As chemist Addy Pross points out, all human cells are replaced many times over but maintain a pattern—the human body—that persists as long as those cells can metabolize energy. And all those human bodies, in turn, constitute the species Homo sapiens sapiens that persists across generations even though the individuals that make up the human race keep dying. (Or, as Xana put it: “Daddy, I know why there is night. So other people can have day.”)
