"[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. The buttercup is replaced by the little poisonous yellow oxalis
with its viviparous buds; the passion-flowers, asclepiads, bignonias,
convolvuluses, and climbing leguminous plants escape both floods and
cattle by climbing the highest trees and towering overhead in a flood of
bloom.
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