176 GNATS OR MOSQUITOES — CHAPTER VlII Now, except when its natural permeability is destroyed by a layer of fine silt, as is the case with the cement-like surface of the Punjab "put," this alluvium is considered merely as a soil, by no means favourable to the development of malaria, as it is for the most part so pervious that it holds water badly. Owing, however, to its unbroken levels the natural surface drainage is everywhere bad, and its artificial improvement difficult or impracticable. In spite, however, of these disadvantages, it may almost be said that the haunts of Anopheles larva are throughout its entire area mainly the work of man, and are therefore to a great extent removable ; always provided that sufficient funds and intelligence be available.
Passing south, the transition from the alluvium to the broken and rocky ground of central India is often well-nigh as abrupt as that from sea to land.
In place of the monotonous plains of Northern India, the surface, at very least undulates and is often mountainous ; and the soil, which is seldom of very great depth, is derived from the decomposition of rocks in the immediate neighbourhood which belong mostly to primary or metamorphic formations. In the northern part of this area much of the cultivable land consists of what is known as the black " cotton soil," which possesses certain characters which have an important influence on the local seasonal incidence of malaria. It absorbs water like a sponge or like so much "black cotton," and once thoroughly saturated holds it well enough to favour the formation of puddles, which are fairly permanent as long as the air remains damp. With the return, however, of the dry weather it dries rapidly, splitting up into a network of deep fissures which render the existence of puddles, whether of domestic or natural origin, well nigh an impossibility.
The greater part of this region is, however, to the south of the line of hybernation lox Anopheles, so that adult insects may be taken at all times of the year ; but the presence of larvse is practically limited to the season of the rains, or from the middle of June to the end of September. As, however, the closure of the fissures by the sweUing
CONDITIONS INFLUENCING PREVALENCE 177 of the spongy soil as it drinks in the rains still leaves the surface irregular, this soil is peculiarly favourable to the multiplication of the species as long as the wet season lasts, so that the physical characteristics of this sort of soil necessarily are such as to favour greatly the development of malaria for this short and limited period ; and as a matter of fact, the seasonal incidence of malaria corresponds well with these facts, as taken altogether, such sites are fairly healthy, though malaria is rife and wide-spread while it lasts.
Owing to the instability of such a foundation the construction of permanent works of all kinds is a matter of the greatest difficulty, and hence the surface drainage of towns is always costly, and even when most carefully designed requires continual regrading.
Below the black soil there is commonly, especially in Kathiawar, a stratum of limestone, locally known as moram (miliolite) intervening between it and the subsoil water, which, though not very dense, is yet sufficiently effective in holding up the surface moisture. It is doubtful if even subsoil drainage would be of any great use in combating malaria in land of this description, as the peculiarity of this soil is that it dries rather by evaporation from above than by the draining away of its moisture from below, and it is obvious that under such circumstances works of this sort would be not only expensive, but probably ineffective.
With a highly civilised population it is possible that some good might be effected by the systematic destruction of the adult insects during the dry season, as the climate and general characters of the country are such as to render the shelter of houses almost indispensable to the maintenance of the species ; but under existing circumstances this appears one of those cases in which the free distribution of quinine and the popularisation of its use can alone effect much benefit, and is therefore the more fortunate that the same physical peculiarities that so favour the development of malaria for three or four months of the year also limit its duration.
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178 GNATS OR MOSQUITOES— CHAPTER YIII It is needless to remark that besides the above ahuost every variety of soil is to be met with in so large a country as the Indian peninsula, but the above are the only two cases with which I have any personal familiarity which are sufficiently extensive and peculiar to require any special mention. In all probability what has been remarked with regard to the Gangetic alluvium will apply also to other tropical and subtropical alluvia, such as those of the Nile and Mississippi, but I have not been able to obtain any special information on such points, except as regards India.
Influence of Vegetation. — As the three essentials to the well-being of all species of Mosquitoes, vegetable food, shelter from the sun during the heat of the day, and the presence of puddles wherein to rear their young, are all either dependent upon or greatly favoured by the presence of vegetation, it is obvious that its amount and character is a factor that must always be considered in estimating the potentialities of malaria in any given place. Open grassy plains have long been known to be unfavourable to malaria. In such situations gnats can find no sufficient shade, and such puddles as form are hidden from them by the closely crowded stems. I am inclined to believe that the hiding of the surface of pools by close vegetation tends to prevent Mosquitoes from using them as breeding places, and that this fact might often be made use of as a cheap and effective means of sanitation. There can be no doubt e.g., that the presence of certain water plants in some way prevents the appearance of Mosquito larvae in the water covered by them, and the only reason I can suggest for this is that the plants act in this way because they hide the water from the female gnat searching for a suitable place in which to deposit her eggs. At any rate it is difficult to otherwise account for the curious fact that in the Benares public gardens, where' there are some scores of the small irrigation tanks described below, Culex and Anopheles larvae, alone or in company, were present in every tank save those that were covered with a peculiar floating water plant, looking much like a
CONDITIONS INFLUENCING PREVALENCE 179 young lettuce, which is spoken of by the natives as the jalkumi.
