MALARIAL PARASITES 149 



discovered a parasitic " germ " which he proved to be the true 

 cause of malarial fevers. Dr. King, of Washington, in 1883 

 suggested the probability of malaria parasites being spread by 

 mosquitoes, adducing much circumstantial evidence in support 

 of his views. It was not until 1898, however, that Sir Ronald 

 Ross, an Englishman in the Indian Medical Service, experiment- 

 ally proved that the malaria parasite is absolutely dependent 

 upon certain species of mosquitoes for its transmission from man 

 to man. Only six years ago (1911) the parasites of malaria were 

 first successfully cultured outside the human body by Bass and 

 Johns at New Orleans, a feat which will eventually lead to new 

 and valuable discoveries. Other workers deserve no less credit, 

 perhaps, for suggestive ideas, or for additional facts concerning 

 the life and control of the malarial parasites. The ultimate 

 results of their discoveries have only begun to be felt, but al- 

 ready such enterprises as the building of the Panama Canal have 

 l)een rendered possible. The Canal could never have been built 

 under the old regime of medical ignorance. Statues of the 

 pioneers in the work of unraveling the truths about malaria 

 and yellow fever might well have occupied conspicuous places 

 at the Panama Pacific International Exposition at San Francisco. 



Malarial Parasites. — Malarial fevers, of which there are 

 several different kinds, we now know to be caused by protozoan 

 parasites which live at the expense of the red blood corpuscles, 

 and are injected into the human body and transmitted from 

 person to person only by the bite of certain species of mosquitoes. 



The malarial parasites belong to the protozoan class Sporozoa, 

 or spore animals, so called from their habit of reproducing by 

 breaking up into a number of small parts or spores, instead of 

 simply dividing into two as do most of the Protozoa. All of the 

 Class Sporozoa are parasitic and have no organs of locomotion 

 when full grown. Although there are many different kinds 

 which live as parasites in other animals, ver3^ few normally attack 

 man and only the malarial parasites, belonging to the genus 

 Plasmodium, are of primary importance. There is still consider- 

 able disagreement as regards the classification of the human 

 malarial parasites. Nearly all workers on the subject agree 

 that there are at least three well-defined species of Plasmodium 

 causing human malaria, and there is some ev^idence that distinct 

 subspecies or varieties of some of these occur. The commonest 



