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VI] India, as pointed out by Christophers and Stephens, and similar districts in Italy, as pointed out by Celli, in which, though surrounded by highly malarious country and though Anophelinæ abound, there is no malaria. The explanation of this anomaly is not apparent. Perhaps in these places there is some local substance on which the mosquitoes feed that is fatal to the parasite. Sambon ingeniously suggests that there may be special local conditions that favour hyperparasitism, that is, the attack and destruction of the malaria parasite by some other kind of parasite. Similar occurrences are common enough in nature. In support of this view he cites Ross's observation of what he designated "black spores" (Fig. 22), bodies which are now known to be protozoal organisms belonging

to the genus Nosema. These "black spores" prey on the malaria oöcyst in the wall of the mosquito's stomach and destroy it, just as they prey on the larvæ of Filaria immitis encysted within the Malpighian tubes of the mosquitoes which subserve their development. Whatever the explanation of the salubrity of these mosquito-haunted plains be, it is well worth searching for; it may be that it would carry a hint of value in attempting the prevention of malaria.

Atmospheric temperature.—One of the most important conditions necessary to the sporogonic phase of the malaria germ is a sustained average temperature of at least 60° F. The malaria parasite will not develop in the mosquito at low temperatures.

Altitude.—Altitude per se has, apparently, no influence on malaria. It is the decrease in temperature, usually implied by an increase in altitude, that is the real determining circumstance in bringing about a diminution in the prevalence of malaria in uplands. In the tropics an elevation of six or seven thousand feet may not secure immunity from malaria unless there be, at the same time, a corresponding and