In the tanks so planted, the water was alive with young leeches and nematodes, but in none of them could be discovered a single Mosquito larva, while the others swarmed with them. Introduced into a tank already containing Mosquito larvse, however, the plants appeared to exercise no hostile influence whatever on their development, and for this reason, I conclude that the plants act mechanically in the same way as an artificial cover. The jalkumi floats on the surface of the water and so forms a most effective screen ; but another plant resembling the Canadian duckwood, which grows completely submerged appeared almost as effectual, though in this case the surface of the water was certainly not hidden in the ordinary sense of the word, though the green coloration may have masked its presence to the defective and short ranged vision of a Mosquito. Whether the presence of such plants would be equally effective in places where no alternative pools are accessible is of course open to doubt, and the vagrant nature of my employment as Sanitary Commissioner prevented my being able to follow the point up, but the matter is certainly worthy of investigation, as if confirmed, it would afford an extremely simple and inexpensive means of diminishing the number of available nurseries for larvae.
A variety of plants, such as the castor oil and eucalyptus have enjoyed the reputation of being protective against either Mosquitoes and malaria, and it is possible that the scent of certain strong smelling species may be obnoxious to the insects; but the observations of Celli (C. M., p. 143) show that so far from being destructive to them such plants may form an excellent refuge for Mosquitoes. Our experience in India is similar, as may be judged by the following extract from the Pioneer (April 4, 1901) : — To the Editor.
" Sir, — Some short time back, there appeared, in the correspondence columns of your paper, I think, a recom-
180 GNATS OR MOSQUITOES — CHAPTER VIII mendation to use the castor oil plant to keep a bungalow free from Mosquitoes. I being a sufferer had six plants placed in pots in my rooms. I fancy we must breed a different variety of Mosquitoes than your correspondent, for the castor oil plants are thickly covered with the insects by day, who, at night time, seem to be actually invigorated by the apparently stimulating effect of their new quarters. — D." The influence of trees in especial has hitherto been greatly misunderstood. A screen of trees was supposed, in some occult way, to be capable of filtering out malarial germs from the air ; and it is just possible that the presence of a convenient shelter of this sort might in certain cases prevent Mosquitoes wandering further to dwellings which might otherwise have been their nearest convenient refuge ; but it would obviously be bad policy to multiply such shelters. Speaking generally indeed there can be no doubt, that trees greatly favour the multiplication of Mosquitoes. This they do in four ways : they afford shelter during the day ; their shade prevents the drying up of puddles ; their flowers often afford the staple food of the insects ; and lastly, they prevent the growth of grass.
In 1884, the Italian authorities instituted an enquiry as to the influence of disforesting on pubhc health (C. M. p. 141), and the result of their investigations was that they were unable to find any proof that disforesting was injurious to health, but that some facts indicated an opposite effect. Popular and professional opinion in India as to the influence of trees on malaria has oscillated, but has generally been in favour of open sites, though no one doubts that trees greatly favour the prevalence of Mosquitoes. The history of the large military station of Meean Meer is both curious and instructive in this respect. After the annexation of the Punjab, the large force quartered close to the great native city of Lahore suffered so terribly from malaria that it became absolutely necessary to remove thein to some more healthy site. At that time the, as we can now see, perfectly defensible view that open sites are least malarious was that in vogue and the General and his
CONDITIONS INFLUENCING PREVALENCE 181 advisers therefore selected Meeaii Meer, a barren plain, almost uncultivated owing to the thinness of the soil. The extensive building operations necessary for the housing of the troops, however, soon honeycombed the level surface with excavations of all sizes and the rising barracks and bungalows soon afforded abundant shelter of the effective sort dearest to the Indian Anopheles ; and hence, like all new places, before long occupation has gradually ameliorated the untidiness inseparable from new and growing places ; the change proved no better than from " frying pan to fire." Changing opinion led to the unhealthiness of the place being ascribed to the absence of trees, and at great trouble and expense, a separate shaft, filled in with soil, having to be sunk through the limestone for each sapling, large numbers of trees were planted, with no very perceptible result. The cantonment still bears an evil reputation for malaria, but half a century of occupation has wrought great improvement in surface drainage and other kindred ameliorations, and though still decidedly malarious, it is to day neither markedly better nor worse than other stations in the Province. In India, as elsewhere, especially during the rains, many species of Mosquitoes certainly habitually harbour in trees during the day, and there can be no doubt that the proximity of trees, however pleasant to the eye, is most undesirable if one wishes to keep a house free from these pests. Celli (C. M., p. 141) states that Aji. bifurcatus, in Italy, lives preferably in thickets, and that persons sleeping in such places, even during the day, are frequently bitten ; though the experience of Dr. Sambon and Low {B.M.J. Dec. 8, 1900, p. 162) were contradictory in this respect. The explanation of this is probably that the observations were made in different months, for in India at any rate our local species of Anopheles do not apparently find that the shelter of trees is sufficient during the fierce heat of the day, and prefer the deeper shade of buildings ; and, as far as my somewhat limited experience extends, are not to be found amongst trees during the day, though it is likely enough that they may do so in parts of India where the summer heat is less fierce than in the North West Provinces.
