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into its own. As yet we are hardly on the threshold of the new biology, but for those who delight in the destruction of error and the advancement of true learning, the protozoological prospect is already full of hope.

The Applied Science. The chief practical appli cations of protozoology are to medicine. Certain of the Protozoa live as parasites in the bodies of men and animals, and thereby cause diseases. Some of these are so important that they are widely known for example, malaria and sleeping sickness and the elucidation of such diseases is one of the most interesting and recent chapters in biology. Protozoology also has certain applica- tions to agricultural science, because many Protozoa inhabit the soil, but their value is still doubtful.

The founder of protozoology was the first to find Protozoa inhabiting the living bodies of other and larger animals. In 1681 he described one such " animalcule " which was living in his own intestine. In 1683 he described and depicted others from the intestine of the frog. All these are recognizable, with fair certainty, at the present day. Leeuwenhoek did not suggest that these " parasites " were in any way concerned in the causation of disease, and it is probable, indeed, that the forms which he observed are not. But already at that date the " mi- crobe " theory of disease-production was in existence, for it was guessed at long before any " microbes " were discovered; and consequently we find that, even in Leeuwenhoek's lifetime, the suggestion was put forward that his " little animals " might be the " causes " of certain disorders. We find, for example, an early fellow of the Royal Society remarking, in 1683, of a " murren " which had raged among the cattle in central Europe, and of which the cause was undiscovered: " I wish Mr. Leewen- hoeck had been present at some of the dissections of these in- fected Animals, I am perswaded He would have discovered some strange Insect or other in them." Mr. Leeuwenhoek's successors have, on many a like occasion, fulfilled the expectations of " the ingenious Fred. Slare, M.D., and F.R.S.," but his " s.trange insects " they now call " Protozoa " or " Bacteria."

From the time of Leeuwenhoek to the present day the parasitic Protozoa have been studied with increasing attention. Their relation to diseases has been gradually elucidated, though we are still very far from finality in our knowledge of this ab- sorbingly interesting subject. The history of our knowledge is long, and the discoveries have followed devious ways too devious and intricate to be more than touched upon here.

Our knowledge of protozoal diseases diseases colloquially said to be " caused " by protozoal parasites really begins as recently as the middle of the ipth century, when Louis Pasteur (1822-1805) began his researches on a disease of silkworms called pebrine. Applying to the investigation of this disease the genius which stamps his work on " microbes " generally, Pasteur first discovered its causes, and then deduced methods for its prevention. The " cause " he found to be a microscopic parasite, now called Nosema bombycis and classified among the Protozoa. Although Pasteur did not know that the parasite was a protozoon, his work on pebrine and other microbic diseases was of fundamental importance for protozoology, because it demonstrated the methods by which such diseases can be studied and elucidated. Pasteur's scientific principles were impeccable, and equalled only by his own practical applications of them. It is common knowledge that he founded modern bacteriology; but in so doing he also laid the foundations of medical proto- zoology. To the casual reader it may seem strange that the study of silkworms can have any bearing upon medicine, or could in any way contribute towards the alleviation of human suffering. But there was another practical result of Pasteur's work which everyone will immediately appreciate, since it can be expressed in pounds, shillings and pence. Before pebrine attacked the silk- worms of France the silk industry yielded an annual revenue of 130,000,000 francs to the State. After the disease had raged for a dozen years the revenue had fallen to 8,000,000, and the industry was on the brink of ruin. To have discovered the causes of the disease, and to have devised, as a direct consequence, means for its control, and, as a further consequence of this, to

have rehabilitated the whole silk industry these are practical results which everyone can understand. And one has but to remember that protozoal diseases may affect man himself and his larger domesticated animals not merely silkworms to realize the practical possibilities of protozoology.

Towards the close of the ipth century medical protozoology became linked up with another branch of zoology entomology, the science which deals with insects. This connexion has nothing to do with the silkworms just mentioned, but arose through the discovery of the part played by certain other insects in the causation of protozoal diseases. The discoveries in this field began, once more, with the investigation of a disease of domes- ticated animals; but the pioneer was not, in this case, the Frenchman Pasteur, but the Scotsman David Bruce. His work is of such importance that we must notice it at this point.

The Work of Bruce. Some parts of Africa are the home of certain large blood-sucking flies called " tsetse." The " Fly Country " is uninhabitable except for wild animals; and long before its full significance was understood, the fly itself was recognized as a serious obstacle to the opening-up of Central Africa. Livingstone, the greatest of all African explorers, was so impressed with the fly's importance in this connexion that he put a vignette of a tsetse on the title-page of his Missionary Travels (ist ed., 1857). Live stock taken into the " Fly Country" rapidly succumbs to a disease which is called " nagana " in Zululand, where Bruce's original investigations were made. The disease was also called " tsetse-fly disease," since it was believed by the European settlers to be, caused by the bite of the fly. The natives believed, however, that it was " caused by the presence of large game, the wild animals in some way con- taminating the grass or drinking-water."

Bruce began his work in Zululand after an abortive attempt in 1894 in Sept. 1895 (the month of Pasteur's death). His full report on his researches is dated May 1896. In this almost in- credibly short space of time he demonstrated that nagana is caused by a protozoal blood-parasite since named Trypanosoma brucei, after its discoverer; that the parasite lives normally in the blood of big game, without harming them; and that it is conveyed from animal to animal by the tsetse. When the fly sucks the blood of an infected animal it becomes itself infected with the trypanosomes, which are subsequently re-inoculated into other animals by the fly when it sucks their blood. If these other animals are domestic stock, such as oxen or horses, they become infected with trypanosomes, contract nagana, and die. If they are wild game, such as antelopes, they also become in- fected, but develop no disease. In nature the trypanosome lives in the game and the flies alternately, the fly acting as an intermediary in the spread of infection from antelope to antelope. The big game indigenous in the country are habituated to and proof against the infection; domestic animals foreigners, introduced by man are not, and when infected usually die.

Bruce thus succeeded in extracting elements of truth from both the European and the native beliefs, and was able to com- bine them into a true theory of the causation of nagana. At the same time he threw a flood of light on many other protozoal diseases, and suggested all sorts of possibilities concerning their causation and prevention. He forged new links between proto- zoology and medicine and between entomology and protozoology. It is true there were other lights and other links before. Try- panosomes were known, and known to cause diseases, before Bruce went to Zululand. Timothy Lewis and Griffith Evans had observed similar parasites in India more than a decade earlier; and Theobald Smith and Kilborne, in America, had demonstrated in 1893 that the disease of cattle known as " Texas fever " a disease also caused by a blood-inhabiting protozoal parasite is transmitted from beast to beast by the agency of ticks. But Bruce's work was solid, complete, and demonstrative. By clean experiments and right reasoning he contributed more to science in a few months than hundreds who have followed up his work have since been able to contribute in many years. In work of this sort it is the quality, not the quantity, that counts. Later researches have but served to enhance the