viii CONQUEST OF DISEASE 227 



a part of their existence in the bodies of mosquitoes, 

 which carry them from one person to another. When in 

 the blood of a human being the parasites are in a certain 

 stage of development, but they can only complete their 

 life-cycle in the body of their insect host. 



To Sir Eonald Ross belongs the honour of tracing the 

 various stages of the existence of the parasite in the 

 body of the mosquito until it was ripe for injection into 

 a human being by the bite of the insect. He proved by 

 numerous experiments that the only means by which 

 a healthy person can acquire malaria is by the bite of a 

 mosquito which has previously bitten someone whose 

 blood contains the particular organisms associated with 

 the disease. In other words, if there were no mosquitoes 

 of the kind required by the malarial parasites to com- 

 plete their life-cycle, there could be no malarial fever. 

 On the eve of this remarkable discovery, Ross offered 

 up a prayer of thanks which makes a beautiful supple- 

 ment to the lines written several years before : 



This day relenting God 



Hath placed within my hand 

 A wondrous thing ; and God 



Be praised. At His command, 



Seeking His secret deeds, 



With tears and toiling breath, 

 I find thy cunning seeds, 



O million-murdering Death. 



I know this little thing 



A myriad men will save. 

 O Death, where is thy sting ? 



Thy victory, O Grave ? 



The cause of the disease having been found, the remedy 

 was evidently to stamp out the mosquito, so far as 

 possible, by searching out its breeding-places and 



