14 THEORIES 



with occasional modification from the fancy of each author, 

 the vegetable theory of miasm became almost an esta- 

 blished dogma of the schools, not often questioned until 

 the very time in which we live. Now, writers, dissatisfied 

 with the inexact condition of the subject, demand proofs 

 in favor of the marsh theory, which they cannot find; and 

 I may, perhaps, feel safe in asserting, that, at the present 

 day, few well-informed physicians accept the theory of 

 the miasmatists, as detailed by McCulloch. 



Whatever view may be taken of the nature of the pes- 

 tilential cause, it is usually most potent in places of a 

 moist and marshy character, such as are the borders of 

 lakes and rivers ; and in such places it commonly most 

 abounds, when accompanied by a luxuriant vegetation and 

 a high temperature. As heat, moisture, and vegetation, 

 so commonly attend the production of malarious influ- 

 ence, careless observers, naturally enough, believe the 

 action of heat and moisture upon the vegetation, to be 

 the efficient cause of miasm; while they refer to con- 

 trasts of temperature and moisture as exhibited by day 

 and night, as the exciting causes of the periodical fevers 

 of such places. 



Such conditions, predisposing and exciting, no doubt 

 cause such maladies ; but inquirers take very different 

 views of the mode of production, and of the immediate 

 agents concerned. Some conceive, as already stated, that 

 by decomposition, a predisposing poison is produced, suf- 

 ficient of itself often to excite disease, whilst dews and 

 change of temperature may occasionally precipitate or 

 ^determine an attack. Others think that the mephitic va- 

 jpors of marshes only enfeeble health, and thus enable the 

 obvious changes of heat and moisture -to excite disease, 

 - which they often produce without any such preparation. 



