Any person operating for hire any public conveyance propelled by steam, gasoline, electricity, or other motive power, for the purpose of affording a means of street transportation similar to that ordinarily afforded by street railways (but not operated upon fixed tracks) by indiscriminately accepting and discharging such persons as may offer themselves for transportation along the course of operation, is declared to be a common carrier, and the business of all such common carriers is declared to be a privilege.