xiv DESIDERATA. 
in the ntericr of the country, Eastern and Asiatic species 
become much more numerous as the American ones disappear. 
In regard to ali these travelled species, we are in want of trust- 
worthy data as to the stations they occupy, how far from the 
sea, from the habitations of man, or from the regions of cultiva- 
tion, their seareity or abundance, the limits of the tracts they - 
occupy, and other circumstances tending to elucidate their mode _ 
of transmission, 1 
The endemic plants of West Tropical Africa are. of the . 
greatest interest, as supplying data for speculations on the laws 
regulating the geographical dissemination of analogous forms. 
Senegal and other drier northern parts of our region, not only — 
have many identical species, but still more of analogous ones to 
those which prevail through Nubia and Arabia, eastward. to 
the hot, dry plains and table-lands of India. In the moist, 
close regions about the mouths and branches of the Niger and 
the island of Fernando Po, some curious analogies may be — 
observed, with corresponding forms in Madagascar, Ceylon, — 
and the Malayan Archipelago, With these Asiatic forms are 1 
mixed, in various parts of the region, African representatives of — 
American genera, which appear to find here their extreme ^ 
Eastern limits, European and South African forms, genera as | 
well as species, are more completely excluded from this than . 
from any other Tropical region. For all data from which any - 
general conclusions under this head can be formed, we must — 
rely entirely upon the geographical notes supplied by local | 
collectors. | 
The practical, economical 
Tropical Africa is less known 
Products of the greatest value 
Similar doubts hang oyer the species or 
