424 
MIGRATION OP PLANTS. 
been recognized among those that were gathered bj M 
Bonpland and myself on the cold table-lands of Mexico, 
along the burning shores of the Orinoco, and in the south- 
ern hemisphere on the Andes and Quito.* How can we 
conceive the migration of plants through regions now co- 
vered by the ocean ? How have the germs of organic life, 
which resemble each other in their appearance, and even in 
their internal structure, unfolded themselves at unequal dis- 
tances from the poles and from the surface of the seas, 
wherever places so distant present any analogy of tempe- 
rature ? Notwithstanding the influence exercised on the 
vital functions of plants by the pressure of the air, and the 
greater or less extinction of light, heat, unequally distri- 
buted in different seasons of the year, must doubtless be 
considered as the most powerful stimulus of vegetation. 
The number of identical species in the two continents 
and in the two hemispheres is far less than the statements 
of early travellers would lead us to believe. The lofty 
mountains of equinoctial America have certainly plantains, 
valerians, arenarias, rauuneuluses, medlars, oaks, and pines, 
which from their physiognomy we might confound with 
those of Europe ; but they are all specifically different. 
'When nature does not present the same species, she loves 
to repeat the same genera. Neighbouring species are often 
placed at enormous distances from each other, in the low 
regions of the temperate zone, and on the alpine heights of 
the equator. At other times (and the Silla of Caracas 
affords a striking example of this phenomenon), they are 
not the European genera, which have sent species to people 
like colonists the mountains of the torrid zone, but genera 
of the same tribe, difficult to be distinguished by their 
appearance, which take the place of each other in different 
latitudes. 
The mountains of New Grenada surrounding the table- 
lands of Bogota are more than two hundred leagues dis- 
tant from those of Caracas, and yet the Silla, the only 
elevated peak in the chain of low mountains, presents those 
singular groupings of befarias with purple flowers, of 
* Cyperus mucronatus, Poa eragrostis, Festuca myurus, AnJropogos 
aTenaceus, l.apago racemosa. (See the Nova Genera et Species Plartaruiu 
vol. i. p. xxv.) 
