51:4.6

The more backward humans are usually employed as laborers by the more progressive races. This accounts for the origin of slavery on the planets during the early ages. The orange men are usually subdued by the red and reduced to the status of servants—sometimes exterminated. The yellow and red men often fraternize, but not always. The yellow race usually enslaves the green, while the blue man subdues the indigo. These races of primitive men think no more of utilizing the services of their backward fellows in compulsory labor than Urantians would of buying and selling horses and cattle.