136 SCIENCE PROGRESS 



here). What brought fame to him was his hypothesis that these filariae 

 may be carried out of the human blood by means of mosquitoes. This 

 notion was bitterly opposed — Heaven knows why, because at that time the 

 great continental parasitologists had proved the law of metaxeny, accord- 

 ing to which a species of parasite can occupy alternate hosts, and had 

 already given many examples in which the law holds good, at least amongst 

 the Metazoa. In or after 1858, Fedschenko, acting on Leuckart's sugges- 

 tion, had shown that embryos of the Filaria medinensis of men, when emitted 

 into water, attack and enter certain species of Cyclops. Manson, who was 

 aware of this law, evidently concluded that a similar metaxeny probably 

 occurred with the F. bancrofii and mosquitoes. In the Lancet of January 12, 

 1878, Cobbold mentioned a suggestion of Bancroft himself dated April 20, 

 1877, that F. bancrofii may be so carried, but at the same time stated that 

 Manson had actually proved the first steps of the transformation. I have 

 not been able to ascertain whether the original suggestion came from Manson 

 or from Bancroft, but really the whole matter was a development of the 

 previous work of the parasitologists. The arguments which led Manson 

 first to conjecture that the mosquito is the host of the F. bancrofii were 

 both sound and profound, but were of the same class as the inductions 

 employed by Kuchenmeister and Leuckart ; and the stages of the life- 

 cycle of F. bancrofii in mosquitoes wliich he subsequently found were very 

 closely similar to those of F. medinensis in Cyclops. Both Fedschenko 

 and Manson carried the development only to a short stage. It is remark- 

 able that at that time and until my malaria work of 1898 Manson thought 

 and taught that mosquitoes suck blood only once, and die in four or five 

 days afterwards on the surface of water in which they have laid their eggs 

 — although really the full development of F. bancrofii in them takes several 

 weeks ; and this extraordinary error prevented his tracing the young filarise 

 into the mosquito's proboscis, as was done later after my malaria work, 

 and it vitiated his conclusions. Nevertheless we, who do so little to support 

 our genuine investigators, should be duly thankful to his memory for what 

 he did. 



After he had returned to England Mr. H. G. Plimmer showed him the 

 parasites of malaria (about 1894, I think). At that time I had been work- 

 ing on malaria for some years in India with the view of finding how the 

 disease is communicated to men. Malaria is caused by the parasites called 

 Plasmodium, discovered by A. Laveran in 1880 ; but I had failed to find 

 them in cases in India owing largely to certain mistakes made there by various 

 observers regarding them. In 1894 I came home on leave, and Manson 

 demonstrated the true parasites to me in London. These organisms undergo 

 a remarkable change when the blood containing them is drawn from the 

 patient's finger, and only then. Near the end of 1894 Manson, whom 

 I was then frequently seeing, informed me that he had just conjectured 

 that this change was meant by nature to occur'in the stomach of mosquitoes 

 — a theoretical induction which had escaped Laveran and all other observers. 



It should be clearly explained, in view of many statements frequently 

 made, that the mosquito theory of malaria was not originated by Manson but 

 is a very old one, dating possibly from the time of the Romans and certainly 

 mooted much more recently by King in America in 1883, by Laveran in 

 1883, S'l^d by Koch in 1884, ten years before Manson thought of the same 

 thing. But the argument for the theory which Manson gave was a very 

 powerful addition to those previously employed ; and I therefore deter- 

 mined to test the matter by experiment on my return to India in 1895. 

 Manson's induction was one of the profoundest ever made in medical 

 science ; but it indicated only some suctorial insect as the carrying agent 

 of malaria, and gave us no clue as to how or where the parasites live 

 in that carrying agent, nor what its species might be. There are many 



