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heavily infected with these disease-producing parasites, and yet enjoying perfect health.

Another curious feature of amoebic dysentery is the circum- stance that it cannot be contracted from a person suffering from the disease. The people responsible for the spread of infection are those who harbour the parasite but themselves suffer no ill consequences from its presence. The explanation of these seemingly contradictory facts is really quite simple, now that we know the life-history of the amoeba and its relation to disease. It is a popular fallacy to suppose that any parasite is the sole " cause " of any disease. A disease is a joint result of many antecedent factors, and in the present case it would probably be nearer the truth to say that the person who harbours the amoeba, rather than the amoeba itself, is the " cause " of amoebic dys- entery. For dysentery results only when the infected person happens to be abnormally sensitive to infection with the amoeba, and the condition is as harmful to the parasite as it is to the patient. Normally man and amoeba fit one another, and there is no trouble. Abnormally there is a misfit, and amoebic dys- entery is the consequence. The disease is really an unimportant side-show in the life-history of the parasite, the result of its being planted in unsuitable soil.

The foregoing considerations will serve to show once more the value of protozoology in the study of human diseases. What hope could there ever be of eradicating a disease such as amoebic dysentery if we remained in ignorance of the life-history of the parasites connected with it? We might cure every case of the disease we might conceivably prevent the death of every patient who contracted it; but even if we did, it is now clear that this would have no effect whatever upon the continuance and prevalence of the disease itself. Such procedure could not possibly stamp out amoebic dysentery, or even reduce by one the annual number of cases of this disorder. This is not to say that protozoology has yet enabled us to do either of these things; but it has enabled us to formulate the problem cor- rectly, and has shown the uselessness of expending our energies in wrong directions. Greater results will follow when our knowledge is greater and more properly and consistently applied.

It has been supposed for so long that the parasites which produce protozoal diseases are peculiar to tropical or sub- tropical countries that the discovery of the dysentery amoeba in Britain may seem surprising. It is really not so surprising as the circumstance that nobody, until quite recently, had thought of looking for it here. And there are many equally remarkable parallels. To mention only those diseases and parasites which we have already noted, we can now say that malaria occurs indigenously in Britain though this was hardly suspected until recently; and that parasites closely similar to those which cause nagana and Texas fever have now been discovered in the sheep and cattle of the United Kingdom. How far these observations are of practical importance the future will show, but already they clearly indicate that protozoology may be studied with profit at home no less than abroad.

Organization and Training of Workers. In conclusion, we shall now note very briefly what has already been done for the promotion of protozoology as a branch of science.

As a profession it still hardly exists. Most of those who have enlarged the science have been zoologists or medical men en- gaged in teaching other subjects and in practising their pro- fessions. Many great discoveries have been made by men who cannot be described as protozoologists^ But the science has now become so vast, from the amassing of myriads of complicated details, that it can no longer be regarded as an occupation for anyone but a highly trained specialist. The amateur toying with his microscope, the ordinary zoologist or physician working in occasional vacations or leisure hours snatched from practice, can no longer expect to make any solid contributions to proto- zoology. In future all great advances in knowledge must come from those who are bred up as protozoologists who not only have the necessary physical and mental gifts for this most difficult study, but who also are prepared to devote their lives and energies to it, and to it alone.

Modern science has already developed to such unwieldy pro- portions that it has ceased to be coherent and has burst asunder into separate segments. The day of the " scientist," with all science for his province, is gone for ever. If men of science are to escape the fate of the builders of the Tower of Babel, it can only be by conscious cooperation. Each worker must do his own special work, but must do it with due regard for his fellow- labourers in adjoining sections, and with the plan of the whole building constantly before his eyes. Protozoology must, accord- ingly, develop along its own lines and by the labour of proto- zoologists, but it must remain in touch with the rest of zoology and with medicine and with all other sciences whose collaboration is likely to be mutually beneficial. We can already observe the bad effects of non-collaboration in the modern school of proto- zoology which originated with Fritz Schaudinn in Berlin. Over-specialization has there led after beginning on an ad- mirable foundation of fact to fantastic speculation and the promulgation of doctrines which are biologically unsound.

One of the good results of the World War was to encourage the collaboration of workers in different branches of science, and in Britain the bonds which previously existed between protozoology and medicine have been greatly strengthened. One of the most obvious conditions necessary for the con- tinuance of this alliance is the growth of protozoology itself. Unless the protozoologists can build scudly, and not too slowly, they will lose their advantages. Unfortunately, no adequate provision has yet been made for the training of workers in protozoology. At present there are in Britain and elsewhere few first-rate professional protozoologists and few competent teachers, but a large number of day labourers and dabblers from other sciences. Protozoologists are still mainly recruited from other professions. The remedy for this state of affairs will be found only when protozoology is recognized as a separate science an occupation for specialists and not for smatterers; and when encouragement is given to its development by the founding of .professorships in the subject or similar appoint- ments in the larger universities. These professorships must be primarily for research, and secondarily for teaching purposes. The professor must have ample time and funds for teaching himself, and for carrying out his own researches. If he is suffi- ciently gifted to do both these things, he will be able at the same time to teach his science to others who would follow in his foot- steps. But the tune has now gone when the junior demonstrator in zoology or the lecturer on general parasitology in the medical schools can expect to " take up " protozoology for a term or two and thereby profit science or himself. Unfortunately, too little had been done up to 1921 to create the necessary facilities.

A professorship in the subject, founded on the right lines, was indeed instituted in London University some years ago, but it had remained unoccupied up to 1921 since the death of its first holder in 1915. At Cambridge the Quick professorship of biology, founded later, at one time appeared likely to develop into a chair of protozoology, but these hopes were not fulfilled. An assistant professorship, chiefly devoted to protozoology, recently existed in the Imperial College of Science in London; but no further appointment was made to this post after it was vacated by its first occupant. The medical schools of Great Britain have, in some instances, lecturers in protozoology, but these are mostly medical men with other work to perform and no special knowl- edge of the science as a whole. The schools of tropical medicine in London and Liverpool have been more fortunate, and have been able to appoint to their staffs protozoologists who can devote their undivided attention to the subject. But here again it is chiefly the practical side of the science, as applied to medi- cine, that is being fostered. Rothamsted Experimental Station has a protozoologist to study the subject in its agricultural aspects, and several universities and other institutions of minor importance have members who have specialized in protozoology. Veterinary medicine in Great Britain has, however, still done little for research or instruction in protozoology.

In the British colonies and dependencies things are no better. A chair of protozoology has recently been created in India;