SUSCEPTIBILITY AND IMMUNITY. 235 
selected individuals and their progeny. The essential difference be- 
tween a susceptible and immune animal depends upon the fact that 
in one the pathogenic germ, when introduced by accident or ex- 
perimental inoculation, multiplies and invades the tissues or the 
blood, where, by reason of its nutritive requirements and toxic pro- 
ducts, it produces changes in the tissues and fluids of the body incon- 
sistent with the vital requirements of the infected animal; while in 
the immune animal multiplication does not occur or is restricted to a 
local invasion of limited extent, and in which after a time the re- 
sources of nature suffice to destroy the parasitic invader. 
Now the question is, upon what does this essential difference de- 
pend ? Evidently upon conditions favorable or unfavorable to the 
development of the pathogenic germ; or upon its destruction by 
some active agent present in the tissues or fluids of the body of the 
immune animal; or upon a neutralization of its toxic products by some 
substance present in the body of the animal which survives infec- 
tion. 
What, then, are the unfavorable conditions which may be supposed 
to prevent development in immune animals? In the first place, the 
temperature of the body may not be favorable. Certain pathogenic 
bacteria are only able to develop within very narrow temperature lim- 
its, and, if all other conditions were favorable, could not be expected 
to multiply in the bodies of cold-blooded animals. Or the temperature 
of warm-blooded animals, and especially of fowls, may be above the 
point favorable for their development. This is the explanation 
offered by Pasteur of the immunity of fowls, which are usually re- 
fractory against anthrax ; and in support of this view he showed by 
experiment that when chickens are refrigerated after inoculation, by 
being partly immersed in cold water, they are liable to become in- 
fected and to perish. But, as pointed out by Koch, the sparrow, 
which has a temperature as high as that of the chicken, may con- 
tract anthrax without being refrigerated. We must not, therefore, 
too hastily conclude that the success in Pasteur’s experiment de- 
pended alone upon a reduction of the body heat. Gibier has shown 
that the anthrax bacillus may multiply in the bodies of frogs or 
fish, if these are kept in water having a temperature of 35° C. 
But the anthrax bacillus grows within comparatively wide tempera- 
ture limits, while other pathogenic bacteria are known to have a 
more restricted temperature range and would be more decidedly 
influenced by this factor—e.g., the tubercle bacillus. 
The composition of the body fluids, and especially their reaction, 
is probably a determining factor in some instances. Thus Behring 
has ascribed the failure of the anthrax bacillus to develop in the 
white rat, which possesses a remarkable immunity against anthrax, 
