A law that is to be passed today by Japan's parliament could doom its 1,500-year-old hereditary imperial institution. This comes as the law insists that only men can become emperor, sparking worry about the shrinking, fast-aging imperial family. Japan's male-only succession rule means the line must move to the emperor's younger brother, then to his 19-year-old nephew Prince Hisahito. In an imperial family that places a premium on male royal babies, Hisahito is the first such boy to be born in four decades.