44 
COMPARATIVE ZOOLOGY. 
These four are possessed by all animals, but in a variety 
of ways. No two species have exactly the same mech¬ 
anism and method of life. We must learn to distinguish 
between what is vital and what is only accessory. That 
only is essential to life which is common to all forms of 
life. Our brains, stomachs, livers, hands, and feet are 
luxuries. They are necessary to make us human, but not 
living, beings. Half of our body is taken up with a com¬ 
plicated system of digestion ; but the Amoeba has neither 
mouth nor stomach. We have an elaborate apparatus of 
motion ; the adult Oyster cannot stir an inch. 
Nutrition , Motion , and Sensation indicate three steps 
up the grade of life. Thus, the first is the prominent 
function in the Coral, which simply “vegetates,” the pow¬ 
ers of moving and feeling being very feeble. In the 
higher Insect, as the Bee, there is great activity with sim¬ 
ple organs of nutrition. In the still higher Mammal, as 
Man, there is less power of locomotion, though the most 
perfect nutritive system ; but both functions are subordi¬ 
nate to sensation, which is the crowning development. 
In studying the comparative anatomy and physiology 
of the animal kingdom, our plan will be to trace the vari¬ 
ous organs and functions, from their simplest expression 
upward to the highest complexity. Thus Nutrition will 
begin with absorption, which is the simplest method of 
taking food; going higher, we find digestion, but in no 
particular spot in the body; next, we see it confined to a 
tube; then to a tube with a sac, or stomach; and, finally, 
we reach the complex arrangement of the higher animals. 
