CONDITIONS INFLUENCING PREVALENCE 161 



unknown in really cold countries, while, given the presence 

 of the malarial blood parasite, no hot climate is free from the 

 disease. If this be absent, there can of course be no malaria, 

 as the disease is no necessary concomitant of a tropical 

 climate, as is shown by the case of the Island of Mauri- 

 tius, which, previously healthy, suddenly became intensely 

 malarious. In this case there can be little doubt that 

 Mosquitoes of the suitable species were already present, for 

 the endemic developed with a rapidity quite inconsistent 

 with the idea of the establishment of the necessary species 

 from a few chance emigrants. All that was required was 

 the importation of the infected man ; and of this, before the 

 introduction of steam navigation, there was but a precarious 

 chance. 



It has usually been suggested that the disease was in this 

 instance imported from India, but the type of the disease 

 seen there is very different. Never in India have I met 

 with cases exhibiting the absolutely classical malarial 

 paroxysm such as I have witnessed in soldiers sent to 

 Natal for change of air from Mauritius. 



In other cases the immunity of places where every con- 

 dition of climate is favourable is due to the absence of 

 Mosquitoes. 



In a letter to the Lancet, dated January 18th, 1901, by 

 Mr. H. D. O'Neill, an interesting observation by Kobert 

 Louis Stevenson is referred to on the subject of Mosquitoes 

 and their association with filaria and malaria : "In Atuona 

 (Marquesas Islands), a village planted in a shore-side marsh, 

 the houses standing everywhere intermingled with the pools 

 of a taro-garden, we find every condition of tropical danger 

 and discomfort, and yet there are not even Mosquitoes, nor 

 even the hateful day-fly of Wuka-Niva, and fever and its 

 concomitant, the island fe'efe'e, are unknown." 



It is a long- established fact that the northern limit of 

 malaria corresponds roughly with the summer maximum 

 isotherm of 76° F., or, according to Hirsch, to a mean 

 summer temperature of 15° — 16' C. (60° F.), which is much 

 the same thing. Recent Italian researches show that the 

 development of the hsemosporidia within the Mosquito 

 11 



