Sexual reproduction makes evolutionary sense despite its complexities because it allows species to evolve new defenses against parasites. (Some lab evidence suggests that sexual reproducers fend off parasites more efficiently than asexual reproducers. ) Also: biologist Leigh Van Valen's idea that a species must develop continuously just to keep up with species co-evolving with it. (Red Queen refers to a chess piece in Lewis Carroll's book Through the Looking Glass who tells Alice that one must run just to stay in place. )