THE RAT THEORY OF PLAGUE EPIDEMICS II 



reached in about a month or six weeks after its commencement. Again, 

 it will be seen that the epizootic reaches a higher level than the epidemic. 



An important point is that the epizootic appears in advance of the 

 epidemic, the latter making its appearance a week or a fortnight later. 

 Further fluctuations in the amount of the epizootic are usually followed 

 by fluctuations in the epidemic, and a general resemblance of the two 

 curves to each other is reproduced. It is interesting to note that the 

 interval of a week or a fortnight between the fluctuations in the two 

 curves is maintained until the maximum amount of both forms of 

 plague is developed. 



The apex of the curve of the epidemic is reached almost exactly a 

 fortnight after the maximum amount of the epizootic. 



With these general remarks, it will be well to examine the chart for 

 1902 more in detail. 



The epizootic curve reaches its maximum within five weeks. The 

 epidemic follows closely behind, reaching its maximum in seven weeks 

 from the date of the commencement of the epizootic, i.e., a fortnight 

 later than rat plague. If the chart be examined closely, it will be found 

 that elevations in the rat plague curve are followed by similar exacerba- 

 tions in the amount of the epidemic. This picture is perhaps not so 

 graphic in the chart for the year 1902, but, when examined along with 

 that for the year 1903, the elevations and depressions of the epizootic 

 curve are found to be followed by similar irregularities in the curve of 

 the epidemic. Indeed, these variations of the one curve, followed by 

 similar changes in the other, are so admirably reproduced (especially in 

 the year 1903), that one would imagine that the epidemic curve in its 

 rise and fall is too good to be true. 



Such an epidemic curve, occurring ten days or a fortnight after that 

 of the epizootic, and reproducing so admirably the variations in the 

 amount of the latter, is, in my opinion, strong evidence of some 

 extremely close relation existing between rat plague and human plague. 



In order to bring the epidemic curve into close relation with that of 

 the epizootic, the defervescence of the curves is of the highest importance. 

 So far as the ascent and maximal elevation of the curves is concerned, 

 the two curves follow each other closely. From this alone, however, one 

 could not claim to have established a connection between the two out- 

 breaks. All that can be said is that both occur about the same time, 



(225) Q 



