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distribution of disease than its periodical return, and the 
only disease, whooping cough, that from our data has a 
two years’ cycle is not included in Dr. Lawson’s observa- 
tions. 
3. It has been suggested as an explanation of the 
recurrence of epidemics after a certain period that only 
certain years of life are prone to the disease, and that 
when these have been cleared away the disease could not 
prevail extensively until there were again persons of fit 
age to receive the poison. 
This simple “ age ” theory will, however, be found to 
involve two important assumptions, in order to account for 
cycles of several years’ interval. 1. It will be necessary 
that the epidemic should in the first instance be capable of 
only attacking persons at the superior limit of age suscep- 
tibility. In the case of whooping cough at the age of two 
years, and in the case of the other complaints at four, six, 
or seven years of age ; and 2, that it should then travel 
down to the lower ages, and so clear out of the way all 
persons capable of taking the disease that the complaint 
could not again appear until the most sensitive superior 
limit was again attained. 
I need hardly say that neither of these assumptions is 
borne out by actual experience during an outbreak of 
epidemic disease. Children of all ages take the complaint 
both at the commencement and end of the epidemic. 
If therefore age alone were the determining element, an 
infectious complaint would be continuous, and not inter- 
mittent in its ravages. 
Owing to the constant occurrence of fresh births in a 
population, there must always be present in it children of 
fit age to receive the virus ; there is always food upon which 
it is able to flourish. The disease would only be periodic 
if the birth-rate were also periodic in its character. 
4. A fourth possible explanation of epidemic cycles might 
