28 
COMPARATIVE ZOOLOGY. 
cartilage; but the ternary compounds help in repairing 
waste, while both produce heat. When oxidized, whether 
for work or warmth, these complex compounds break up 
into the simple compounds—water, carbon dioxide, and 
(ultimately) ammonia, and as such are returned to earth 
and air from the animal. Both plant and animal end 
their life by going back to the mineral 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 combined and arranged. 
Plants compress the forces of inorganic nature into chem¬ 
ical compounds; animals liberate them. Plants produce; 
animals consume. The work of plants is synthesis, a 
building-up; the work of animals is analysis, or destruc¬ 
tion. The tendency in plants is deoxidation; the tenden¬ 
cy in animals is oxidation. Without plants, animals would 
perish; without animals, plants had no need to be. There 
is no plant which may not serve as food to some animal. 
CHAPTER IV* 
LIFE. 
All forces are known by the phenomena which they 
cause. So long as the animal and plant were supposed to 
exist in opposition to ordinary physical forces or indepen¬ 
dently of them, a vital force or principle was postulated 
by which the work of the body was performed. It is now 
known that most, if not all, of the phenomena manifested 
by a living body are due to one or more of the ordinary 
physical forces — heat, chemical affinity, electricity, etc. 
There is no work done which demands a vital force. 
The common modern view is that vitality is simply a 
* See Appendix. 
