Chapter 2: The Law of Club and Fang:
“…His development (or retrogression) was rapid. His muscles became hard as iron and he grew callous to all ordinary pain. He achieved an internal as well as external economy. He could eat anything, no matter how loathsome of indigestible; and, once eaten, the juices of his stomach extracted the last least particle of nutriment; and his blood carried it to the farthest reaches of his body, building it into the toughest and stoutest of tissues. Sight and scent became remarkably keen, while his hearing developed such acuteness that in his sleep he heard the faintest sound and knew whether it heralded peace or peril…The domesticated generations fell from him.”
Tis the season to dust this one back off, always worth the re-read. For a change, you can catch the free domain audiobook here, or watch Charlton Heston's take here.