The background against which the early history of the South American animals unrolls includes the food supplies, the climate, and the migration possibilities. The time when the various groups begin to assume modern aspects and when the mammals begin to flourish is the latter part of the Cretaceous and the beginning of the Eocene.
The prime influence controlling any group of animals is the food supply. While the origin of mammals goes at least back to the Comanchean, it is not until the Eocene that they flourish and spread out into luxuriant lines of adaptation. The evolution of flowering plants started in a manner which, to those of us who study mostly mammals, is strange; for the early angiosperms are trees of considerable size, while the herbaceous plants coming later are smaller by far. Just before the beginning of the Eocene, sedges and grasses . . .