In colonies of eusocial animals, some animals are sterile. They can not pass on their genes at all. How can these animals evolve and persist? Since they do not breed, their fitness should be zero and any genes causing this condition should be eliminated from the population immediately. In "On the Origin of Species", Darwin called this eusocial behavior the "one special difficulty, which at first appeared to me insuperable, and actually fatal to my theory". Darwin thought that the resolution to the paradox would lie in the close family relationship.