AND THE NEIGHBOURING DISTRICTS. 189 
none of the native ones possess so hardy a constitution, or such powers of propagation, as 
these strangers. It is, as Mr. Darwin remarks, a parallel case to that of the horse and 
ox, which have, within the last three centuries, spread themselves in such countless num- 
bers over the same countries. 
It appears to me that this wide diffusion of naturalized plants, originally foreign to the 
country in which they now grow, bears in some degree upon the question of specific cen- 
tres; or at least is adverse to the views of those who consider the natural distribution of 
species as determined solely by favourable local circumstances. These introduced plants 
have established themselves so readily and so completely, that it is quite evident, the soil, 
climate, and other circumstances affecting their distribution, must be highly favourable ; 
yet they did not exist in those countries until introduced by the indirect agency of man. 
Therefore it would seem that they were not created indiscriminately in all the situations 
naturally adapted to their constitutions. But the general question of the distribution of 
plants is too wide for me to enter further upon it in this place. 
The social character which is so eminently conspicuous in many of the naturalized plants 
above noticed is not confined to them, but is observable also, though in a less degree, in 
several of the indigenous plants of the Pampas of Buenos Ayres. The most remarkable 
in this respect, as far as I observed, are Verbena erinoides and chamedrifolia, Mitracar- 
pum Sellovianum, and a dwarf Solanum ; besides a few grasses, which, as they were not in 
flower at the time of my visit to Buenos Ayres, I could not determine. This social growth 
of some particular plants, and consequent uniformity of vegetation, has, I think, been 
noticed by various naturalists as characteristic of extensive plains. ! 
Tropical forms of vegetation are not wanting in the Argentine region, but occur chiefly 
on the banks and islands of the principal rivers, much more rarely in the open country. 
They are principally woody climbers, such as Passiflora cerulea, Stigmaphylion littorale, 
two or three species of Paullinia, a Cardiospermum, and a Bignonia; or Leguminose of a 
tropical character,—species of Mimosa, Inga, Calliandra, and Cassia. Of the Melastoma- 
cee, a family so eminently characteristic of tropical South America, and especially of 
Brazil, one solitary species (an Arthrostemma) reaches to the north bank of the Plata, 
. but does not cross it. Colonia, opposite to Buenos Ayres, seems to be the most southern 
locality of that beautiful order. One Macherium, a very tropical form, grows in the 
islands of the Uruguay, near its mouth, and is probably the most southern representative 
of the Dalbergia tribe of Leguminose. A few Monocotyledonous genera which have their 
head-quarters within the tropics appear for the last time, as we go southwards, on the 
banks of the Plata; such are Canna (of which there is one species at Buenos Ayres), On- 
cidium, and Tillandsia. 
Of the range of Palms in the region in question I have no knowledge. It would appear 
from Mr. Darwin’s statements, that they occur here and there as far as 35° 8. lat., which 
seems to be likewise their southern limit in Chile. 
The southern limit of the Argentine vegetation seems to be determined mainly by soil; 
the northern, by climate alone. To the south its extension seems to depend upon that of 
the Pampean formation; that is to say, where the calcareous mud and marl of the Pam- 
pas are succeeded by the arid gravel or shingle of Patagonia, the character of the vegeta- 
