THE COMING OF EVOLUTION 199 



most careful experiments, carried out by the inocula- 

 tion of living animals, can their properties be investi- 

 gated. The hosts themselves sometimes remain un- 

 affected by the invading microbe, and this immunity 

 makes it exceedingly difficult to forecast the direction 

 in which the source of infection must be sought. The 

 final conquest of malaria or ague is an excellent ex- 

 ample of the difficulties and dangers which surround 

 research in microbic disease. Malaria, as the name 

 shows, had always been associated with the bad air 

 of swampy districts and undrained tracts of country. 

 Many ingenious theories had been constructed and 

 experiments undertaken, in order to show the connec- 

 tion between the poisonous gaseous exhalations of 

 the marshes and the fever that they were assumed to 

 produce. The most convincing proof of the connection 

 lay in the fact that those men who undertook to study 

 the disease in its favourite localities invariably fell 

 victims to the infection. At last, the possibility of 

 a secondary dependence only on the swampy nature 

 of the district occurred to some of the investigators, 

 and volunteers were found ready to spend the most un- 

 healthy season of the year among the Italian marshes, 

 having first provided themselves with wire and gauze 

 protections against the insect life of the district. 

 The experiment was successful : it was clear that the 

 marsh alone was not the primary cause of the disease. 

 The germ of malaria was discovered by Laveran, 

 a French army surgeon, about 1880. Five years 

 later, Italian observers had shown that infection 

 reached man from the bites of mosquitoes, and in 

 the years 1894-7 Manson and Ross proved that one 

 special kind of mosquito (Anopheles] was infested 



