Not that Buck reasoned it out. He was fit, that was all, and unconsciously he accommodated himself to the new mode of life. All his days, no matter what the odds, he had never run from a fight. But the club of the man in the red sweater had beaten into him a more fundamental and primitive code. Civilized, he could have died for a moral consideration, say the defence of Judge Miller's riding-whip; but the completeness of his decivilization was now evidenced by his ability to flee from the defence of a moral consideration and so save his hide. He did not steal for joy of it, but because of the clamor of his stomach. He did not rob openly, but stole secretly and cunningly, out of respect for club and fang. In short, the things he did were done because it was easier to do them than not to do them. 
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 or 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. He learned to bite the ice out with his teeth when it collected between his toes; and when he was thirsty and there was a thick scum of ice over the water hole, he would break it by rearing and striking it with stiff fore legs. His most conspicuous trait was an ability to scent the wind and forecast it a night in advance. No matter how breathless the air when he dug his nest by tree or bank, the wind that later blew inevitably found him to leeward, sheltered and snug. 