LIMITATION. 99 



morbific agent whose action is so uniformly and irregu- 

 larly postponed. 



Nothing more startles the student, who has been taught 

 to believe in marsh or other exhalations as being the grand 

 cause of autumnal diseases, than when told, that often a 

 low wall, a common road, or a screen of trees, can, and 

 does, arrest the progress of marsh miasmata, though the 

 wind from the marsh whistles freely past them, bringing 

 with it even the paludal odor. He is also told by McCul- 

 loch, the great advocate of the vegeto-aerial theory, that 

 sometimes agues prevail exclusively on one side of a 

 street, and that inch by inch, and foot by foot, the site 

 of the Roman capital is invaded by malarious diseases. 

 The absurdity and inconsistency of these various posi- 

 tions strike at the very root of all the old theories. 

 On the other hand, when we suppose that the poison is a 

 fungous one, progressively marching over the soil, sus- 

 tained by the rich air and pregnant moisture from the 

 marsh', we can readily suppose that the wall, or the road, 

 or the wood, may limit its progress. Besides this, the 

 spores of all fungi are more or less electrical, and are, 

 therefore, likely to be arrested by the trees of a wood. 



Authors have admitted that malaria appears to act in 

 many instances as if it could exert no power, except when 

 close to the spot where it originated, whilst in other cases, 

 it seems to be wafted to a great distance from its appa- 

 rent source. If we suppose the existence of germs sus- 

 ceptible of reproduction, and progressive growth, these 

 seeming contradictions fall at once. The interruption of 

 progress by a road or wall justifies this view of the mode 

 of conveyance, and the many facts which show the narrow 

 limits of the poisonous activity, enforce it strongly. The 

 place, the very spot, where the disease is found, must re- 



