MALARIA AND MOSQUITOES 63 



ing- to the districts where ag-ue was formerly preva- 

 lent. 



4. Since the disappearance of ague does not depend 

 upon the extinction of Anopheles, it is probahly due to 

 several causes operating together : 



(«.) A reduction in the number of these insects conse- 

 quent upon drainage of the land, this being in accord 

 with all the older authors, Avho attributed the disappear- 

 ance of ague largely to this cause. 



{h.) Keduction of the population in infected districts as 

 the result of emigration, about the time when ague disap- 

 peared from Eng-land. This would naturally reduce the 

 number of infected individuals, and thus lessen the 

 chance of the Anopheles becoming- infected. 



(c.) It is possible that the use of quinine has reduced 

 the chances of infecting the Anopheles, through checking 

 the development of the parasites in the blood of subjects 

 affected with ague. 



Of these, the first-mentioned cause seems to have been 

 chiefly operative. The possibility is not yet excluded of 

 there being another intermediary host besides man, capa- 

 ble of harboring the parasite, and, assuming that this 

 were so, this host may have become extinct in the low- 

 lands where it is known that the fauna and flora have al- 

 tered. 



5. The coincidence of the geographical distril)utiou of 

 ague and Anopheles as claimed by Grassi for Italy, and 

 as probably holding good for other parts of the world, is 

 hereby disproved for England, and consequently the gen- 

 eralizations are proved to l)e premature whereby he ex- 



