1888 - 89 .] Dr R. W. Felkin on Tropical Diseases. 
277 
not at the height of summer, but only when the high temperature 
is declining in late summer and in autumn, and, for the tropics 
in particular, at the end of the hot season. As many observers 
state, this is directly due to the great diurnal range of temperature 
that occurs at that season.” 
The influence of rain or moisture has undoubtedly much to do 
with both the production and spread of malaria. With regard to 
the rains, we may say that the malarial poison is most virulent 
either when they set in after a long period of heat, or when the rains 
cease and give place to warm, dry weather. An endemic outbreak 
of malaria, and its epidemic spread are both notably diminished at 
the height of the rains, if they are very abundant, and it has been 
proved over and over again that the malarial process is developed 
more abundantly in wet than in dry years. These remarks hold 
good both as to tropical and temperate climates. But it is not 
with rainfall alone that we have to do, for moisture must be present in 
the soil in order for the production of malaria, and this saturation 
of the ground may be produced in various ways apart from atmo- 
spheric precipitations. Drainage from rivers, lakes, and pools, may 
constantly saturate the soil, and so may inundations either periodic 
or irregular. The irrigation of the soil exerts an undoubted 
influence on the production of the poison ; its effect is very marked 
in Egypt, and in the irrigation districts of India. Lastly, the soil 
may be saturated by sub-soil water. This point is of importance, 
because it explains the occurrence of malaria in localities remote 
from river basins, and where the soil cannot become saturated in 
other ways. The occurrence of malaria in the Sahara, in Spain, 
Greece, Algiers, Tripolis, and Darfour, is in all probability due to 
this sub-soil water, arising either from springs or in other ways and 
resting upon impermeable strata of rock or marl. 
Apart from the moisture in the soil, it must possess other physical 
characteristics, although the geological characters of the country 
would appear to exert little or no influence on the production or 
non-production of the disease, except in so far as they affect the 
physical nature of the soil. Clay, loam, clayey marl, and marshy 
soil are most favourable to the production of malaria; a porous 
chalky soil is less favourable and sandy soil least so, provided 
always that the chalk or sand does not rest either upon clay or firm 
