Workers are not technically sexless; they are females, but are incapable of reproducing. Their supposed altruism remained a mystery in Miles' day and posed a serious problem to Darwin's theory of natural selection. In Origin of Species Darwin admitted that social insects were an exception to his theory--if hymenoptera workers did not reproduce, how did their altruistic "worker-trait"get passed along to their descendents? In 1964 British biologist W. D. Hamilton posited an answer consistent with natural selection. He proved that all females in a brood share roughly three-quarters of the same genes. Thus, when the queen, their sister, reproduces, a large percentage of each worker's own genes appear in the offspring. As O'Toole explains: "By being a worker, she reproduces by proxy. . . . [Hamilton's theory] solves Darwin's dilemma and gives us an important insight into social behavior in bees, wasps and ants: far from being a selfless altruist, the worker hymenoptera is, or rather its genes are, utterly selfish" (111).