Natural selection therefore explains why wheels have not appeared. A wheel without one or more of its important parts would probably not help an organism very much. However, the same is not true for the flagellum discussed above. As the flagellum evolved, different parts of other structures were used for a new purpose. The part that is now a rotating motor, called a basal body, may have evolved from a part that was used by bacteria to inject poisons into other cells. This use of existing parts is called "exaptation".