A carrier is a person or company that undertakes to carry, or makes a business of carrying, persons or goods for hire. They may be either private, or public or common, carriers. A private carrier is one who carries only occasionally and not as a public business or employment. Such a carrier is not bound to serve all who offer to employ him. He is liable for negligence in transporting the goods, but is held only to the exercise of ordinary diligence. Public or common carriers are those whose regular business it is to carry goods from place to place for all persons who choose to employ and remunerate them, such as railroad, express, and steamship companies. They may be carriers of passengers or carriers of goods, but it is to the last-named class that the term is usually applied. To constitute one a common carrier, he must have held himself out to the public as ready to carry, for hire, as a business goods of the sort he professes to carry.