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Rh isms subtending them, are kept under by natural enemies which prey on them, just as fishes prey on and keep down water-haunting insects, or as mice do humble-bees. The geographical range of such disease germs, therefore, will depend, not only on the presence of favourable conditions, but, also, on the absence of unfavourable ones. Herein lies a vast field for study, and one which, as yet, has not been touched by epidemiologists.

In these and similar ways the peculiar distribution of tropical diseases is regulated. The more we learn about these diseases the less important in its bearing on their geographical distribution, and as a direct pathogenic agency, becomes the role of temperature per se, and the more important the influence of the tropical fauna.

Whatever may have been the original source of the pathogenic parasites of man, it is certain that many of those which have a wide distribution at the present day were much more restricted originally. The extension of many of them has occurred within historical, and of some even in recent, times. Thus, in the last century, cholera spread over a great part of the world from its reputed home in India. Small-pox and other Old- World diseases have crossed the Atlantic; and some originally American diseases, such as syphilis and the chigger, have appeared in the Old World; measles, whooping-cough, tuberculosis, and leprosy have been introduced into the Pacific Islands. The process of diffusion is still proceeding, assisted, doubtless, by the vastly increased rapidity and frequency of modern travel, and by the breaking down in recent times of social, political, and physical barriers that formerly isolated many communities, some of which had been from time immemorial the sole repositories of particular disease germs. Thus the sleeping sickness of West Africa is passing to East Africa, and thus, most probably, the yellow fever of America will pass to Asia.

There is one factor which undoubtedly has contributed powerfully to delay the diffusion of certain