November 5, 19 14] 



NATURE 



271 



with marshes and stagnant water in warm countries. 

 Later, when it was seen that the disease is not con- 

 fined only to the proximity of marshes, the theorists 

 conceived that theie is a telluric poison which causes 

 malaria, and is especially abundant in damp places. 

 All this was a ver}- general proposition ; and in order 

 to prevent the disease, it was necessary- to undertake 

 very extensive drainage. Now, however, the new 

 knowledge obtained enabled us to particularise the 

 exact route of infection. We believe no longer that 

 the poison is spread uniformly in the air of warm 

 countries, but know that it is alwaxs contained in the 

 minute bodies of certain insects, and, more than that, 

 in the still more minute salivary glands of those 

 creatures. Here then, science had given us knowledge 

 which could not fail to be of immense practical import- 

 ance to the world. 



The discover}- of the full life-cycle of the parasites 

 enables us, not only to exercise the route of infection, 

 but to determine exactly which species of mosquitoes 

 are concerned. My failures with numerous undeter- 

 mined species of the genera Culex and Stegomyia had 

 shown that 'hese are probabh' innocuous as regards 

 malaria ; and my ultimate success with certain 

 Anophelines had show"n that these were inculpated. 

 Since then, the work of many observers has proved 

 that, out of about five hundred Culicidae only about 

 twent}- species carr\- malaria, .and that all these 

 belong to the Anophelines. So that for the prevention 

 of malaria we are not obliged to deal with mos- 

 quitoes in general, but only with particular species. 



Still further, the knowledge so obtained enabled us 

 to study exactly the habits of the culpable species. In 

 fact, I necessarily began such studies during the whole 

 of my researches. The genera Culex and Stegomyia 

 breed most commonly in artificial collections of water 

 round houses ; but I saw as early a^ 1897 that the 

 dappled-winged mosquitoes (Anophelines) breed prin- 

 cipally in different sites ; that is,chiefly in natural col- 

 lections of water, such as marshes, puddles, streamlets, 

 and the edges of lakes, ponds, and rivers. This had 

 been known before in the case of certain species; but 

 I now saw the great epidemiological and sanitary- 

 bearing of the phenomenon. The reason why malaria 

 is connected with marshes was now fully established 

 by quite independent work. Humanity had explained 

 the fact by supposing that the poison of malaria itself 

 rises from the marsh ; it was now seen that it is not 

 the poison itself that rises from the marsh but the 

 carrier of the poison. The net result was the same, . 

 t-xcept that we now knew not only the source of the 

 poison but the exact method of transference. After 

 ah, humanity had reached the truth by empirical ob- 

 servations made during thousands of years ; but 

 science, in confirming those obser\ations, has brought 

 them to a fine point of exact theory. 



But these were by no means the only fruits obtained. 

 To return to our militarA- parallel, when a command- 

 ing position of the enemy is carried, victon,- extends 

 to a large area round it. The principal function of 

 men of science is not merely to observe, desc^-ibe, and 

 catalogue phenomena, but, above all, to solve difficult 

 problems ; and the solution of one such problem fre- 

 quently gives us the solution of many more. Such 

 has proved to be true also in this case ; for so soon as 

 we had solved the malaria problem we were able to 

 apply the same theorem to a number of other diseases. 

 As already stated, Manson had shown in 1879 that the 

 embr\-os of F. hancrofii can live in certain species of 

 Culex — but he had not shown how they pass back 

 from these insects to man. Now, however, James and 

 Low, working independently, showed that the embryos 

 enter the insect's proboscis, thus suggesting that the}- 

 return into the human circulation by a route similar 

 to that which is used by the parasites of malaria, and 

 NO. 2349, vol/ 94] 



this work has been well followed up by Bahr and 

 many others. 



.Another discover}', concerned with one of the most 

 important of human diseases, namely, yellow fever, 

 was made by Reed, Carroll, Lazear, and Agramonte 

 during the last days of last centur}-. Without know- 

 ing the causative agent of that disease, they yet showed 

 by direct e.xperiments on human beings that the infec- 

 tion is carried directly from man to man by another 

 species of mosquito, Stegomyia fasciata, or calopus. 

 It had long been stated by epidemiologists that malaria 

 differs from yellow fever in that the former is con- 

 nected with damp and decaying vegetation and the 

 latter with insanitary conditions round houses. The 

 former hypothesis was verified by the observation that 

 Anophelines breed in terrestrial waters, and the latter 

 was now explained by the fact that Stegomyia breed in 

 artificial collections of water round houses. A little 

 later Graham gave strong evidence in favour of the 

 theory that dengue fever is carried by a species of 

 Culex. 



Thus mosquitoes have now been incriminated as 

 the carrying agents of no fewer than four important 

 disease of man. But this is by no means 

 all. I have mentioned that before my work 

 Bruce incriminated Glossitia morsifans as the 

 carrying agent of nagana ; and he and others 

 now showed that the deadly sleeping sickness of Africa 

 is carried by other tsetse-flies. Various Spirochaetes, 

 especially that of tick fever, have been shown to be 

 conveyed by ticks. A peculiar type of comparatively 

 mild fever of which the cause, like that of yellow 

 fever, is still unknown, has been proved to be conveyed 

 by sand-flies. Several diseases of animals have been 

 proved to possess a similar histor\- ; and others, both 

 of animals and men, are suspected to lie in the same 

 category. Perhaps, however, the most important and 

 dramatic result was that obtained in the case of plague 

 — the most terrible of epidemic diseases, the wonder 

 and the despair of humanity since the beginnings of 

 history, the scourge which was so often attributed to 

 the direct action of God. It is due really to the rat 

 flea. And this discovery signals another advance, 

 because plague is, as we all know, due, not to an 

 animal, but a vegetable parasite ; and we therefore see 

 that bacteria also may adopt precise routes of entr}\ 

 A similar case is that of Mediterranean fever, which 

 is carried principally by the milk of infected goats ; 

 and leprosy (another supposed scourge of heaven) has 

 been attributed to the bites of bed-bugs; while some 

 are even beginning to think that measles is due to 

 fleas. 



Gentlemen, I have now completed my task. We 

 have seen at least another instance of how strongly 

 recent advances in science bear upon medicine — how 

 they confirm facts previously guessed at, achieve vic- 

 tories formerly undreamed of, and establish great 

 theorems which will be of value to humanity so long 

 as civilisation exists. But they affect not only the 

 theory and the treatment of disease, but. what is per- 

 haps still more important, its prevention ; and it is 

 especially in this line that the new theorems affect us. 

 A whole great epidemiological group of diseases has 

 been separated out, the so-called insect-borne diseases, 

 and they are perhaps on the whole the most important, 

 at least in the tropics. But, more than that, these 

 discoveries give us practical methods of prevention, 

 which jnay be summed up in the two words — no 

 vermin. We now have a great sanitar}- ideal put 

 before us : so to manage our habitations, villages, 

 towns, and cities that the vermin in them shall be 

 reduced to the lowest possible figure. Scores of ento- 

 mologists and medical men are now dealing exactly 

 with the habits of these creatures and showing us how- 

 to effect the required object. It demands only intelli- 



