“Citizens, we shall say to them in our tale, you are brothers, yet God has framed you differently. Some of you have the power to command, and these he has composed of gold, wherefore also they have the greatest honor; others of silver, to be auxiliaries; others again who are to be husbandmen and craftsmen he has made of brass and iron; and the species will generally be preserved in the children. But as you are of the same original family, a golden parent will sometimes have a silver son, or a silver parent a golden son. And God proclaims to the rulers, as a first principle, that before all things they should watch over their offspring, and see what elements mingle in their nature, for if the son of a golden or silver parent has an admixture of brass and iron, then nature orders a transposition of the ranks; and the eye of the ruler must not be pitiful towards his child because he has to descend in the scale and will become a husbandman or artisan, just as there may be others sprung from the artisan class who are raised to honor, and become guardians and auxiliaries.” As the safeguard of this principle of natural selection of functions, Plato proposed a system of common marriage. “The wives of these guardians are to be common, and their children are also common, and no parent is to know his own child, nor any child his parent. Starting from the day of the hymeneal, the bridegroom who was then married will call all the male children who are born ten and seven months afterwards his sons, and the female children his daughters, and they will call him father.... And those who were born at the same time they will term brothers and sisters, and they are not to intermarry.” One of the features of this system is that the best stocks--the strongest and wisest and most beautiful--are to be encouraged to reproduce themselves. But this is not worked out in detail. There is to be complete freedom of sexual selection among the guardians; and those who are most distinguished in their services are to have access to a great number of women; but beyond encouraging the guardians to be prolific, Plato did not apparently consider the possibilities of cross-breeding between the various classes.