422 
Mosquitoes and Malaria 
satisfactorily dealt with except in particular cases where the necessary statistics 
are available ; neither would results in general terras be of much practical use. 
It is probable that the risk of infection may depend partly on the age and 
past history of an individual ; for example, a person who has completely recovered 
from one or more attacks may become p;irtially or entirely immune. So far, 
however, I have not found any statistics which deal with this phase of the 
question. 
The combined effect of the birth and death rates will, as a rule, be small 
compared with the effects under (a) and (b) ; e.g., the birth rate for Mauritius 
in 1906 was 2-8 per thousand per month and the death rate 3'3 ; of the deaths, 
1"2 per thousand were due to malaria. These figures do not differ appreciably 
from the average taken over a long period of years. 
I propose, therefore, in the present paper, to confine my attention to the effects 
on the malaria rate of the first two of the above causes and to leave for a later 
consideration the minor contributory causes. 
I shall also assume in this preliminary survey that both the human population 
and the anophelines are uniformly distributed over the area in question ; that 
persons of all ages, whether they have previously suffered from malaria or not, 
are equally liable to infection ; that the number of anophelines per unit of the 
population remains approximately constant during the period under review ; and 
further that infected and non-infected persons are equally exposed to the risk of 
being bitten. 
It is obvious that the results so obtained will require modification in practical 
application and ought to be taken into account when the theory is developed to 
a higher approximation ; they will be, at the best, tentative and suggestive, but 
it is hoped that they will tend to throw light on the problems which arise when 
the specialist has to consider remedies for the disease. 
Infection Rate. The number of new infections in a locality during any 
particular month has been called the Monthly Infection Rate. This depends 
chiefly on the number of infected anophelines in the neighbourhood, that is, the 
number which have bitten previously infected persons. 
I shall in general follow the notation adopted by Professor Ross {vide Report, 
p. 31, et seq.). 
Let p denote the average population of the locality during the enquiry, m the 
number of infected persons*, h the number of healthy persons at the beginning of 
the first month and im the average number of the infected persons whose blood 
contains enough of the malaria parasites to infect anophelines ; evidently i is a 
fraction. Next, let a denote the average number of anophelines in the locality 
capable of carrying malaria and ha the average number of these which succeed in 
biting a single person during the month ; here h must be a very small fraction, 
* This differs from Professor Ross' use of m. 
