All the elements of the crews of later times are found in the ships of the Middle Ages. The mariners and "grometes" are the able seamen and ordinary seamen. There were boys then also. The archers were the predecessors of the marines, and of those drafts from the line regiments which were frequently used to make up the complement of men-of-war. The modern officers, too, have their representatives in the vessels of the Plantagenet kings. The _Rector_, afterwards called in official Latin _Magister_, is the master, the constable is the ancestor of the gunner, there was a carpenter, a "clerk," who was renamed the purser later on, and the boatswain. The nature of the work to be done would dictate the formation of these different offices. So soon as regular ships' companies began to be formed, it would be found indispensable to have someone to conduct the navigation--the master; someone to supervise the arms--the constable; someone to serve out the stores--the clerk. As ships' companies grew larger and ships more complicated, it would be necessary to increase the number of officers, and little by little the staff of a modern warship was formed. The title of captain appears at first to have been given to an officer who held what we should call flag rank. In the fifteenth century it began to be applied to the commander of a single ship. He was primarily a military officer, who might or might not be a seaman, but who in either case had a master under his command whose function it was to navigate the ship.