DISTRIBUTION AND HABITAT OF THE BACTERIA. 709 



siderable distances than in finely porous soil ; and this 

 must be the result, more especially when copious rains 

 thoroughly soak the soil, and when there is no drying zone. 

 In such a grossly porous soil the drying zone assumes 

 a special importance ; for it is only while it exists that 

 there is any chance of the bacteria being transported by 

 winds, or by men or things. Finely porous soil, on the 

 other hand, keeps back the bacteria in the superficial 

 layer to a much greater extent, and even possibly when 

 the dry zone is absent ; in this case the bacteria may be 

 distributed, not indeed by winds, but in other ways, for 

 example, by contact with man, &c., even when the sur- 

 face is moist, and hence the difference between the dry 

 and the moist stage is not so sharply marked in finely 

 porous soil. 



We must, however, for the present leave it to experi- 

 mental investigations made with bacteria to confirm or 

 to correct these ideas, and to give us definite information 

 as to the special disposition of the individual kinds of 

 soil for the spread of infective germs. 



From what has gone before we can at all events come Resume. 

 to the definite conclusion that the soil does not lead 

 to any special ripening, or even to active multiplication 

 of the pathogenic bacteria, but on the other hand, that 

 it possibly aids in their preservation, and in the distri- 

 bution of the bacteria so preserved. Both preservation 

 and distribution are probably subject to local and seasonal 

 variations ; for it is only a porous soil, and only at a time 

 when a superficial dry zone exists, and where the ground 

 water is as a rule sinking, that a preservation and more 

 especially a distribution of the infective germs can take 

 place. 



By this property of preserving the pathogenic agents influence ot' 

 in an unaltered condition, and allowing their return oAfeeoU on 8 

 again under certain conditions in large numbers to the 

 surroundings of man, the soil probably takes part in the disease 

 spread of many infective diseases ; and the dependence 

 of these properties of the soil on local and seasonal in- 

 fluences will ultimately find its expression in the local 

 and seasonal variations of many epidemics. The most 



