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 chamcedrifolia, Mitracar- 

 pwm 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 ccerulea, Stigmaphyllon littorale, 

 two or three species of Paullinia, a Cardiospermttm, and a Bignonia ; or Leguminosce of a 

 tropical character, — species of Mimosa, Inga, Calliandra, and Cassia. Of the Melastoma- 

 cece, 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 Machcerium, 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 Legwminosce. 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° S. 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- 



