A change that is scarcely visible, but is of considerable economic importance, thus takes place in the vegetation of the prairie owing to the presence of herds. The _pasto fuerte_, composed of rough grasses, which is the natural vegetation, is being succeeded by the _pasto dulce_, in which annual species, soft grasses, leguminous plants, etc., predominate. It is mainly composed of plants of European origin. The difference between the _pasto dulce_ and the _pasto fuerte_ or _duro_ is so important for the farmer that there is hardly a single work on Argentina which does not dwell on it. The idea, however, that the _pasto dulce_ has advanced steadily westward, starting from the vicinity of Buenos Aires and constantly enlarging its domain, is not strictly accurate. In 1895 Holmberg[3] traced the western limit of the zone of the _pasto dulce_ through Pergamino, Junin, Bragado, Azul, Ayacucho, and Mar Chiquita. When we compare this with earlier observations, we see that in the course of the nineteenth century the zone of the _pasto dulce_ has extended by about a hundred miles on the southern Pampa. When Darwin travelled from Bahia Blanca to Buenos Aires in 1833, he found no _pasto dulce_ except round Monte, on the right bank of the Salado. Further north, on the other hand, the extent of the _pasto dulce_ does not seem to have altered appreciably. The expedition to the Salt Lakes in 1778 found that there were already thistles beyond the line of the ranches, and these are characteristic of the _pasto dulce_ in the Chivilcoy region on the Salado, which was then abandoned to herds of wild cattle. "There was thistle enough to cook," says the journal of the expedition. The difference is connected with the history of colonization in the province of Buenos Aires, where ground was gained only toward the south between 1800 and 1875. Since 1895 the _pasto duro_ has been eliminated by agriculture rather than by the feet of the herds. Hence the advance of the _pasto dulce_ is no longer in a continuous line moving toward the west. It is sporadic, depending upon the construction of new railways which open up the plain to the plough.[4] [3] Holmberg, "La Flora de la Republica Argentina," in the _Secundo Censo de la Republica Argentina_, vol. i. (Buenos Aires, 1898). [4] _Diario de la expedition de 1778 a las Salinas_ (Coll. de Angelis, iv.). Colonization does more than emphasize the individuality of each of the natural regions. It connects together different features, and blends them in a complex vital organism which goes on evolving and renewing itself.