It is possibly not justifiable to say that the distributions of malaria and of the Anophelince are identical, for there are many regions, or at least districts, of the earth where malaria is known to exist, and as yet observations are lacking as to the presence of the Anophelince, but it is absolutely true to say that there is no focus of malaria anywhere which, when examined for Anophelince, has failed to reveal them. It is hardly necessary to consider statements to the contrary: they result simply from ignorance. Such statements have been made only too often by those who do not know a sand fly from a mosquito, much less a Culex from an Anopheles. They do not require serious refutation. If, however, we consider the distribution of the known Anophelince, we find that, although it is true that no malarial country or place exists without them, yet the converse is not true, and Anophelince can exist in a country without the occurrence of malaria. The well known disappearance in England of malaria in the fen district and elsewhere has not been dependent on the disappearance of Anophelince, for these exist there now. And this is not an isolated instance, for elsewhere, as in parts of Italy, Anophelince exist where there is no malaria; similarly in the environs of Paris, in Holland, in northern Europe, Anophelince exist, where there is no malaria at the present day, nor, in many cases, has, as far as is known, ever existed. We shall see that this non parallelism between Anophelince and malaria exists also in the tropics (India), but to this we shall return. We mention it here, as it makes us accept with great caution the common explanations given of the disappearance of malaria from certain areas, e. g., the fen district of England. In this particular case the disappearance has been attributed to-(1) a diminution in the number of Anophelince consequent upon drainage. To this reason, in our opinion, little importance can be attached, for we know of no good reason to believe that the number of Anophelince has any great influence on malaria, provided, of course, that they do not become positively scanty, and this is not the case at the present day in these districts. (2) Emigration from the district. There is no evidence to show that this cause has been operative in districts in northern Europe where malaria has disappeared. (3) Quinin. This is a cause which, we know, could produce the effect, but we still have many cinchonized populations and yet malaria nourishes. It appears to us that none of these reasons is the real one, and that until explanations can be given of similar conditions found also in the tropics where none of these reasons apply, we must be content to assign these phenomena to unknown factors. We are not, then, in a position to explain the disappearance of malaria from a place where it formerly existed, nor can we say why malaria never has, so far as we know, existed and does not exist to day in regions where there are Anophelince. It is hardly an explanation to say that malaria does not prevail there because there are no parasites there for the mosquitos to transmit. The difficulty is to explain why malaria has become established in one place and not in another in both of which Anophelince exist. Bearing in mind, then, the negative limitations of the main proposition, " No malaria without Anophelince" we may consider broadly-and even at the present day the facts at our disposal are far from enabling us to do so closely- some of the epidemiologic features of malaria. We can to day explain most, if not all, the epidemiologic facts which common experience has accumulated during past centuries, and, indeed, where we cannot explain satisfactorily every statement that has been generally accepted, we are, since Laveran's discovery, in a position to put to the test the accuracy of the diagnosis of malaria by a microscopic examination of the blood. It is not until this has been done in these doubtful cases that we are certain of our facts, and whether we are not really attempting an explanation of what is not really a fact, but a misconception.
Malaria has a distribution roughly of 40° S. to 60° N. latitude, and is limited by the isothermal of 15° to 16° C. But there is a closer relationship than this to temperature, in the well known seasonal outbreak of malaria which varies in ternperate climes according to the temperature of the particular country. Thus in Germany and England the maximum temperature coincident with the outbreak of malaria occurred in August and September, while in Italy the malarial year, so to speak, begins in June. Wanzel, quoted by Ruge, who had made a close study of the epidemiology of malaria, had noticed that in Germany a mean temperature of 15° C. was necessary for the development of an epidemic, and that with higher temperature the malarial curve began to rise twenty days afterward, while at lower temperatures it rose twenty five days later. Taking twelve to fourteen days as the incubation period for an attack, this gives six to eleven days as the time taken for the development of the virus. These facts showed that there was a close dependence on temperature, and further they correspond in a remarkable way with what we now know to be the effect of temperature on the time taken for the mosquito cycle, and also to the now established fact that a certain temperature is necessary before sporozoites develop at all.