Quote 34
One night he sprang from sleep with a start, eager-eyed, nostrils quivering and scenting, his mane bristling in recurrent waves. From the forest came the call (or one note of it, for the call was many noted), distinct and definite as never before--a long-drawn howl, like, yet unlike, any noise made by husky dog. And he knew it, in the old familiar way, as a sound heard before. He sprang through the sleeping camp and in swift silence dashed through the woods. As he drew closer to the cry he went more slowly, with caution in every movement, till he came to an open place among the trees, and looking out saw, erect on haunches, with nose pointed to the sky, a long, lean, timber wolf. (7.12)
Buck’s internal desires to be in the wild are manifested in the physical call that he literally hears from the woods.
Quote 35
On the opposite slope of the watershed they came down into a level country where were great stretches of forest and many streams, and through these great stretches they ran steadily, hour after hour, the sun rising higher and the day growing warmer. Buck was wildly glad. He knew he was at last answering the call, running by the side of his wood brother toward the place from where the call surely came. Old memories were coming upon him fast, and he was stirring to them as of old he stirred to the realities of which they were the shadows. He had done this thing before, somewhere in that other and dimly remembered world, and he was doing it again, now, running free in the open, the unpacked earth underfoot, the wide sky overhead. (7.16)
The affinity Buck feels for the natural world is tied to his instinctual feelings and his relation to the past.
Quote 36
For two days and nights Buck never left camp, never let Thornton out of his sight. He followed him about at his work, watched him while he ate, saw him into his blankets at night and out of them in the morning. But after two days the call in the forest began to sound more imperiously than ever. Buck's restlessness came back on him, and he was haunted by recollections of the wild brother, and of the smiling land beyond the divide and the run side by side through the wide forest stretches. Once again he took to wandering in the woods, but the wild brother came no more; and though he listened through long vigils, the mournful howl was never raised. (7.19)
Buck’s conflict over his love for Thornton and his love for the wild are tied to his internal conflicts—the clash between the domestic dog he used to be and the wild wolf he is becoming.