XI. 
EXPERIMENTS UPON ANIMALS. 
THE pathogenic power of various bacteria has been demonstrated 
by injecting pure cultures into susceptible animals. As a rule, the 
herbivora are more susceptible than the carnivora, and this is per- 
haps to be explained in accordance with the theory of natural selec- 
tion. Carnivorous animals often feed upon the bodies of animals 
which have succumbed to infectious diseases, and upon dead animals 
in which putrefactive changes have commenced. In their struggles 
with each other they are wounded by teeth and claws soiled with in- 
fectious material which would cause a fatal disease if inoculated into 
the more susceptible herbivorous animals. As this has been going 
on for ages, we may suppose that, by survival of the fittest, a race 
tolerance has been acquired. The lower animals have their own in- 
fectious diseases, some of which are peculiar to certain species and 
some common to several. Asa rule, the specific infectious diseases 
of man cannot be transmitted to lower animals, and man is not sub- 
ject to the diseases of the same class which prevail among animals. 
But certain diseases furnish an exception to this general rule. Thus 
tuberculosis is common to man and several of the lower animals ; 
relapsing fever may by inoculation be transmitted to monkeys ; 
diphtheria may be transmitted to pigeons and guinea-pigs. On the 
other hand, anthrax and glanders may be contracted by man as a 
result of accidental inoculation or contact with an infected animal. 
Nearly allied species sometimes present very remarkable differ- 
ences as to susceptibility. Thus the bacillus of mouse septicaemia is 
fatal to house mice but not to field mice, while, on the other hand, 
field mice are killed by the bacillus of glanders and house mice are 
immune from this pathogenic bacillus. 
The animals most commonly used for testing the pathogenic 
power of bacteria are the mouse, the guinea-pig, and the rabbit. 
Domestic fowls and pigeons are also useful for certain experiments. 
The dog and the rat are of comparatively little use on account of 
their slight susceptibility. 
