256 THE Microscope. 
haps here for that very purpose), the acids of the stomach, and the 
strongly antiseptic bile of the liver in animals ; or, finally are beaten 
in the struggle for existence by the peculiar powers of vital re- 
sistance possessed in greater or less degree by all living things, 
plants as well as animals, man as well as bacteria. 
Against such defences and barriers our minute, simply con- 
structed, senseless bacteria must make their way if they are going 
to gain their own sustenance from living plants or animals. The 
tact is that most kind of bacteria are never able to overcome these 
vital barriers. 
We hear it said, bacteria can not be the cause of disease, 
since they are about us and within us in great numbers in our best 
states of health. As well might be said, animals will not eat 
flesh, since we know great pastures full of flocks and herds that 
cannot be made to touch it as food. Most species of bacteria are 
confined to a regimen of dead organic matter. They riot in the 
carcass of a dead animal that they were wholly unable to injure in 
the living state. Other species, still greater in number than those to 
be next mentioned, manage to grow and multiply on or in living 
things only when the ramparts of the latter are broken down, when 
the defences are weakened, when the ebb and flow of the vital cur- 
rent is hy other means reduced or deranged. Bacteria that with- 
out help can not get through the skin upon one’s arm, may readily 
do so by the aid of croton oil or cantharides. Those which ordi- 
narily cannot stand the ordeals of the stomach, may do so in cases 
when the latter fails to excrete its normal juices by the untoward 
influences of unhygienic food. 
Finally, certain species much fewer in number than the pre- 
ceding, seemed to have powers of invasion altogether surpassing 
the defences normally offered by many individuals. These are the 
organisms of true contagions, like small pox, scarlet and yellow 
fevers, measles and cholera. Yet even with these last the struggle 
is ever going on. In the worst epidemics certain persons remain 
unharmed, not because the disease producers are not wanting, but 
because their physiological activities are more than matched by 
those of the exempt party. And the same person may offer far 
greater resistance to these destroyers at one time or in one condi- 
tion than at, or in another, just as he may against every other in- 
fluence or thing. Let a man, not accustomed to hard labor in the 
