164 GNATS OR MOSQUITOES — CHAPTER VIII 



cannot take place at a lower temperature than 20" C. 

 (68° F.), or at a higher than 30' C. (86° F.), and in the 

 existence of this upper limit we find an explanation of the 

 fact that the hot dry weather in northern India, where for 

 months together the temperature rarely falls as low as 

 this, is, in spite of the uuhearable heat, by far the healthiest 

 season of the year, and that during it, primary cases of 

 malaria are practically unknown. 



As in these parts of the Peninsula the winter temperature, 

 for considerable periods falls considerably below the lower 

 limit, extreme climates such as we have there, are blessed 

 with two consecutive periods of immunity, malaria being 

 confined to the season of the " rains " and early autumn. 

 Strictly speaking, however, Northern India is sub-tropical. 

 In the Tropics, and especially in the Equatorial region the 

 lower limit is practically never reached and the existence of 

 the higher is of far greater practical importance, as during 

 the drier portions of the year it is, except in certain specially 

 favoured localities, always surpassed for considerable periods. 

 As, however, in the truly Equatorial regions there are two 

 dry and two rainy reasons in each year, the periods of 

 immunity are too short to bring about any very distinct 

 diminution in the amount of sickness due to malaria, as their 

 occurrence is masked by recurrent attacks which always 

 continue to vitiate the statistics of any period of immunity 

 for a longer period than the entire duration of these short 

 intervals. 



While then in temperate climates the duration of the 

 winter is the most important climatic factor in securing a 

 prolonged period of immunity, in tropical and sub-tropical 

 regions it is the hot dry weather that exercises a sanitary 

 influence. 



Second only to temperature is the amount and distribu- 

 tion of rainfall. It is needless, however, to go into any 

 lengthened considerations of its effects, as these naturally 

 follow from the premise that malaria is dependent on the 

 Mosquito for transmission from man to man ; beyond 

 pointing out that besides being the main natural puddle- 

 producing agency it has the additional effect of keeping 



