988 The American Naturalist. [December, 
of the earth covered with the accumulations of the growth of 
forests in past ages that would have tumbled upon each other 
until there would be such an accumulation of dead trees and 
dead leaves’and dead vegetation of all kinds on the surface of 
the earth that plants would not be able to grow. The dead 
bodies of all the animals that have lived in the past would 
have been piled up until the whole surface of the, world would 
have been so covered by the dead bodies of animals and plants 
that life would have become impossible. These scavengers, 
these bacteria, are absolutely necessary to us. It is through 
the agency of certain bacterial organisms that the tree is soft- 
ened so that insects can get atit. It is through the agency of 
bacteria that the tissues of the bird are decomposed and gases 
produced which pass off into the air. It is these bacteria 
which cause all the changes in the bodies of animals and veg- 
etables, decomposing them until they gradually sink down in 
the soil and disappear. So it is through their agency and this 
alone that the surface of the earth is kept in a condition which 
renders it possible for life to continue to exist. Of course you 
have all had experience of the value of bacteria as scaven- 
gers in removing bad odors. We speak of scavengers as 0 
value in removing decaying material, but it is the bacteria 
which produce the decay, and it is through their agency that 
all of these dead bodies are broken to pieces and brought into 
a condition in which they can be either incorporated into the 
soil or passed off into the air. 
Perhaps I may here also say a word in regard to the agency 
of bacteria as scavengers in the human body. We look upon 
bacteria in our bodies as causes of disease rather than things 
which are of any value, and yet a healthy person always has 7 
bacteria in large quantities in his mouth, in his stomach, and 
in his intestines. The bacteria are always migrating in the — 
body to places of abnormal growths, and there is considerable — 
reason for thinking that to a certain extent these bacteria act 
as scavengers in the human body. Some of them unques — 
tionably act as producers of disease, but to a certain extent it 
seems that these bacteria are of a value in assisting in the 
decomposition of tissues that should be decomposed, and there 
