CONDITIONS NECESSARY TO SUPPORT HYPOTHESIS 255 



found the transmitting agent sought for. If the house-fly is the 

 transmitting agent in summer diarrhoea, the following conditions 

 should be fulfilled : — 



"(1) («) There should be evidence that the house-fly carries 

 bacteria under the ordinary summer conditions ; (b) house-flies 

 should be present in suflicient numbers in houses invaded by fatal 

 diarrhoea. 



" (2) There should be a close correspondence between the 

 aggregate number of house-flies in houses and the aggi^egate 

 number of deaths from diarrhoea week by week. 



"(3) The life-history of the house-fly should explain any 

 discrepancy between the observed number of flies and the observed 

 number of deaths. 



" (4) The minority of breast-fed children not apparently 

 accessible to infection should receive explanation. 



"(5) There should be a closer correspondence of diarrhoeal 

 fatality with the number of flies than with any other varying 

 seasonal fact. 



" (6) Any other closely-corresponding seasonal fact should be 

 capable of interpretation in terms of the number of house-flies. 



"(7) Any variation ft-om district to district in the annual 

 curve of deaths should be accompanied by a similar variation in 

 the curve of flies. 



"(8) It will be at once manifest when we come to enteric 

 fever that the house-fly plays but a minor direct part in the 

 production of the annual wave. Such part, however, should have 

 reference to the number of flies and of pre-existing centres of 

 infection. If it can be shown that that portion of the enteric 

 wave which is connected with flies changes from one period of 

 time to another in such a manner as to be explainable in terms of 

 flies, but not of meteorological conditions, the evidence in favour 

 of flies will be greatly strengthened. 



" (9) No other available hypothesis must be capable of 

 explaining the course of summer diarrhoea." 



For the purposes of this study daily counts of the flies cap- 

 tured at the different observation stations which were selected 



