MYZOMYIA ROSSII AS A MALARIA CARRIER. 283 



the probability that earlier experiments were unsuccessful because they 

 were made with insects taken from fresh water. It may be presumed with 

 some certainty that the liabilitj' to malarial infection is variable in the 

 same anopheline species. We are quite in the dark when we attempt to 

 point out the causes influencing this liability. Without any known 

 reason we sometimes succeed, time after time, in our infection experiments, 

 and then again, under apparently similar conditions, we are entirely 

 unsuccessful. 



There are districts where formerly malaria was extremely prevalent but 

 where now the disease is rarely encountered (as in Tuscany), notwith- 

 standing the fact that there is no decrease in the number of anophelines, 

 and malarial subjects are imported into these regions from other parts, 

 a fact which would lead one to expect an increase of malaria. Where no 

 immunity has set in among the inhabitants, the causes of this phenomenon 

 must be sought in a state of immunity in the prevailing anopheline 

 species, arising from outward circumstances which did not exist before. 

 A knowledge of the causes of such immunity might be a powerful weapon 

 in combating malaria in epidemic form. 



The results obtained from these few experiments seem to me to point 

 a way which offers a chance of determining one of these causes with 

 certainty. Finally, they confirm what has been shown by the experiments 

 of Gualdi and Martirano and of Schaudinn, nameh r , that the gametes 

 from blood containing quinine develop to oocysts in the stomach of the 

 anopheles mosquito. 



