The idea of Pleistocene rewilding in North America is provocative, but it need not be treated only in the abstract. The return of beavers (Castor canadensis) to almost every region of the continent has shown us that the behavior of these creatures was, in many ways, originally responsible for the contours of the landscape and many rich soil deposits. Left to their own ways, beavers could reintroduce us to a North America that disappeared when they did.

James M. Bryant
Riverside, Calif.

The article presented an absolutely hair-raising scenario of potential ecological disaster. Introduction of plants or animals at a rate faster than can occur naturally causes a wave of parasitic inoculation that can rapidly devastate naïve local populations.

W. Geisler
Pelham Manor, N.Y.

The animals would have to be engineered to be inedible and to have instinctive knowledge of property and trespass law and an innate passion for strict compliance, or they would not survive. I would think that pigs with wings would be less challenging.

McClellan Blair
Indiana, Pa.

The magazine’s cover, with megafauna on one side of a fence and a highway on the other, perfectly illustrates a more pressing environmental problem: fragmentation of habitat.

Peter Wilson
Simi Valley, Calif.