In one of these 'siccos,' at the time of my visit, no less than 50,000 head of oxen and sheep and horses perished from starvation and thirst, after tearing deep out of the soil every trace of vegetation, including the wiry roots of the pampas-grass.

Under such circumstances the existence of an unprotected tree is impossible.

The only plants that hold their own, in addition to the indestructible thistles, grasses, and clover, are a little herbaceous oxalis, producing viviparous buds of extraordinary vitality, a few poisonous species, such as the hemlock, and a few tough, thorny dwarf-acacias and wiry rushes, which even a starving rat refuses.
"Although the cattle are a modern introduction, the numberless indigenous rodents must always have effectually prevented the introduction of any other species of plants; large tracts are still honeycombed by the ubiquitous biscacho, a gigantic rabbit; and numerous other rodents still exist, including rats and mice, pampas-hares, and the great nutria and carpincho (capybara) on the river banks."[9] Mr.Clark further remarks on the desperate struggle for existence which characterises the bordering fertile zones, where rivers and marshy plains permit a more luxuriant and varied vegetable and animal life.
After describing how the river sometimes rose 30 feet in eight hours, doing immense destruction, and the abundance of the larger carnivora and large reptiles on its banks, he goes on: "But it was among the flora that the principle of natural selection was most prominently displayed.
In such a district--overrun with rodents and escaped cattle, subject to floods that carried away whole islands of botany, and especially to droughts that dried up the lakes and almost the river itself--no ordinary plant could live, even on this rich and watered alluvial debris.

The only plants that escaped the cattle were such as were either poisonous, or thorny, or resinous, or indestructibly tough.

Hence we had only a great development of solanums, talas, acacias, euphorbias, and laurels.