Children are a relatively modem invention. Until a few hundred years ago they did not exist. In medieval and Renaissance painting you see pint-sized men and women, wearing grown-up clothes and grown-up expressions, performing grown-up tasks. Children did not exist because the family as we know it had not evolved. Children today not only exist they have taken more attention, in no place more than in America, and at no time more than now. It is always Kids' Country here. Our civilization is child-centered, child-obsessed. A kid's body is our physical ideal. In Kids' Country we do not permit middle age. Thirty is promoted over 50, but 30 knows that soon his time to be overtaken will come. We are the first society in which parents expect to learn from their children. Such a topsy-turvy (颠倒)situation has come about at least in part because, unlike the rest of the world, ours is an immigrant society, and for immigrants the only hope is in the kids. In the Old Country, that is Europe, hope was in the father, and how much wealth he could accumulate and pass along to his children. In the growth pattern of America and its ever-expanding frontier, the young man was ever advised to go west the father was ever inheriting from his son. Kids' Country may be the inevitable result. Kids' Country is not all bad. America is the greatest country in the world to grow up in because it is Kids' Country. We not only wear kids' clothes and eat kids' food, we dream kids' dreams and make them come true. It was, after all, a boys' game to go to the moon. If in the old days children did not exist, it seems equally true today that adults, as a class, have begun to disappear, condemning all of us to remain boys and girls forever, jogging and doing push-ups against eternity. The author uses the example of the Renaissance painting to show that ______.