236 MOSQUITOES OF NORTH AMERICA 



in certain years. In these years of malaria it was found that the sirocco, a hot 

 wind from the desert, blew continuously for nine or ten days. Sergent reaches 

 the conclusion that usually the very low night temperatures in these localities 

 inhibit the development of the malarial plasmodii within the mosquito; the 

 continuous high temperature during the sirocco, however, brings about the de- 

 velopment of the parasites. 



Recently Hehir, in India, has confirmed the observations of Hirsch that in 

 India and Ceylon malaria occurs up to altitudes of 2000 meters. 



Eeferring again to the United States, it may briefly be stated that malaria 

 rarely exists in this country above an elevation of one thousand feet or in the dry 

 climates of certain of the western and southwestern States, except where irriga- 

 tion has been introduced. This is quite to be expected, from the well-known 

 habits of the species of Anopheles that carry the disease in this country. There 

 seems to be little doubt, moreover, that malaria has practically died out in many 

 localities where once it was rife. In the early settling of the Central and 

 Western States, malaria was one of the most serious obstacles in the path of the 

 pioneers, but with the gradual improvement of agriculture, with the cutting 

 down of the trees in swampy forests, increased value of land for agriculture and 

 the consequent reclamation of swamps, the disease has disappeared in many 

 wide-spread regions; yet Anopheles still retains its foothold in many of these 

 regions, although in vastly smaller numbers than in former years. Over other 

 large regions the disease, while vastly less in relation to the population, still 

 exists and is carried from one person to another by the reduced supply of 

 Anopheles. In such large regions there are often smaller areas, where, through 

 local breeding-places, a large supply of Anopheles is kept up and the disease 

 maintained at a higher rate through their instrumentality. 



THE APPEARANCE AND DISAPPEARANCE OF MALARIA. 

 In the section on economic loss from mosquitoes we have referred to the fact 

 that the development of the United States in many sections has been retarded 

 for years by the prevalence of malaria. The army of pioneers pushing west, 

 stopping at various apparently favorable spots, clearing farms and settling for 

 a time at least to build homes, encountered malaria in many places. Some of 

 them were killed off or disabled by this disease, others, discouraged, were 

 driven back east, while others pushed further west. But in very many of the 

 localities where malaria was once existent in severe form, it has gradually 

 lessened more and more, until localities once extremely unhealthy are now among 

 the healthiest regions of the countiy. This means the gradual disappearance of 

 malaria under the influence of civilization. We shall briefly discuss this class of 

 phenomena from the viewpoint of the breeding places of malarial mosquitoes, 

 and will show by contrast that under other conditions advanced civilization 

 may bring malaria into localities in which it formerly did not exist. 



DECREASE OF MALARIA FOLLOWING SETTLEMENT AND CIVILIZATION. 



It has been shown in our discussion of the geographic distribution of malaria 

 that the disease formerlv flourished in Great Britain, and it is well known that 



