There seems never to have been a civilization without toys, but when and how they developed is unknown. They probably came about just to five children something to do. In the ancient world, as is today, most boys played with some kinds of toys and most girls with another. In societies where social roles are rigidly determined, boys pattern their play after the activities of their fathers and girls after the tasks o~ their mothers. This is true because boys and girls are being prepared, even in play, to step into the roles and responsibilities of the adult world. What is remarkable about the history of toys is not so much how they changed over the centuries but how much they have remained the same. The changes have been mostly in terms of craftsmanship, mechanics, and technology. It is the universality of toys with regard to their development in all part of the world and their persistence to the present that is amazing. In Egypt, the Americas, China, Japan and among the Arctic(北极的) peoples, generally the same kinds of toys appeared. Variations depended on local customs and ways of life because toys imitate their surroundings. Nearly every civilization had dolls, little weapons, toy soldiers, tiny animals and vehicles. Because toys can be generally regarded as a kind of art form, they have not been subject to technological leaps that characterize inventions for adult use. The progress from the wheel to the oxcart to the automobile is a direct line of ascent(进步). The progress from a rattle(拨浪鼓) used by a baby in 3000 B.C. to one used by an infant today, however, is not characterized by inventiveness. Each rattle is the product of the artistic tastes of the times and subject to the limitations of available materials. The reason why the toys most boys play with are different from those that girls play with is that ______.