Malaria has long been known as marsh fever, and its connection with low lying water is universally admitted; but malaria also exists in apparently arid dry places. Even in pre mosquito days this was explained by finding water in such places which had been overlooked, and instances of this still occur. The Anophelince require water to breed in, and, as we have said, no exception to the rule of their presence in malarial regions has yet been found. Now, temperature alone could not account for an outbreak of malaria, for in the tropics we have the converse condition, viz., a temperature uniformly high, and perhaps not varying more than a few degrees throughout the year, and yet a definite malarial season. Here again we find the explanation adequately supplied by the mosquito cycle. It is, at least in some parts of the tropics, not during the maximum of the rains that malaria most prevails, but a month or two later, in the season intermediate between the rains and the dry season. When the rains are decreasing in extent and violence, we find that there has been an immense increase everywhere in breeding grounds, streams, ditches, swamps, pools, etc. This increase, indeed, occurs in the early part of the rains also, but it is perhaps at the end of the rains that it is greatest. The result is an increase in Anophelince, which, allowing a month for their full development, would occur later than the rain maximum. If, now, we consider that these Anophelince have to become infected (in seven to fourteen days), and further we have to allow time for the incubation period (seven to fourteen days) after they have bitten a fresh person, we see how it is that the maximum fever period should fall some time after the maximum of the rains. It would be well if this point were again accurately investigated by a determination of the "endemic index" month by month, for one objection that can be raised to the above data is the doubtfulness of the figures for the malaria morbidity month by month, as in popular opinion, at any rate, the most deadly malarial season in the tropics is most diversely stated, and by some as occurring even at the height of the dry season. Anophelince containing sporozoites were at all periods of the year found by us in the native huts. Gosio gives the following data with regard to monthly infection of A. maculipennis in Tuscany:
Further, as we shall point out elsewhere, it is not so clear in the tropics that there is a direct dependence between the number of anophelines at any particular time and the intensity of malaria , and, indeed, it would be hard to predict what would be the effect of an increase in anophelines; possibly more may become infected, yet this may be counterbalanced by the increased number also of those not infected.
This is, in my opinion, one of the older epidemiologic observations on which the mosquito cycle does not shed any light. The general explanation given is that this leads to the formation of pools, and so to the breeding of anophelines, and then to the infection of these, though by what previously infected cases it is generally not stated. Now, over and over again have planters, engineers, etc., informed us that working on virgin soil or opening up a jungle was especially a cause of " fever " among laborers or coolies. Moreover, this fever attacked these men in a few days, and not some weeks or a month later. That we are dealing here with a factor dependent on anophelines is, I think, extremely doubtful, and, indeed, it is doubtful also whether the so called "fever" is malaria at all. Until this point has been established it would be useless to attempt an explanation, nor can we attribute much importance to the statements so often made that malaria broke out in certain places coincident with the formation of a few new pools during engineering work. If larvae are found in these pools, it is evidence that anopheles exist in the neighborhood, and, moreover, if malaria breaks out, the explanation seems to me to lie in the fact that infected Anophelinae also exist in the neighborhood rather than that the pools started the epidemic, though it is, of course, possible that the infection started among the workmen themselves; though in the case of the turning up of soil mentioned above, frequently the coolies live a mile or so from where they worked, and the pools, if any are made by them, cannot influence the "fever" one way or another.
This is an old observation, completely in harmony with modern views, and is explained by the nocturnal habits of the anophelines.
This, which was difficult to explain on the water or miasm views, is, of course, adequately explained by the mosquito cycle. Moreover, we know that anophelines do not travel far in search of food and water, and, in fact, for all we know to the contrary, return to the houses they leave. An infected anopheline in such a house might and often does infect all the inmates. These are a few of the more important epidemiologic facts which the mosquito cycle has been found completely to satisfy, and we may say that there is no fact which clashes with the mosquito view or which suggests that there is any further cycle but that in the mosquito.