 
are resolved into carbonic acid, water, and ammonia, and 
returned to the earth and air through the kidneys. Both 
plant and animal end their life by going back to the min- 
eral world: and thus the circle is complete—from dust to 
dust. Carbonate of ammonia and water, a blade of grass 
and a horse, are but the same elements differently com- 
bined and arranged. Plants compress the forces of inor- 
ganic nature into chemical compounds; animals liberate 
them. Plants produce; animals consume. The work of 
plants is synthesis, a building-up; the work of animals is 
analysis, or destruction. The tendency in plants is deoxi- 
dation; the tendency in animals is oxidation. Without 
plants, animals would perish; without animals, plants had 
no need to be.” 
CHAPTER IV. 
LIFE. 
An impenetrable veil hides the nature of life. We 
know nothing of it except by the phenomena it mani- 
fests; and as these manifestations differ from those of 
any known physical force, they have been attributed to 
a “special principle.” But the existence of this assumed 
cause has never been demonstrated. The biologist can 
