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 inoculated 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 temperature is lowered somewhat. Even for the final ingestion of bacteria by the phagocytes of alien and insusceptible species the plasma principles 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 herbivora and carnivora, and still less between some closely related species which yet show marked differences in susceptibility to bacterial 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 explain why they do not. In the first place, there are very great differences between the bacteria which seek to enter the body. Some species are never very harmful and are readily combated, excluded or destroyed; other species often possess only a moderate degree of virulence 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 bacterial epidemics it is probable that this factor, virulence, plays a considerable rôle. Of course in epidemics the bacterial causes are by the exigencies of the situation more widely diffused than at other times, so that more individuals come under their influence; but with even such a common bacterium as the diplococcus which causes pneumonia and the bacillus which produces influenza, there arise conditions in which severe and often very extensive outbreaks, or localized epidemics, occur which are probably to be attributed to an accession in virulence of these germs, although the precise causes leading to the increase may not be discovered.
Now this quality of virulence, which is often evolved so quickly
