1900.] on Malaria and Mosquitoes. 299 



constitute a stnge in the history of the parasite. Antolisei, Grassi, 

 Biguami, and others of the Italian school, fell back upon the old theory 

 — which we always like to employ when we cannot explain a pheno- 

 menon — that it is a regressive phenomenon, a disintegration of the 

 parasite due to its death in vitro. Here, however, the controversy 

 practically stayed. While the Italians, in conformity with their 

 views, attached no signification to the motile filaments, Laveran, 

 Danilewsky and Mannaberg, who held an opposite opinion, did not 

 expressly or exactly state what their signification is. Mannaberg, 

 indeed, held that they are meant to lead a saprophytic existence, but 

 did not explain how they could escape from the body in order to 

 do so. 



It was reserved for Manson to detect the ultimate (though not the 

 immediate) function of these bodies. He asked why the escape of the 

 motile filaments occurs only after the blood is abstracted from 

 the host (a fact agreed upon by many observers). From his study of 

 these filaments, of their form and their characteristic movements, he 

 rejected the Italian view that they are regressive forms ; he was con- 

 vinced that they are living elements. Hence he felt that the fact of 

 their appearance only after abstraction of the blood (about fifteen 

 minutes afterwards) must have some definite purpose in the life- 

 scheme of the parasites. What is that purpose ? It is evident that 

 these parasites like all others must pass from host to host ; all known 

 parasites are capable not only of entering the host, but, either in 

 themselves or their progeny, of leaving him. Manson himself had 

 already pushed such methods of inductive reasoning to a brilliantly 

 successful issue in discovering by their means the development of 

 Filaria nocturna in the gnat. He now applied the same methods to 

 the study of the parasites of malaria. Why should the motile fila- 

 ments appear only after abstraction of the blood ? There could be 

 only one explanation. The phenomenon, though it is usually observed 

 in a preparation for the microscope, is really meant to occur within the 

 stomach cavity of some suctorial insect, and constitutes the first step in the 

 life-history of the parasite outside the vertebrate host. 



It is perhaps impossible for any one, except one who has spent 

 years in revolving this subject, to understand the full value and force 

 of this remarkable induction. To my mind the reasoning is complete 

 and exigent. It was from the first impossible to consider the subject 

 in the light in which Manson placed it without feeling convinced that 

 the parasite requires a suctorial insect for its further development. 

 And subsequent events have proved Manson to have been right. 



The most evident reasoning — the connection between malarial 

 fever and low-lying water-logged areas in warm countries — suggested 

 at once that the suctorial insect must be the gnat (called mosquito 

 in the tropics) ; and this view was fortified by numerous analogies 

 which must occur at once to any one who cousiders the subject at all 

 and which it is not necessary to discuss in this place. 



Needless to say, since Manson's theory was proved to be right, it 



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