CRITICISM OF FLY-CARRIER HYPOTHESIS 259 



In the investigation which Hamer (1908, 1909) carried out 

 in London with a view to determining the rektionship which 

 the presence of accumulations of refuse and offensive matter bears 

 to the fly nuisance, opportunity was afforded, and wisely taken 

 advantage of, to study the question of the possible relationship 

 of flies to summer diarrhoea. Hamer indicates what appears to 

 him to be a difficulty in the way of accepting this theory. He 

 states : " It should be pointed out that there are certain difficulties 

 in the way of accepting the thesis that the correspondence exhibited 

 in the curves (he refers to the fly curve and diarrhoea curve) affords 

 reason for concluding that flies and summer diarrhoea stand to one 

 another in relation of cause and effect. At the commencement of 

 the hot summer weeks, when the number of flies has begun to 

 show marked increase, the diarrhoea curve is rapidly rising. After 

 some weeks the number of flies reaches the maximum, and then 

 diminishes, and so, in almost precise correspondence, does the 

 amount of diarrhoea. A period is later reached, towards the close 

 of the hot weeks, at which the number of flies is still as markedly 

 excessive as at the earlier period when the amount of diarrhoea 

 was increasing, but at the later period the amount of diarrhoea is 

 declining; it even anticipates decline in the number of flies. If 

 the fly is to be regarded as the carrier of the organism which 

 causes diarrhoea, it might perhaps have been anticipated that at 

 the later period — the number of flies still being excessive and 

 infective material being then presumably more widely distributed 

 than ever before — the amount of diarrhoea, instead of showing early 

 and rapid decline, would still be increasing. It would almost 

 appear that the advocate of the ' fly-borne diarrhoea hypothesis ' 

 must necessarily fall back in support of his theory upon the 

 hypothetical organism, conveyed by the fly, which he may claim 

 is affected by temperature in such a way as to bring about 

 correspondence between the diarrhoea curve and the fly curve. 

 The very closeness of the correspondence between these two 

 curves may indeed from this point of view be thought of as 

 constituting a difficulty rather than a point in favour of the 

 hypothesis that summer diarrhoea is caused by flies." 



Against Niven's suggestion that one of the explanations of 

 the decline of the diarrhoeal curve while the number of flies still 



17—2 



