tute one of the wonders of nature. Some scatter their spores and eggs broadcast in the soil, water, or air, as it were, in the hope that some of them will alight by accident on a plant or animal suitable for their further growth. Many parasites employ in various ways a second species of animal as a go-between. Thus, some tape-worms and the worms which cause trichinosis spend a part of their lives in the flesh of swine, and transfer themselves to human beings when the latter eat this flesh. To complete the cycle, the parasites return to swine from human offal; so that they propagate alternately from men to swine and from swine to men. The blood parasites which cause the deadly tsetse-fly disease among cattle in South Africa, are transferred from one ox to another on the proboscis of the ox-biting tsetse-fly. The progeny of the flukes of sheep enter a kind of snail, which spreads the parasites upon grass. The progeny of the guinea worm of man enter a water flea. progeny of Filaria nocturna-the worm which produces elephantiasis-live in the human blood. When certain kinds of mosquitoes suck this blood, the young worms take up their quarters in the insect and burrow their way into the proboscis. When the insect, some weeks later, bites another man, they work their way from its proboscis into his skin, and then complete their development in him. At least such is inferred to be the case from the strongest circumstantial evidence. The progeny of the parasites which cause Texas cattle-fever-and which are very like the malaria parasites-live in cattle-ticks, and are transferred by the young of these ticks into healthy cattle. Lastly, we have just discovered by the most conclusive experiments that the dreaded disease, yellow fever, is carried from man to man by a brindled kind of mosquito called Stegomyia.

The

All these are not theories or fancies, but hard facts, won by science at the cost of immense labours carried out by many scientific men during many years.

V. HOW THE PARASITES OF MALARIA PASS FROM ΜΑΝ ΤΟ ΜΑΝ

Like other parasites, those which produce malarial fever, the Haemamoebae, must be able to spread from one man to another. How do they do it?

Noting the connexion between malarial fever and stagnant water, referred to in section I, many people thought that the parasites must come from the soil or water. But all efforts to find them in soil or water proved unavailing; and at one time it seemed probable that the great problem as to how men get malarial fever would remain unsolved for years. Fortunately, in 1883 and later, four distinguished men, King in America, Laveran in France, Koch in Germany, and, especially, Manson in England, suggested that the mosquito carries the disease. Working at the problem from this point of view, by dissecting mosquitoes fed on malarial patients, I succeeded, between 1895 and 1899, in demonstrating that the parasites of malaria found by Laveran pass a hitherto undiscovered stage of their existence in mosquitoes, and are then inoculated into our skin by the bites of the insects.

The story is briefly as follows:-When a mosquito bites a malaria patient, it sucks up a number of malaria parasites with the patient's blood. The parasites burrow into the insect's tissues, grow rapidly, and, after a week or two, produce a number of spores. Most remarkable to relate, these spores enter the poison or salivary

gland of the insect. This gland secretes a minute drop of fluid, which the insect injects through its proboscis into our skin when it bites us, and to which the itching produced by the bite is due. The spores of the malaria parasites actually lie in this fluid and are injected with it into our skin, where they mix with our blood and produce infection.

Thus the mosquito acts in regard to malaria almost precisely as it acts in elephantiasis. It takes up the germs from a patient, and after a week or two puts them into the blood of a healthy man.

All this may appear to the unitiated reader to be too wonderful for credence; but it is absolutely and certainly true. Every stage of development of the parasites in the mosquito has been followed over and over again by the most careful and exacting microscopists; and, I may add, the dissection of mosquitoes and their examination by modern microscopes is not so difficult a task as some may think. But, apart from

the microscope, there is a method of obtaining a crucial proof of the whole matter-by actually producing the disease by means of mosquitoes. This was done by me in 1898, when I infected twenty-two out of twentyeight healthy sparrows by mosquitoes previously fed on diseased sparrows. Subsequently, healthy men have been infected in precisely the same manner. Thus, in 1900, Manson infected two gentlemen in the middle of London by mosquitoes brought from the Campagna in Italy; and similar successful results have been obtained by Ziemann, Fernside, and Bignami. The whole work has been confirmed and amplified by Koch, Daniels, Christophers, Stephens, Bignami, Bastianelli, Celli, Zieman, Ruge, Gosio, Van der Scheer, Van Berlekom, Thayer, Woldert, Fernside, Annett, Ould, Dutton, Nut

tall, Shipley, and others; has been fully accepted by such authorities as Lord Lister, Koch, Laveran, Metchnikoff, Sir Michael Foster, Ray Lankester, Osler, Blanchard, Mesnil, Lühe, and others; and has lately received circumstantial confirmation by the discovery of the fact that yellow fever is communicated in the same way.

VI. SOME ADDITIONAL INFORMATION

During the last three years investigations made in many parts of the world have given the following results:

1. Up to the present, the parasites of human malaria have been found to develop only in mosquitoes of the kind called Anopheles. Even of these, not all species appear equally receptive, because one at least, namely, Anopheles rossii, does not seem to harbour the germ easily. Anopheles costalis and Anopheles funestus are known to carry the parasites in Africa; Anopheles maculipennis in Europe and the United States; and Anopheles culicifacies and two other species in India.

2. Up to the present, Anopheles have always been found where fresh cases of malaria are occurring, and where the facts have been properly investigated by competent persons.

3. The germs have been frequently found in a large percentage of Anopheles caught in infected houses; and occasionally even in Anopheles caught in the bush In the latter case it is assumed that the insects became infected by having previously bitten villagers living in the neighbourhood.

4. The germs have never been found in mosquitoes captured in the larval stage, and examined before being

fed on infected persons. For this and other reasons it may be assumed that Anopheles can become infected only by biting infected people.

5. The kinds of mosquitoes called Culex and Stegomyia always proved negative in my Indian researches; in those of Daniels and others in Africa; and in those of some Italian writers who followed my work. At the same time, we are not yet quite certain that some species of mosquitoes other than Anopheles cannot carry human malaria.

6. The principal facts connected with the habits of mosquitoes as they affect the malaria question, were made out by myself in India, and by Annett, Austen, and myself in Sierra Leone; but many important details have been added by Daniels, Christophers, and Stephens in Africa and India, Howard in the United States, Nuttall and others in England, and Strachan, Annett, and Dutton, and others, elsewhere. The anatomy of mosquitoes has been minutely studied by Christophers, Nuttall, Shipley, Dutton, and others.

7. In 1900, Manson infected two gentlemen in London by mosquitoes brought from Italy, and at the same time kept several free from fever in a mosquitoproof house, in the most deadly part of the Campagna ; thus giving a most striking demonstration of the facts cited above.

8. The Liverpool School of Tropical Medicine has made a detailed survey of the malaria question in West Africa, by sending out numerous experts to that region.

9. Nuttall, Cobbett, Shipley, and Strangeways-Pigg have studied Anopheles in Britain and their relation to former malaria in this country, and their preference for certain colours. Laveran has studied the malaria question in France.