tropical diseases—the circumstance that most of them depend on protozoal or some other kind of animal germ requiring for its transmission an animal intermediary. Diseases which depend on bacterial germs, if their special bacterium be introduced, social and sanitary conditions being favourable, will spread in any country or climate, and thus it is that all bacterial diseases, with hardly an exception, are found, or are capable of existing, everywhere; in the passage from host to host their germs are not killed by ordinary atmospheric conditions, and they require no second intermediary. Diseases depending on protozoa or other animal germs, in many though not in all instances, will not establish themselves thus universally, because their germs in the passage from host to host demand, through their intermediaries or otherwise, very special and climatically restricted conditions. Tropical diseases belong for the most part to this category, and therefore their successful introduction and spread to new ground are attended with more difficulty than bacterial diseases, demanding, as the former generally do, the double condition of the successful introduction not only of the germ itself but also of the intermediary.
Although this double necessity has undoubtedly operated powerfully against the spread of certain tropical diseases, there is reason to believe that in time this difficulty will disappear; for, so far as we know, there is no reason why, if introduced into new places, these animal intermediaries should not obtain a permanent footing and spread.
There are many instances of exotic insects, for example, which have established themselves after either accidental or intentional introduction into new countries. There is no reason, therefore, for thinking that disease -germ insect intermediaries could not be similarly established in countries in which they are unknown at present. Thus, if the tse-tse flies were successfully introduced into India, sleeping sickness might appear there in due course; or, if appropriate anopheles were introduced into
