How is the wolf cub different from his brother and sisters both physically and his personality?

As the five wolf cubs begin to grow and develop, it is the gray cub who most closely resembles his father and their wolf heritage. He is also "the fiercest of the litter. The cubs major fascination is with the "wall of light," the way his father could disappear into the wall of light, and how his mother's slap with a paw kept him from it. This was also a time of famine and all the cubs died, with the exception of the gray cub stronger than the others, another example of London's recurring theme of survival of the fittest.