170 GNATS OR MOSQUITOES — CHAPTER VIII 



The case too of the French transport " Argo," which has 

 so often been quoted as an example of malarial infection 

 through the agency of water, has been conclusively shown 

 by Celli (CM., p. 95) to have been probably an outbreak of 

 acute poisoning, but certainly not malaria. Putting aside 

 then the possibility of water acting as the direct vehicle of 

 infection, let us see how far the prevalence of malaria is 

 indirectly influenced by its agency. 



The presence of water is absolutely essential to the 

 multiplication of Mosquitoes, and further, it is now fully 

 recognised that it is not large bodies of water, such as 

 marshes, lakes and rivers, that form their favourite nurseries, 

 but small stagnant collections such as puddles and pools of 

 no great size that are favoured by them. 



Further, these puddles must be sufficiently permanent to 

 persist for at least ten days. It is therefore the surface 

 distribution of water that is primarily of importance ; but as 

 this is largely influenced by the disposition of the ground 

 water the study of the depth and movements of the latter is 

 of the greatest importance, and the prevalence of malaria is 

 therefore influenced by this in exactly the same way as if it 

 were a truly water-borne disease. Of the older generalisa- 

 tions on the subject, perhaps the only one that can be said 

 to have stood the test of recent knowledge is the often 

 demonstrated fact that localities where the ground water 

 lies close to the surface are generally malarious. The 

 reason of this is that in such places the surface is easily 

 saturated and badly drained ; because, wherever the ground 

 water lies high, it is usually also either sluggish or stagnant. 



Now a marsh is usually a place where the ground water 

 comes 10 the surface, the level of the soil being below that 

 of the ground water of the higher land around it ; but the 

 marsh itself, per se, is rarely concerned in favouring the 

 multiplication of Mosquitoes, the larvie of which are only 

 very rarely to be found in such situations. It is the stretch 

 of country immediately surrounding the marsh, where the 

 ground water level is not below, but almost coincides with 

 that of the soil, that is the true focus of " marsh miasmata," 

 for in such places the smallest depression must needs 

 remain a puddle, in spite of the most rapid evaporation. 



