1892.] Some Uses of Bacteria. -989 
is reason for thinking that they assist in the digestion of food. 
There is no question that bacteria may assist in the process of 
digestion and it is doubtless a fact that the bacteria which we 
take into our alimentary canal are not wholly injurious. 
They may be possibly beneficial to us either in the line of 
scavengers in removing material which ought not to remain 
in our bodies, or in assisting digestion. This point, however, 
is not yet demonstrated, and I merely allude to it as a possi- 
bility. 
Did it ever occur to you to ask why nature is perpetual? 
You know animals and plants have continued to live on the 
surface of the earth for hundreds and hundreds of centuries. 
The vegetation that has been growing on the surface of the 
earth has been constantly taking food out of the air and 
taking food out of the soil, and animals have been constantly 
feeding upon the plants. But the process seems to be a never- 
ending one. It would seem that the material for plant food 
and animal food would sometime be used up; and yet nature 
is perpetual. Now, the reason that nature is perpetual is, 
because animals and plants are enabled, by certain processes 
of nature, to use the same material over and over and over 
again. They can use material for food, and eventually that 
same material gets in a condition in which they can use it for 
food once more. Let me take a single illustration, one that 
you are probably all familiar with. Plants, as the result of 
their life, use up carbonic acid of the air, and, in return, send 
off into the air an equivalent amount of oxygen. Now, 
animals in their life, take out of the air a considerable amount 
of oxygen and send off from their bodies an equivalent 
amount of carbonic acid. You see here one of the adjust- 
ments of nature. Animals use the excretions of plants, plants 
use the excretions of animals. The animals take oxygen and 
give off carbonic acid, and the plants take carbonic acid and 
give off oxygen. This process goes on continually and thus 
the condition of the atmosphere, so far as oxygen and carbonic 
acid are concerned, is kept in the same normal state. Thus, 
so far as these gases are concerned, nature is enabled to be 
