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perhaps be found in Dr. Farr’s observations on the form of 
the epidemic wave and its probable cause. 
In the year 1865 Dr. Farr showed that the curve traced by 
the course of the cattle plague was remarkably regular, and 
that other epidemics showed traces of a similar formation. It 
proceeded by continually decreasing increments from the first 
week to the last, and by noting the rate of diminution of 
the growing power of the disease in its first stages, Dr. Farr 
believed that it was quite possible to foretell its probable 
course and ultimate decline. He was actually able to do 
this for the rinderpest, and his forecast of the mode in v>^hich 
the disease would work itself out was proved to be singularly 
correct by the result, tie accounted for this curve by the 
hypothesis that the poison of the disease was itself so 
diluted by successive transmissions through the bodies of 
its victims that it became gradually less and less able to 
propagate the complaint. 
This hypothesis was supported by the history of most 
epidemics, by the shape of the curve it describes, by the 
fact of its greater fatality at the commencement of the out- 
break, and by the observation that the longer the disease 
was absent from the community, the greater was its viru- 
lence when it did recur. 
This theory would certainly account for most of these 
phenomena, though they are also susceptible of a simpler 
explanation, which I ventured to give in a paper “ on epi- 
demics studied by means of statistics of disease,” which 
was read before the Oxford Meeting of the British Medical 
Association in 1868. 
It is to be remarked also that no explanation is given by 
this hypothesis of the resurrection of the disease after a 
certain lapse of years. It would require the further 
assumption that after a period of latency the poisons gather 
strength enough again to attain an epidemic violence. 
It has indeed been shown by Dr. Thiersch and by Dr. 
