This was in chapter 7.
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He means that Buck was one of a kind. Buck was a dog the likes that they had never seen before. He was strong beyond comprehension, loyal, smart and a natural leader of dogs.
Here is a quote from the chapter,
He perceived and determined and responded in the same instant. In point of fact the three actions of perceiving, determining, and responding were sequential; but so infinitesimal were the intervals of time between them that they appeared simultaneous. His muscles were surcharged with vitality, and snapped into play sharply, like steel springs. Life streamed through him in splendid flood, glad and rampant, until it seemed that it would burst him asunder in sheer ecstasy and put forth generously over the world.
"Never was there such a dog," said John Thornton one day, as the partners watched Buck marching out of camp.
"When he was made, the mold was broke," said Pete.
Cal of the Wild chapter 7