182 GNATS OR MOSQUITOES — CHAPTER VIII All considered, I fear that great as is the solace of a shady garden to the eye jaded with the fierce glare of a tropical sun, it is a most undesirable adjunct to a residence, alike for health and comfort, for besides sheltering and in every way fostering the multiplication of Mosquitoes, whether simply irritating or noxious, the trees, which practically never shield the house from the direct rays of the sun, cut off the breeze and so interfere with that free ventilation, which is a sine qua non of tolerable existence. Finally, it must be remembered that trees are in some way capable of modifying the climate of a locality by increasing the rainfall. How or why they are capable of doing so is by no means clear, but the connection is generally admitted, and some recent observations in India appear to show that such an effect may be produced within the limits of a comparatively restricted area. While they, however, generally favour malaria, Celli justly points out, (C. M., p. 142) that trees should be respected on hilly ground, as by retarding the rapidity of drainage they tend to prevent the flooding of the plains below by heavy rainfallson the hills.
Assuming then it to be possible to chose the site of a tropical residence, it should certainly be placed on open ground, and coolness and rest for the eye should be aimed at by surrounding it with a stretch of well- watered grass, the watering being conducted on some plan that does not involve the use of tanks and other collections of standing water ; but such a plan is always difficult and expensive, and is in many parts of India, impossible.
The presence of thickets and undergrowth has long been recognised as favourable to malaria. Growths of this sort sufficiently luxuriant to be unmanageable, in and about inhabited sites, presuppose a moist climate, free from the extreme heat of the drier parts of the tropics, and in which the habitations alike of Europeans and natives are constructed with an eye rather to the freest possible ventilation than to keeping out the heat ; so that as far as that is concerned, there is little to choose between the shade of a tree and the best built of bungalows.
CONDITIONS INFLUENCING PREVALENCE 183 In such climates, e.g., in Assam and Burmah, the miiltiphcation of all species of Mosquitoes, Anopheles included, can hardly be otherwise than greatly favoured by growth of this sort, and as a matter of fact, the clearing of jungle from village sites is one of the few measures that come within the range of practical pohtics in village sanitation of the provinces in question. Costing as it does nothing more than a little official persuasion, it is pretty generally carried into effect, and is undoubtedly most beneficial.
When the District Officer in Assam has induced his villagers to clear away the jungle from their huts and to give their wells the annual cleaning, he justly feels that he has done all that is humanly possible for the sanitation of the unpromising human material he has to deal with ; for the Government resolution that converts the Assamese peasant to sanitary decency will have to be framed with a stringency far beyond the ingenuity of any legislator, past or present.
The consideration of the role played by plants in the propagation of malaria leads naturally to the effects of cultivation. As far as India is concerned the question of the influence of cultivation on malaria resolves itself mainly into that of the effects of irrigation. At any rate it is the only one in which remedial measures on a large scale can be attempted by Government, as the methods of the indigenous cultivator are too ingrained and detailed to be capable of modification by legislative measures, to say nothing of the fact that agricultural experts are agreed that in the main his system is but little capable of improvement, and can therefore be hardly profitably meddled with. The total area under irrigation in India is about 29,000 square miles, rather more than the area of Greece, or just over 2 per cent, of the entire country ; but as the irrigated areas are also very densely populated, their influence on public health is much greater than such a proportion would suggest, to say nothing of the fact that such areas serve as foci from which the disease is constantly spread abroad by human agency.
It is needless for me to quote here any evidence as to the