When a medicine man, or wicasa wakan, is old and nearing the end of his life, who does he try to pass on his vision and knowledge to and what happens if that doesn't work?

When a medicine man is old and nearing the end of his life, he tries to pass on his vision and knowledge to his son, but sometimes after much effort, the son doesn't receive a vision. Sometimes the power of being a medicine man will skip a generation and actually fall on a grandson instead of a son.