NATURAL RESISTANCE TO DISEASE—FLEXNER. 729 
the introduction of bacteria, incapable of causing infection, into im- 
mune species is followed by immediate phagocytic ingestion and 
destruction of the microorganisms. The rapidity and perfection of 
the phagocytic reaction in insusceptible animals are very impressive 
and might readily lead to the decision that they suffice to explain the 
resistance or immunity. However, the matter does not permit of such 
summary disposal, since there appear to be other factors that enter 
into the phenomena. The frog that does not become tetanic when in- 
oculated with tetanus bacilli or poison, develops tetanic spasms when 
the temperature is raised somewhat; the hen that does not respond 
to an anthrax inoculation develops the infection when the tempera- 
ture is lowered somewhat. Even for the final ingestion of bacteria 
by the phagocytes of alien and insusceptible species the plasma prin- 
ciples are required. 
Undoubtedly the phenomena of racial and species immunity are 
affected by phagocytosis. But our present knowledge does not justify 
us in disregarding other possible and contributing agencies. We are 
still so little informed of even the grosser features of the body’s 
metabolism that it would be premature to deny to it influence on 
susceptibility to infection. Between the metabolism of birds and 
mammals there is such wide disparity that an influence could easily 
be conceived; but the metabolic disparity is less between the her- 
bivora and carnivora, and still less between some closely related 
species which yet show marked differences in susceptibility to bacte- 
rial infection ; and as between individuals of the same species it could 
only be the finer intramolecular variations that conceivably could 
come into play. 
Although the properties of the defensive mechanisms of the blood 
have not been exhausted, yet they have been defined in such detail as 
to suffice for the moment and to permit us to turn attention, for a brief 
space, to some of the properties of the intending invading bacteria. 
It is matter of common experience, which each of us has suffered, that 
the elaborate mechanisms provided for our protection from bacterial 
infection do not always suffice, and now it becomes necessary to ex- 
plain why they do not. In the first place, there are very great differ- 
ences between the bacteria which seek to enter the body. Some spe- 
cies are never very harmful and are readily combated, excluded, or 
destroyed ; other species often possess only a moderate degree of viru- 
lence or potential power of doing injury and can also, as a rule, be 
overcome; while these second species sometimes acquire such highly 
virulent or invasive powers that the defenses prove quite inadequate 
to exclude or combat them. During the prevalence of great bacte- 
rial epidemics it is probable that this factor, virulence, plays a con- 
siderable role. Of course, in epidemics the bacterial causes are, by the 
exigencies of the situation, more widely diffused than at other times, 
