The Indians could not undertake any widespread cultivation of the plains not only because they lacked iron tools but also because they had no draft animals. ↗
They weren't immigrating to some existing society; indeed, they often did whatever they could do to destroy whatever existed here in the way of Indian society. ↗
There the wild animals wandered and fed as though they were in a pasture that stretched much farther than a man could see, and there were no settlers. Only Indians lived there. ↗