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Rh transmitting medium—be it air, water, or food—be too high or too low for the special requirements of the germ in question, that germ dies and ceases to be infective. In this way may be explained the absence from the tropics of a class of directly infectious diseases represented by scarlet fever, and the possible absence from temperate climates of a similar class of diseases. In the one case, during the short passage from one human being to another, tropical temperature is fatal to the air-borne germ; in the other it may be that the lower temperature of higher latitudes has the same effect.

In another type of disease, of which tropical scaly ringworm (tinea imbricate) is an excellent example, the germ vegetates on the surface of the body, and is thus exposed to the vicissitudes of climate. One of the requirements of the germ referred to is a high atmospheric temperature and a certain degree of moisture. Given these it nourishes; remove these and it dies out, just as a palm tree or a bird of paradise would die on being transferred to a cold climate.

Many diseases require for their transmission from one individual to another the services of a third and wholly different animal. The propagation and continued existence of a disease of this description will depend, therefore, on the presence of the third animal. If the latter be a tropical species, the disease for the transmission of which it is indispensable must necessarily be confined to the tropics. Thus the geographical range of malaria and of filariasis is determined by that of certain species of mosquito which ingest and act as alternative hosts to the respective germs, and, so to speak, prepare them for entrance into their human host. The distribution of a large number of animal parasitic diseases depends in this way on the distribution of these alternative hosts. When this animal happens to be a tropical species, the disease it subtends, so to speak, is, in natural conditions, necessarily tropical also.

Certain diseases are common to man and the lower animals. If these latter happen to be tropical species