We should be far too from the discouraging persuasion, that man is fixed, by the law of his nature, at a given point: that his improvement is a chimæra, and the hope delusive of rendering ourselves wiser, happier or better than our forefathers were. As well might it be urged that the wild & uncultivated tree, hitherto yielding sour & bitter fruit only, can never be made to yield better: yet we know that the grafting art implants a new tree on the savage stock, producing what is most estimable both in kind & degree
This says that men are not completely bound by the laws of nature and that through education, man can change their future. If humans become educated they can improve and become better like how a chimæra can shape shift into other animals. It also compares man to a tree and how cultivating a tree can make a new tree that is better than it was before.