DESIDERATA. xiii 
including even some of American origin, which like the Maize 
and the Arachis, were so early cultivated in Asia as to give rise 
to doubts whether they were not known there before the dis- 
covery of the New Continent. Among those now cultivated in 
America as well as Afriea, and not known, or of recent intro- 
duction into Asia, the majority (as for example, some species 
of Panicum, Amomum, &c.) have been carried over to America 
from Africa; and few only (Manihot ?, Indigofera Anil, some 
species of Dioscorea, &c.) appear to have been introduced from 
America into Africa, or are among the naturally indigenous 
species to both countries. Additional facts tending to elucidate 
these points are much wanted. 
The weeds of cultivation, and other plants which accompany 
man in his migrations, are mostly, like the cultivated plants, of 
Eastern origin, although many are likewise now common in 
cultivated parts of Tropical America. The principal points to 
be attended to in respect to this class, are, how far they become 
really naturalized by maintaining themselves and spreading 
` beyond the crops with which they were originally introduced. 
The cosmopolite indigenous species are, within the tropics, 
chiefly aquatic or marshy, or belonging to the glumaceous or 
cryptogamic orders. The laws which are supposed to regulate 
their diffusion, being deduced from the number of recorded 
stations, the collector should never neglect them on the ground 
of their being common elsewhere. The preserving specimens 
of widely spread species is the more important, as it is often 
difficult for the most experienced botanist to be certain of the 
identity of plants observed at different periods, without an actual 
comparison of specimens, and it is only by a careful observation 
of variations of form occasioned by diversity of soil and climate 
that any satisfactory judgment can be formed of the systematic 
limits of species and races. 
The indigenous species common to West Tropical Africa and 
Tropical America, appear to be chiefly found near the sea, or at 
any rate do not penetrate beyond the first hills ; few, indeed, of 
the really maritime species are Asiatic or East African, with the 
exception of such as are diffused over all Tropical sea coasts. 
