G. F. Petrie and C. R. Avari 
313 
the curves in the two charts show a completely inverse relation—the 
off season in the one case corresponding to the season of prevalence in 
the other. 
Before attempting to compare these charts from the point of view 
of a possible causal relationship, it is necessary to take into account the 
mean interval elapsing between the date of infection of the rats and the 
date on which examination revealed the parasites in their blood. It is 
impossible to determine this interval with any degree of accuracy, but 
it may be pointed out that all the rats examined were young, probably 
Chart III. Showing Rat flea prevalence in Bombay, 1906, 1907. (From the Plague 
Commission’s Reports, Journ. Hygiene, Yol. vm.) 
-= Fleas on Mas rattus. 
.= Fleas on Mus decumanus. 
on an average 4—6 weeks old. It may be noted also in this connection 
that Laveran and Mesnil (1901) have found experimentally that in 
young rats (30—100 grammes) the incubation period, i.e. the interval 
between inoculation of infected blood and the appearance of the para¬ 
sites in the circulation, is very short—usually'only 24 hours. For these 
reasons we think that a general comparison may justly be made between 
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