topographical factors in epidemiology of MALARIA 
141 
deltas of both rivers, particularly in their 
lower portions. Propagation also occurred 
in isolated pools in the upper portions of 
these rivers during the dry season. 
Since 1915 there has been a conspicuous 
decline in malaria in California, due to con¬ 
trol measures. However, most of the cases 
still occur in these river valleys (California 
Department of Public Health 1940), al¬ 
though much of it is attributed to A. maculi- 
pennis propagation in irrigated fields in 
these districts. 
A. maculipennis finds suitable environ¬ 
ments for propagation elsewhere in Califor¬ 
nia and in the Pacific Northwest (Stage and 
Gjullin 1935) in spring and seepage water 
and along the grassy edges of permanent 
ponds and small lakes. Webber (1920) re¬ 
ports endemic malaria at an altitude of 
5482 feet. However, it appears that ma¬ 
laria and anophelism elsewhere than in the 
river valleys named above are of relatively 
little consequence. 
2. Mexico, Central America, and the 
West Indies. The elevated plateau of cen¬ 
tral Mexico is flanked on either side by a 
high chain of mountains, the Sierra Madres, 
which slope down to the Pacific Ocean on 
one side and to the Gulf of Mexico on the 
other. The peninsula of Southern Cali¬ 
fornia is a continuation of the American 
coast range, while the peninsula of Yucatan 
is composed entirely of low calcareous 
plains. The central Mexican plateau 
reaches an elevation of 8000 feet in the 
vicinity of Mexico City; its southward slope 
is abrupt and its northward slope is very 
gradual. River systems are scarce, and 
much of the northern part of the country is 
arid or semi-arid. Breeding sites for the 
several species of malaria vectors present 
in the country are spread widely, and oc¬ 
cur in topography varying from the flat 
coastal plains to the high central plateau. 
The basic topography of the Central 
American countries is not uniform through¬ 
out, but in general low-lying coastal plains 
rise to mountain ranges in the interior. 
Notable exceptions are the San Juan River 
and Lake Nicaragua regions in the south¬ 
ern part of Nicaragua, where a wide strip 
of lowland cuts the mainland from the Mos¬ 
quito Bank to the Pacific Ocean. Large 
areas of swampland border the eastern 
Kiironoeim 
f 
