158 GNATS OR MOSQUITOES — CHAPTER VIII 



while some of the most deadly spots in the world have but 

 a scanty population. The reason why the conditions of 

 urban life are unfavourable to the spread of malaria will be 

 dealt with further on, and it is sufficient to point out that 

 the reason why such tracts of country as the notorious 

 Indian Terai are so deserted is that they are too malarious 

 for human occupation. In other words, the local conditions 

 are so favourable to the multiplication of the species of 

 mosquito concerned in the transmission of the disease that 

 the presence of a very small number of infected persons 

 suffices to infect an enormous number of Mosquitoes, and 

 to render well nigh certain the infection of any visitor who 

 exposes himself to the same conditions. Moreover, every 

 place of the sort that I have heard of has always been a 

 tract of close jungle or swamp, in which it is impossible 

 for the traveller to stray from certain beaten tracks, or to 

 make his camp elsewhere than at certain definite halting 

 places, where, however small may be its number, there is 

 always a permanent population, or at the least, passers 

 through are sufficiently numerous to maintain the infection. 

 That in such deserted tracts these foci of intense malaria are 

 purely local I do not entertain a doubt, and that a healthy 

 man who landed from a balloon a mile or two away from 

 them would, though equally pestered by Mosquitoes, take no 

 other harm ; but in such country man can push his way 

 but slowly, and there is always time for those who attempt 

 to open it up to carry the infection with them. 



Moreover, in comparing the relative salubrity of neigh- 

 bouring places, it must be remembered that a certain tem- 

 perature is essential to the development of the parasite in 

 the body of the Mosquito, and that on this account a 

 difference of a few thousand feet above the level of the sea 

 is quite sufficient to account for a place being quite healthy, 

 though but a few miles of horizontal distance from foci of 

 intense virulence. It is this factor that is overlooked by Mr. 

 Guy Marshall, in his paper on " Mosquitoes and Malaria," 

 in the Entomologist, August, 1900, p. 218, who points out 

 that Salisbury, in Mashonaland, though comparatively 

 thickly populated, is much less malarious than the sparsely 



