1905-6.] Studies in Immunity : Theory of an Epidemic. 499 
the case where smallpox invades a school in which there are a 
great number of unvaccinated children. All these factors and 
many other minor ones make their influence felt with more or less 
regularity. These, however, cannot he taken as the complete 
reasons for the difference noted. It is likely that there is a 
biological factor in addition. The infecting powers of an organism 
cannot be expected to obey completely regular laws of growth or 
Diagram XV. 
decay. As with children : some have a slow, regular growth ; others 
make little increase in size for some years, then shoot to their full 
stature in a very short time. While these variations comport 
themselves so that in the sum the different groups adjust them- 
selves closely to the law of error, yet in individual cases a law can 
only be expected to give a rough approximation, and the application 
of a law of averages to an individual instance cannot be expected 
to give good examples of curve fitting. I intend at some future 
time, with reference to some special disease, to make an investiga- 
