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104 sufficient lowering of temperature. In Italy there are many malarious spots high up among the hills; the same is the case in India and elsewhere in those elevated valleys which are also narrow, imperfectly ventilated, and imperfectly drained. Moisture.—— Another important condition for the production of malaria is the .presence of water. In the Sahara there is no malaria unless in the oases; in many of these it is rife—— in Biskra, for example. A large expanse of water is not favourable to malaria. The mosquito thrives best in sluggish streams with grassy banks and many backwaters; still better, in small pools or other collections of water, as in broken bottles, empty tins, etc., where there are no fish to prey on their larvæ, and where the surface is not agitated by winds. One such puddle near a house may suffice to render that house unhealthy.

Decomposing vegetable matter.—— It is customary to add yet another condition as being necessary to the existence of malaria—— namely, the presence in the soil of a notable amount of decomposing organic matter, particularly vegetable matter. But that this is not an indispensable condition is proved by the fact that there are many almost barren spots in which malaria abounds.

Other conditions necessary.—— The concurrence of these conditions, high temperature and moisture, even though associated with abundant vegetation, is not sufficient to generate or support malaria; for there are many places in the world the Argentine and the islands of the South Pacific, for example in which high temperature, moisture, and decaying vegetable matter are present but in which malaria is almost unknown. Manifestly there are other and more complicated conditions which are equally indispensable, and which must concur with heat and moisture in order to secure the presence of malaria-bearing mosquitoes. What these conditions are it is as yet impossible to say.

Influence of subsoil moisture.—— The state of the subsoil as regards moisture appears to have considerable influence on its malaria-producing properties.