PART I.] Current Theories as to the Causation of Cholera. 289 



■within the bodies of those suffering from the disease. It holds that the drinking 

 ■water determines prevalence, both by its effect on personal susceptibility, and by 

 being the essential medium in which the specific poison is developed. In any case, 

 the quality of the drinking water of a locality is regarded as the essential determinant 

 of the fluctuations in the prevalence of cholera in it at different periods. Any 

 fluctuations of prevalence in localities in which cholera is endemic are regarded as 

 depending on corresponding fluctuations in the quality of the water, increase and 

 decrease in prevalence being due to increase and decrease in the impurity of the 

 water. 



4^/i. — The theory which regards the soil as the essential site of the processes 

 resulting in the production of the material inducing cholera. The majority of the 

 adherents of this theory regard the air as the vehicle, by means of which the 

 material produced in the soil is conveyed to human beings, so as to cause the disease 

 in them ; but it is obvious, that questions of vehicle are really subordinate here, and 

 that materials manufactured in the soil may reach the human subject by more than 

 one path. According to this view, fluctuations in prevalence of cholera are dependent 

 on corresponding fluctuations in the condition of the soil, which may act either 

 on the manufacture or the diffusion of the specific materials. 



We have now to inquire how far each of these theories is capable of accounting 

 for the phenomena of seasonal fluctuations exhibited by the prevalence of cholera in 

 Calcutta. 



\8t. — The theory of direct contagion. It may be deemed superfluous, at the 

 present day, to enter into the serious consideration of this doctrine, still as it is yet 

 advocated by some whose opinions are entitled to respect, it is necessary to determine 

 the bearing of our data upon it. The normal course followed by the fluctuations in 

 the prevalence of cholera in Calcutta is not merely inexplicable by this theory, but 

 is strongly opposed to it. If prevalence were dependent on direct contagion, the 

 maximum prevalence should occur at those times of year when the population is 

 most liable to close association. As a fact, the maximum prevalence in Calcutta 

 occurs during a period when there is less crowding together, or close association 

 of the population, than in either of the periods immediately preceding and following 

 it. The native population is more crowded together during the height of the cold 

 weather and of the rains than at any other time of year ; for it is then that the 

 people are obliged — by the temperature in the one case, by the rainfall in the other 

 — to pass the nights packed together in their houses, instead of spending them 

 largely in the open air. According to this theory, the prevalence of cholera ought 

 to attain its maximum in December and January, and in July and August ; but, in 

 place of this being the case, these are precisely the periods during which prevalence 

 of the disease is at its lowest ebb, 



"Ind. — The " water-theory," as ordinarily understood. This theory, also, fails 

 to explain the phenomena of seasonal fluctuation in prevalence, or to gain any 



