6 Major Ronald Ross [March 2, 



has been shown to be not entirely original. Nuttall, in his admirable 

 history of the mosquito theory, demonstrates its antiquity. Eleveu 

 years before Manson wrote, King had already accumulated much 

 evidence, based on epidemiological data, in favour of the theory. A 

 year later (1884), Laveran himself briefly enunciated the same views, 

 on. the analogy with Filaria nocturna. Koch, and later, Bignami and 

 Mendini, were also advocates of the theory partly on epidemiological 

 grounds and partly because of a possible analogy with the protozoal 

 parasites of Texas cattle-fever which Smith and Kilborne had shown 

 to be carried by a tick. Hence many observers had independently 

 arrived at the same theory by different routes. But I feel it most 

 necessary to point out here that there is a difference between a 

 fortunate guess and a true scientific theory. Interesting and sug- 

 gestive as were many of the hypotheses to which 1 have just referred, 

 they were to my mind far from convincing. Filaria nocturna, and 

 even Apiosoma bigeminum, are not in close enough relationship with 

 the Haeuaamoabidse to admit of very forcible analogies in regard to 

 the respective life-histories. The epidemiological arguments of King 

 and Bignami (some of which were also used by Manson) were scarcely 

 solid enough to support by themselves a theory of any weight. All 

 these were hypotheses little more: I can scarcely conceive a prac- 

 tical man sitting down to laborious researches on the strength of 

 arguments like these. On the other hand, Manson's theory was what 

 I have called it an induction a chain of reasoning from which it 

 was impossible to escape. 



I have wished to defend this work of Manson's because it has been 

 much misunderstood and much misrepresented, and even (in a 

 somewhat amusing manner) completely ignored by some who, though 

 they once strongly opposed his theory, now, as soon as it has done 

 its work, wished to forget it. It is true that he endeavoured to 

 predict the history of the parasites a little too far, and that he was 

 in error (as will presently appear) regarding the immediate nature of 

 the motile filaments ; but the core of his theory was invaluable. I 

 have no hesitation in saying that it was Manson's theory, and no 

 other, which actually solved the problem ; and to be frank, I am 

 equally certain that but for Manson's theory, the problem would 

 have remained unsolved at the present day. 



Dr. Laveran's theory was unfortunately enunciated with great 

 brevity ; but it appears to me to have been really founded on many 

 if not all the arguments independently advanced by King and Manson. 

 To him we owe not only the discovery which made all these re- 

 searches possible ; but also an early and correct prediction as to the 

 future life-history of the organisms with which his name will be 

 inseparably connected. 



To leave these interesting theories and to return to actual observa- 

 tions I should begin by remarking that Manson thought the motile 

 filaments to be of the nature of zoospores that is, motile spores 

 which escape from the gametocytes in the stomach cavity of the gnat, 



