Interbreeding

"Your visitors seek to establish themselves ... through interbreeding. They cannot live in your environment. They need your physical stamina. They need your natural affinity with the world. They need your reproductive abilities. They also want to bond with you because they understand that this creates allegiance. This, in a way, establishes their presence here because the offspring of such a program will have blood relations in the world and yet will have allegiance to the visitors."