What is the “ call of the wild “ that Buck experiences ?

This happens at the end of the last chapter (chapter 7). That night Buck hears the call once more, and this time he knows he must leave for good. Nothing remains to hold him. The wolf pack rushes into the clearing, but they come to a halt upon seeing Buck. The boldest one strikes Buck and instantly has his neck broken. The rest descend, but Buck holds them off for half an hour. The wolves draw back, discomfited. One steps forward and whines softly. Buck recognizes his wild brother, and they touch noses. When the pack howls, Buck joins them. Obeying the law of club and fang, they accept his fellowship, and he runs with them into the woods.

After some years, the wolves of the valley are seen to have splashes of brown and white. Yeehats speak of a Ghost Dog that haunts the woods and slays the bravest hunter. One wolf returns to the valley each year. He sits and muses, remembering, howls once and then rejoins his pack.