278 Proceedings of Royal Society of Edinburgh. [sess. 
rock. The exemption from malaria of some of the islands in the 
West Indies with a chalky soil is remarkable when contrasted with 
its special prevalence in islands of volcanic formation. The amount 
of organic material too, contained by the soil is bound up with the 
production of malaria. All other circumstances being equal, the 
greatest amount of malaria will be found where the amount of 
organic matter in the soil is greatest, the prevalence of the disease 
diminishing as the organic matter is found in less abundance. 
Changes in the soil produced by cultivation, the neglect of cultiva- 
tion, and by excavations, also affect the presence or absence of 
malaria, but space forbids us entering into these details. In 
reference to marshes, however, it may be noted in passing that 
malaria will disappear from a marshy district if it is completely 
drained and dried, or if a marsh is converted into a pool or lake. 
The amount of malaria, as also its severity, is affected by 
altitude and the configuration of the ground. The altitude at 
which malaria can be produced varies in different regions, being 
higher in the tropics than in the temperate zones. Thus in Central 
Africa we find that a height of some 3000 feet must be attained 
before one reaches a district free from malaria, whereas in the 
Apennines a height of 1500 feet only is required, and farther north 
only 500 feet. This, however, must be explained more by variation 
in temperature than by mere altitude. On the other hand, however, 
examining the configuration of the ground in plains, it is found 
that the disease is distinctly more virulent the lower the level of 
the country. This fact is so marked that often even 50 or 100 feet 
less makes a considerable difference to the salubrity or otherwise of 
a given spot. 
Although winds do not exert any direct influence upon the pro- 
duction of the malarial poison, they act indirectly, as, for instance, 
by moderating temperature, &c. They act, however, directly in 
the diffusion of the poison or in preventing it exercising its potent 
effects. Wind may carry the malarious poison from a marsh to a 
healthy district, but it is probable that it can only thus convey it 
for a distance of some two or three miles. Malaria may rise to a 
height of some 700 or 800 feet in a calm atmosphere ; wind will 
prevent this vertical diffusion. Probably on some islands, where 
from analogy we should expect to find malaria present, the constant 
