494 Proceedings of Royal Society of Edinburgh. [sess. 
type (diagram XI., Table A, No. 18). The measles epidemic of 
the present day has exactly the same form. 
The theory of endemic disease is more difficult. The degree of 
endemicity, of course, varies in different instances. Sometimes, 
when a disease invades a territory, there is a period of some years 
during which it is never absent. In some such cases we have 
obviously to deal with two independent epidemics, the tail of the 
first of which runs into the beginning of the succeeding. More 
Diagram XI 
often, however, as with scarlet fever, enteric fever, malaria, etc., 
there is a minimum below which the amount of disease never 
falls. One disease, namely, zymotic diarrhoea, partakes of both 
characters, being truly endemic, occurring year after year at the 
same season, and yet in characteristic outbursts, so that it may 
conveniently be considered with first instance. The statistics 
chiefly refer to deaths, and only in a few instances to cases. 
The latter are in this disease specially important, as diarrhoea 
claims its victims mainly at the two extremes of life, and con- 
sequently nothing can be inferred a priori as to the relationship 
