March 29 1900] 



NA TURE 



527 



in various parts of the world. The facts already collected would 

 fill a small volume ; and every month witnesses additional 

 publications on the subject. I shall therefore, in conclusion, 

 content myself with a brief reference to three points of leading 

 importance. 



I shall first try to indicate how completely the recent dis- 

 coveries explain the well-known laws regarding the diffusion of 

 malaria. As mentioned at the beginning of this lecture, malarial 

 fever has long been known to be connected with the presence of 

 stagnant water. That is to say, we generally, though not in- 

 variably, find that the disease is associated with low-lying flat 

 areas, where water tends to collect to a considerable extent. It 

 was indeed the general appreciation of this law which led to the 

 old miasma-theory of the disease — the theory on which the word 

 "malaria" was based. We assumed that the poison is one 

 which rises from marshy areas in the form of a mist, and which 

 thence infects all living within a given distance. Later, when 

 the pathogenetic parasite was discovered in the blood of febrici- 

 tants, many observers, still clinging to this conception, thought 

 that the parasite is an organism which in its free state dwells in such 

 places, and diffuses itself in such mists. It is interesting to note 

 how near to the truth this almost instinctive conception took us. 

 It is right in idea, wrong in fact. It is not the parasite itself which 

 springs from the marshy ground, but the carrier of the parasite. 

 This was one of the many interesting points made by King in 

 his mosquito-theory of seventeen years ago. But King fell into 

 an error which could have been used as a powerful argument 

 against his hypothesis. He seemed to have assumed that all 

 mosquitoes rise from marshes. Hence, he said, malaria exists 

 in the presence of marshes ; hence it is a disease of the country 

 rather than of towns, and so on. As a matter of fact, mosquitoes 

 as a rule do not rise from marshes at all ; they do not all even 

 rise from pools of water on the ground ; the commonest species, 

 at least of those which habitually annoy human beings, spring 

 from tubs and pots of water in the vicinity of houses, and are 

 indeed more common in cities than in country places, at any 

 rate in the tropics. Now it is not the least interesting feature of 

 recent researches that they have shown where the error lay. As 

 soon as I had succeeded in cultivating the human parasites in 

 my " dappled- winged m.osquitoes," which were really gnats of 

 the genus Anopheles, I began to study the habits of these insects, 

 and soon ascertained the remarkable fact that while gnats of 

 genus Ctikx generally breed, in India, in vessels of water round 

 houses, gnats of genus Anopheles, which I had just connected 

 with malaria, breed in small pools of water on the ground. This 

 point was made the subject of a special investigation by the recent 

 expedition to Sierra Leone ; and we found that the law holds 

 good there as in India. While Culex larvse were to be seen in 

 almost every vessel of water, or empty gourd or flower-pot in 

 which a little rain-water collected, in only one case did we find 

 Anopheles larvte in such. On the other hand. Anopheles larviie 

 occurred in about a hundred small puddles scattered through the 

 city of Freetown — puddles mostly of a fairly permanent descrip- 

 tion, kept 'filled by the rain, and not liable to scouring out 

 during heavy showers. What was almost equally significant, 

 the larva" seemed to live chiefly on green water-weed. Hence 

 it follows that while Culex, the apparently innocuous genus of 

 gnats, are essentially, or at least often, domestic insects, 

 Anopheles, the malaria-bearing genus, are essentially gnats 

 which spring from stagnant water on the ground. And numerous 

 other facts in the history of malaria can be explained by the 

 same discovery. It is supposed, for instance, that malaria 

 originates from freshly-turned earth ; now we actually noted 

 examples where railway embankments and the like had pro- 

 duced Anopheles pools ; and it is easy to see that disturbance of 

 the soil may often produce depressions in the ground capable of 

 holding a little rain-water suitable for the larvK of these insects. 

 Again, malarial fever often appears on board vessels which have 

 touched at malarious ports ; as an explanation of this we ascer- 

 tained that Anopheles visit ships from the shore. In short, on 

 studying the matter from every point of view, I must confess to 

 being ignorant of any well-established fact about malarial fever 

 which is not explained by the mosquito theory. 



