SOILS 383 



We now know there are no such things as malarial 

 vapours, and that though the above soil conditions asso- 

 ciated with malaria still exist, yet they are not the cause 

 of the disease, but only the breeding and feeding ground of 

 the insect that carries the parasite, inoculation with which 

 produces the disorder. 



This example of recent research is given as showing how 

 liable one may be, even with moderately exact data, to 

 arrive at a wrong conclusion ; it is not the soil but the 

 mosquito which gives fever, but the mosquito requires as 

 its breeding ground a certain kind of soil, and for years 

 that soil has been recognised as unhealthy. 



It does, however, appear quite clear that apart from this 

 there are soils which are healthy and others unhealthy, 

 and the chief factor in deciding this point seems to be the 

 question of water in the soil. It is by no means certain how 

 damp buildings and damp soils act, but they seem to lower 

 the general powers of resistance ; otherwise it is hardly 

 possible in any other way to account for the reduction in 

 human tuberculosis as the result of draining towns and 

 cities. Eespiratory affections and rheumatism are well 

 known to be more prevalent in damp than in dry districts, 

 but why it should be so we are at present in ignorance. 



Great stress has in the past been laid on the question of 

 the air in soils, and the rise and fall in the level of the 

 ground water, as being the cause of certain diseases like 

 human enteric ; we shall presently study this question 

 and see how it stands, but our object here is to sound a 

 word of warning, and to suggest that unless we are prepared 

 to believe that pathogenic organisms come out of the soil 

 like a dust cloud, we ought to be careful in accepting as 

 evidence of soil implication what may only prove to be one 

 link in the chain of causation. 



Pollution of the earth occurs mainly from vegetable 

 debris and the excreta of men and animals ; in places like 

 towns and cities where the population is dense the soiling 

 is greatest, in some places, as in the old countries of the 

 world it has been going on for thousands of years, and yet 



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