166 GNATS OR MOSQUITOES — CHAPTER VIII 



Hill stations are, of course, as a rule free from malaria, 

 but the reason of this is not that because the barometer 

 stands some inches below the instrument in the plains, but 

 because they are cool and well drained. Given other favour- 

 fible conditions, and malaria will develop at any elevation, 

 and as a matter of fact, the writer has observed and treated 

 undoubted malaria in Wakham in the upper Oxus Valley, at 

 an elevation of 9,000 feet above the sea, among the Tajik 

 tribesmen who rarely or never visit places at a lower level. 



The Influence of Water. — In a certain sense, the 

 malariousness or otherwise of any locality is intimately 

 connected with its water supply ; but the question is one 

 of its relative abundance and distribution, and not of its 

 quality. As has already been pointed out, it is, for biologi- 

 cal reasons, in the last degree improbable that there can 

 exist any alternative route of malarial infection than that 

 through the Mosquito, and it was not proposed to enter here 

 into the question on the possibility of drinking water form- 

 ing a vehicle of infection ; but old fallacies die hard, and as 

 an attempt has recently been made by Captain Leonard 

 Eogers, I. M.S., to resuscitate this one, it maj^ be well to 

 devote some space to the consideration of the arguments 

 brought forward by him. Captain Kogers attempts to 

 show that those parts of Calcutta and neighbourhood which 

 receive a filtered water supply are less malarious than 

 neighbouring suburbs which draw their supply from the 

 river, from tanks, or from wells. He estimates the relative 

 malariousness of the compared localities by what he terms 

 their spleen ratio, a factor which he determines by counting 

 the number of enlarged spleens in a given number of persons 

 whom he regards as fairly representative of the general 

 population. 



Now such a method of estimation is obviously open to a 

 variety of sources of fallacy, and is, at least, misleading. 

 As pointed out by Drs. Christophers and Stephens, and 

 confirmed by Koch, the true index of the malariousness of 

 a place, is the average length of time required for the in- 

 fection of new comers ; and the most convenient class of 

 immigrant, because always ready to hand, are the young 



