30 THEORIES 



"It would appear that the materials of vegetation abound- 

 ing in excess, acted upon by a powerful cause, give out a 

 principle, which, not being expended on the growth and 

 nourishment of plants, is diffused to a certain extent in 

 the atmosphere, causing a derangement of such bodies as 

 come within the sphere of its action." 



Mr. Doughty offers a modification of this sentiment, in 

 the supposition that by the separation of their nutrition 

 from the soil, especially when their growth is very rapid, 

 plants cause in the earth new combinations of rejected 

 elements, which thus become aerial, and poison the neigh- 

 boring atmosphere. As many highly malarious places 

 are barren, and naked of apparent vegetation, the theory 

 of Jackson falls at once to the ground. But if not, then 

 is there the additional difficulty of explaining by such a 

 cause, the existence of malarious diseases when the season 

 of active phenogamous vegetation has passed. It is also 

 a pure hypothesis unsustained by facts or reasoning. 



The theory of Ferguson is received now by the profes- 

 sion more favorably than perhaps any other. It narrows 

 the malarial question down to this, that the only condi- 

 tions essential to the production of miasmata are soil and 

 water, especially a porous soil. And the only relation 

 . between these elements is, that of successive moisture and 

 dryness. Stating it in the words of Dr. Watson, of Lon- 

 don: " There is reason to believe that the flooding of 

 a porous earthy surface with water, and a subsequent dry- 

 ing of that surface, under a certain degree of heat, consti- 

 tute the sole or main conditions of the generation of the 

 poison." 



If these are the sole conditions, only moisture can come 

 from the soil, for if anything else does, it must be a miasm, 

 and we revert to the old opinion of Lancisi, If only moist- 



