magnitude and difficulty of the problem which confronted Bruce in 1895; and to find a just parallel to the masterly manner in which he solved it, we must go back to Pasteur. There is, indeed, the same simplicity, the same directness, the same insight in the work of both these men. Their works are enduring demon- strations of the method of science: they are a delight to read, and illustrate on every page the favourite maxim of Boerhaave: Simplex sigillum veri.
The following-up of Bruce's discoveries and the working-out of details and consequences have led to the accumulation of an immense amount of new knowledge protozoological, ento- mological, and medical. We can do no more than mention it here. We must, however, notice one of the first-fruits of his labours the application of his results to the study of human dissasss. This application was made mainly by Bruce himself. A few y^ars after he had done his great work on nagana he attacked the problem of sleeping sickness, a human disease which has depopulated large areas of Central Africa. Bruce and his collaborators were able to show that this disease is similar to nagana. It is likewise caused by a trypanosome, which is conveyed to man by the bite of a tsetse-fly, and which is capable of living in other animals. In this case the parasite had been previously seen by Forde and Button, and by Castellani. But its relation to human disease and the part played by the tsetse in its transmission were first clearly demonstrated through the work of Bruce.
Malaria and Other Diseases. We must now notice another disease, which is known by name to all malaria, " the scourge of the tropics." This disease, as we now know, is also carried from man to man by the agency of a blood-sucking fly in this case a mosquito, and it is also caused by a blood-inhabiting protozoal parasite, though it is one very different from that which causes nagana. Moreover, this parasite lives in men and mosquitoes only. After undergoing a peculiar development in the blood of a human being, it is sucked up with his blood by a mosquito when it feeds upon him. Provided that the mosquito is of the right sort, the parasites in the blood if they are in the proper stage of development undergo further remarkable changes in the mosquito's body. Thereafter the mosquito is able to infect other men with the parasites, which it injects into their blood in the process of sucking. And so the life of the para- site continues.
The foregoing is the briefest synopsis of a very complicated story, in which almost every event has been worked out in great detail. Hundreds have contributed to this work, though some of them can hardly be said to have cooperated in it. Indeed, such bitter fights have taken place among them that it has now become almost impossible to mention the names of some workers without offending others. The history of these discoveries would give an unpleasant shock to anybody simple enough to believe that men of science always labour for truth and the advancement of knowledge rather than for fame and personal gain. Fortunate- ly the names of the leading discoverers are now known to almost everybody, and their individual achievements are no longer in dispute. Even the " general reader " is familiar with the name of Laveran, the great Frenchman who, in 1880, discovered the malarial parasites in human blood; of Patrick Manson, the founder of modern tropical medicine, who divined, in 1894, the part played by the mosquito; of Ronald Ross, who, inspired by Manson, first worked out in 1898 the complete development of the malarial parasite of birds, and thus solved the general problem; and of Grassi and his fellow- workers in Italy, who immediately confirmed Ross's work and extended and success- fully applied his results to the study of malaria in man. When the igth century ended the story was almost complete.
It will be evident that malaria, nagana, and similar diseases are not purely protozoological problems. It will also be obvious that such diseases might be stamped out and prevented by attacking either the protozoal parasites which " cause " them, or the insects which transmit them, though there could have
een but little hope of success in coping with such diseases be- fore the life-histories of the parasites were discovered. When
protozoology, entomology and medicine have solved their re- spective parts of such problems, then many tropical regions which are now forbidden ground will become habitable for man and beast. The practical importance of protozoology in cases such as these is self-evident. The facts speak for themselves.
Malaria is a far commoner disease than nagana, and the dis- coveries relating to it have therefore made a far wider appeal to the public. It intrigues the public to hear that there would still be no Panama Canal but for the great discoveries in con- nexion with malaria. It would excite them but little to hear that some obscure tribe of Zulus could now keep cattle in places where it was previously impossible. But the advancement of science is not measured in such terms, and science values most highly those who discover and enunciate new principles. Already we can observe that the problems presented by nagana and malaria are similar, and that most of the generalizations which their solution can give us are, indeed, the same. We can see, too, that history, in the end, is generally just. Consequently, we may hazard a guess that in years to come the historian of science, in his impartial search for beginnings and great names, will not fail to note the sequence of the discoveries which we have just considered, and will apportion his praise accordingly.
The World War Period. Medical protozoology, like many another branch of science, received a powerful stimulus from the World War of 1914-8. Not only was much of the previously acquired knowledge put into practice, but this practical applica- tion in turn revealed or emphasized the gaps, defects, and errors in many current conceptions, and so led ultimately to the prosecution of new researches and the acquisition of much new knowledge. Surveyed from the most general standpoint, the war appears to have taught us little that was new regarding malaria and the other protozoal diseases already mentioned. Its chief ' protozoological contribution has been to our knowledge of those Protozoa which live in the human intestine, and more especially to the elucidation of the disease called amoebic dysentery. We may therefore say a few words on this subject at this point.
The Protozoa known as " amoebae " form a large and inter- esting group. Most of the species live independently in such places as ponds, ditches, or the soil; but some of them live in the bodies of other animals, and one of them called Entamoeba hislolytica was already known before the war to live in the human bowel and " cause " amoebic dysentery. The parasite was discovered by Losch in Russia as long ago as 1875. Its real relation to dysentery, however, was not made clear, though much debated, until just before the war, when the admirable researches of two American workers in the Philippine Islands E. L. Walker and A. W. Sellards were published. During the war their results have been confirmed and greatly extended, chiefly by the investigations of British workers. As a con- sequence, we now know as much about amoebic dysentery as we do about malaria or the diseases due to trypanosomes. There are several points here which are worthy of mention.
We now know that no less than five different species of amoebae may live in the intestine of man, though only one of these the " dysentery amoeba " already mentioned ever does him any harm. Moreover, we now know also that amoebic dysentery is a comparatively rare disease. There are many different kinds of dysentery, and the kind due to amoebae is far from being the commonest. Before the war amoebic dys- entery was generally recognized as a disease more or less re- stricted to the tropics, though certain other kinds of dysentery occur all over the world. "The curious fact brought into prom- inence by the war is that the dysentery amoeba itself is very common almost everywhere. This parasite, which can cause, by its presence in the bowel, a violent and sometimes fatal form of dysentery, usually does no such thing. Very many people, in all parts of the world, are infected with it, but very few ever suffer any appreciable harm from its presence. The parasite and the person who harbours it are usually suited to one another in such a way that they can live together comfortably, oblivious of the existence of one another. There are, for instance, in the British Isles at this moment many thousands of people who are
