506 Proceedings of Royal Society of Edinburgh. [sess. 
constants, though the former has a time unit of two days and the 
latter of two weeks, the same loss of infectivity taking place for 
each epidemic in these different periods. 
The explanation of endemic prevalence which scarlet fever and 
enteric fever display may he sought in the fact that two factors are 
in action, one the true endemic prevalence, and the other the 
seasonal epidemic prevalence, the former varying according to a 
Diagram XXII. 
different series of causes from the latter. There may be constantly 
present a certain amount of infection of a low grade which keeps 
up the endemic supply, while if the seasonal conditions in the 
early summer he suitable, some portion of this acquires a high 
grade of infectivity, and alone produces the true autumnal outburst. 
Thus a severe autumnal epidemic might readily occur though the 
endemic prevalence was small, and with the latter at a considerably 
higher level the presence of unfavourable seasonal conditions might 
