462 Bulletin American Museum of Natural History [Vol. LIIT 
being almost everywhere over 40 inches yearly and more than 60 inches 
in much of the territory (See Map 8). The seasonal variation of moisture 
then becomes the deciding factor in the ecology and distribution of 
plants and animals. In Africa, as in other tropical regions, it rains during 
the hottest season of the year, when the sun nears the zenith. Therefore 
a a, 
ae. 
2S CO 
Mean Annual Rainfalf 
{nehes [1 millim. 
UT WTOP worn noon === TROPIC OF, CAPRICORN 
600 800 1000 Miles 
ado 
“m0, 200) 400 600 _800 ay ee 
Map 8. Distribution of mean annual rainfall on the African continent (after 
Knox). 
at the equator there are two seasons of heavy rains at intervals of about 
six months, while during the intervening periods the rains are scarcer or 
may even stop altogether. Farther north or south, the dry seasons be- 
come more marked, but, on the other hand, the two rainy seasons come 
closer together until near the tropics there is only one rainy season of a 
few months’ duration. At least in the lowlands of the Congo, latitude 
thus gives a good clue to the yearly amount and seasonal distribution of 
rainfall. In the mountains, however, not only must one take into account 
