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RECENT discovery has clearly demonstrated the importance of the Protozoa as factors in the etiology of disease, especially of tropical disease. Manifestly the subject is still in its infancy. Many as yet unknown species of blood protozoa will doubtless be added to the already considerable list of those attacking man and the lower animals. As regards man, it is doubtful if we have as yet completed the list even of the much-studied malaria parasites, much less determined their respective life histories and zoological relationships. Moreover, there are certain genera of blood protozoa which, although occurring in other mammals, have, so far, not been found, or, at all events, recognized in man, Babesia and Hæmogregarina for example. It is not unreasonable to conjecture that, although hitherto unrecognized, representatives of these genera do occur in man, and that ere long they will be discovered. The investigator must not lose sight of this probability.

The entire subject is one of the utmost importance to the medical man in the tropics, and, whether he would perfect himself in existing knowledge or seek to add to it, he should study it practically and from the standpoint of comparative pathology. I would therefore strongly recommend him to avail himself of the abundant opportunities he is sure to enjoy to study the protozoa not in man only but in other mammals, in birds, in fishes, and in reptiles. As a guide and help in such studies I append a brief summary, for which I am indebted to Dr. Philip Bahr, of our present knowledge of this important class of