Description of the Landes of Acquitania. 5 
themselves deep furrows in the sand, laying bare the roots, and 
carrying away the shrubby plants, leaving beds of white micaceous 
sand behind them. : 
Having given the idea, then, of an extensive tract of land be- 
longing to the upper marine formation, consisting of a sandy soil, 
lying on a quartzose-ironstone (grés ferrugineux) sands, and shelly 
marle ; of little elevation, and of a very continuous level, and limit- 
ed in its extent by hydrographical and geognostical boundaries, we 
shall proceed to delineate the characters of its vegetation, which 
are always of the greatest interest in the study of uncultivated 
Jands. 
Between the vast tracts of sand entirely deprived of vegetable 
or animal life, which occur in the tropical regions, and the eternal 
snows of the polar lands, the plains, vallies, and mountains, and 
the contrasted configurations of which the continental spaces con- 
sist, are generally clothed with a vegetation more or less luxuriant, 
and presenting characters in whose constancy are to be traced the 
elements of their physical distribution, and the harmony of their 
natural laws. It is in the tendency which certain plants have of 
g-owing always in the society of one another, and inhabiting only 
determinate situations, that originates the characteristic vegetation 
of all uncultivated tracts; as in the lines of equal annual heat, 
or of similar hyemal, or summer seasons, must be sought the ex- 
tended zones of vegetation which characterize different geographi- 
cal latitudes. 3 
When subjected to determinate laws of this nature, plants have 
been denominated social; but while some grow in the society of 
one another, others seem to prefer the society of different species. 
If the latter were always the same, such facts would assist in in- 
ductions on the vegetation of a district; but a vagabond plant, 
that in one latitude accompanies a certain set, will traverse one or 
more zones accompanying other families of plants, which in these 
zones grow on tracts of a similar nature; while others, perma- 
nently social in one climate, become dispersed and nomadic when 
they traverse the limits of their natural stations. 
While all plants may be said to be special, with respect to their 
primitive site, whether parasitical, aquatic, or the inhabitants of fo- 
rests or plains, still the same characters of lake or marsh, and moun- 
tains of the same geognostic structure, will, in different climates, 
be clothed with plants of different species, and often of different 
families ; while these circumstances becoming conditions of exist- 
ence, render the plants growing on these tracts slaves to the socie- 
ty of one another, causing them to perish when borne beyond certain 
limits of their natural stations ; and in these facts consists the loca- 
lity of plants, or the laws of special and local creations. Amidst 
these, numerous flowering plants will appear, whose representa- 
tives, are oftentimes found at the antipodal points of the globe, and 
which gradually advance by regular means of dissemination, or 
suddenly traverse immense intervals, by accidents connected with 
