524 Introduction to the Study of Science 



About that time Laveran discovered that malaria is caused 

 by a very minute parasitic animal, called plasmodium, of the 

 class of sporozoa belonging to the protozoa. This plasmodium 

 infests the blood, consuming the red corpuscles and filling the 

 circulatory system with its offspring and with waste products. 

 Golgi, 1 an Italian scientist, discovered many, but not all, of 

 the essential stages in the life history of the parasite. He failed 

 to find some of the most important stages in the human blood, 

 and he was forced to conclude that its life history is not com- 

 pleted there but elsewhere. The question then was : If not in 

 the human circulation, where do the necessary changes take 

 place ? This involved such questions as these : How is the 

 parasite removed from man? Where does it develop through 

 the missing stages, and how is it introduced again into man? 

 Or it may be inquired, How is the germ transmitted from a 

 malarial patient to a healthy individual ? 



Discovery of the missing facts was necessary in order to master 

 the disease and to block the ways of its transmission. 



255. The work of Ross. Ronald Ross, a surgeon in the 

 English Army in India, attacked the problem. He accepted 

 the hypothesis suggested by Sir Patrick Manson, the greatest 

 authority on tropical diseases, that the malarial parasite 

 is distributed by blood-sucking insects, and that the missing 

 stages in the life of the parasite take place in the body of the 

 insects. Ross set out to prove or disprove this hypothesis. 



For more than two years he searched patiently, examining 

 thousands of mosquitoes ; but he found nothing to support the 

 belief that mosquitoes transmit the malarial parasite. Failure 

 seemed inevitable. It happened, however, that he had been 

 examining only one species, the Culex, perhaps because all mos- 

 quitoes were then considered the same. 



1 The current classification of the different kinds of malaria was made by 

 Golgi, who also named the three varieties of malarial parasite that he discovered. 

 The plasmodium malarias causes the four-day or quartan type of the disease ; 

 the plasmodium vii-ax, the three-day or tertian type ; and the plasmodium falci- 

 parum, the remittent or recurrent type. 



