In The Origin of Species (1859), he wrote, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.
En "A orixe das especies" escribiu: "se fose posíbel demostrar que calquera órgano complexo exista, que non puidese formarse por pequenas mutacións numerosas e sucesivas, a miña teoría estaría destruída.