This brings me to the subject of objections to the mosquito- 

 theory. In view of the exact and copious microscopical and 

 experimental evidence which has now been collected in proof 

 of the theory, it is no longer permissible to doubt the main 

 facts ; and the objections which one still finds, both in the lay 

 and the medical press, are generally based on a complete 

 ignorance of these facts, and need not be discussed here. But 



NO. 1587, VOL. 61] 



there is one objection — frequently made, in spite of corrections 

 as frequent, by persons who reside ip malarious places —which 

 deserves comment. This is, that malaria exists where there are 

 no mosquitoes, and that so-and-so has had fever without being 

 bitten by gnats at all. Generally speaking, we must always 

 remember that malarial fever is a disease in which relapses 

 occur perhaps for years after the first infection, and that it is 

 this first infection and not the relapses which are due to the 

 bite of Anopheles. It is thus possible to suffer from any number 

 of attacks of fever without being bitten by Aiwpheles (except on 

 one occasion), and without invalidating the theory — a fact of 

 which those who argue in this manner are generally ignorant. 

 Again, it is well known that one may be bitten without 

 perceiving it ; that some persons are singularly callous to 

 the punctures of these insects ; and, lastly, that many others 

 have very limited powers of observation. I may say at once 

 that, personally, I cannot accept any statement to the effect 

 that gnats are absent in any locality in the tropics, until such a 

 statement is made by a competent observer after direct search ; 

 because I have never been in any place in the tropics — and I 

 have been in a large number — where ttiere were no gnats. On 

 the other hand, I have often found numerous gnats in localities 

 where I was previously told there were none. I was once 

 actually informed that there are no mosquitoes in Sierra Leone ! 

 The fact is that those who will trust the statements of the 

 general public on such matters must be very credulous. 



I turn lastly to the all-important subject of prevention, but 

 can do no more than touch upon it here. Two methods sug- 

 gest themselves at once. I need not refer to that of guarding 

 against the bites of these insects by the use of mosquito-nets 

 and so on — an obvious and, I believe, an exceedingly useful 

 measure, which may reduce the chances of infection to a small 

 fraction. Unfortunately such methods will never be employed 

 on a large scale in the majority of malarious localities ; and we 

 must resort to the destruction of malaria-bearing species of 

 gnats. Early in 1892 I reported to the Government of India 

 that it may be possible to exterminate Anopheles in some locali- 

 ties — especially some towns, cantonments and plantations — 

 owing to the habit the insects have (in some places) of breeding 

 only in selected pools. Since then, a considerable literature 

 has already grown up round the subject. Reviewing this litera- 

 ture, it seems probable that we may be able to exterminate 

 Anopheles, or at least largely reduce their numbers, in towns 

 where, owing to the conformation of the ground, the low level 

 of the subsoil water or the small rainfall, surface pools suitable 

 for the insects are comparatively few. The methods which can 

 be adopted against the \ax\x are numerous — such as brushing 

 out the pools with a broom, draining them away, filling them 

 up, or treatiug them with various culicides, such as paraffin and 

 numerous other substances (recently investigated by Celli and 

 Casagrandi). On the whole, the most promising method which 

 suggests itself is the employment of some cheap solid material 

 or powder which dissolves slowly, which kills the larvae without 

 injuring higher animals, and which renders small pools unin- 

 habitable for the larva" for some months. If, for instance, a 

 cartload of such a material would suffice to extirpate the larva; 

 from a square mile of a malarious town, the result would be a 

 large gain to its healthiness. Dr. Fielding-Ould has lately re- 

 ported favourably on tar. Grillet recently reports a case in 

 France where a large district was rendered free of malaria by 

 the extensive use of lime for agricultural purposes. Gas-lime, 

 or even common sal^, may be suggested. In short, though the 

 question of the possibility of attacking these insects with suc- 

 cess is still entirely in the experimental stage, we may reason- 

 ably hope that the mosquito-theory of malaria may some day 

 prove to be as useful to humanity as it certainly ha» proved 

 interesting to the student of science. 



In conclusion, however, I should add that this result is not 

 likely to be attained unless we, as a nation, determine to pay 

 more attention to scientific discoveries in the field of tropical, 

 medicine than hitherto we have done. During the last fifty 

 years discovery after discovery in this field has been made with- 

 out finding any adequate reflex in medical and sanitary prac- 

 tice in our tropical possessions. The discoveries, for instance, 

 of Losch, Davaine, Dubini, Bilharz, Bancroft, Koch, Laveran, 

 Manson, Carter and Giles, though nearly concerned with the 

 lives of thousands of human beings, have been generally treated 

 either with scepticism or neglect — have been neither sufficiently 

 followed in the laboratory nor sufficiently acted upon in the 

 region of practical sanitation. 



