620 Insects and Disease 



long quotation from Ross, taken from a lecture delivered by him on March 2, 

 1900, before the Royal Institution of Great Britain, gives a detailed account 

 of this work, answers both the questions asked above, and at the same time 

 serves to reveal a typical instance of the faith and persistence of the men 

 to whom we owe scientific progress. 



"It was reserved for Manson," says Ross, "to detect the ultimate (though 

 not the Immediate) functions of these bodies [the motile filaments]. 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 w-as convinced 

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

 only ajter abstraction from the blood (about fifteen minutes afterwards) 

 must have some definite purpose in the life-scheme of the parasites. What 

 is that puri)ose? It is evident that these parasites, like all others, must pass 

 from host to host; all known parasites are capable of not only 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 bril- 

 liantly 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 filaments appear 

 onlv 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 williin the stomach-cavity oj some suctorial 

 insect, and constitutes the first step in the lije-history oj the parasite outside 

 the vertebrate host. 



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

 revolving the subject, to understand the full value and force of this remarkable 

 induction. To my mind the reasoning is com])lete and exigent. It was 

 from the first impossible to consider the subject in the light which Manson 

 placed it without feeling convinced that the parasite requires a suctorial 

 insect for its further development. .\nd subsequent events have proved 

 Man.son 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 considers the subject at all, and which it is not necessar}' to 

 discuss in this place. 



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

 one it has been shown to be not entirely original. Nutlall, in his admirable 

 history of the mosquito theory, demonstrates its antiquity. Eleven years 



