Exactly the same fact has been observed in the prairies of
North America, where coarse grass, between five and six feet high,
when grazed by cattle, changes into common pasture land. (6/7. See
Mr. Atwater's "Account of the Prairies" in "Silliman's North
American Journal" volume 1 page 117.) I am not botanist enough to
say whether the change here is owing to the introduction of new
species, to the altered growth of the same, or to a difference in
their proportional numbers. Azara has also observed with
astonishment this change: he is likewise much perplexed by the
immediate appearance of plants not occurring in the neighbourhood,
on the borders of any track that leads to a newly-constructed
hovel. In another part he says, "Ces chevaux (sauvages) ont la
manie de pr?f?rer les chemins, et le bord des routes pour d?poser
leurs excr?mens, dont on trouve des monceaux dans ces endroits."
(6/8. Azara's "Voyage" volume 1 page 373.) Does this not partly
explain the circumstance? We thus have lines of richly manured land
serving as channels of communication across wide districts.
(PLATE 26. GIANT THISTLE OF PAMPAS.)
(PLATE 27. CYNARA CARDUNCULUS OR CARDOON.)
Near the Guardia we find the southern limit of two European plants,
now become extraordinarily common.
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