1.  Field of the Invention
The present invention relates to an image forming apparatus and an image forming system to which one of a first cartridge containing a developer, and a second cartridge containing the developer more than the first cartridge is selectively installable. 
2.  Description of the Related Art
Conventionally, there is known an image forming apparatus configured such that one of two types of cartridges, which contain different amounts of a developer when the cartridges are unused, is selectively installable.  In particular, one of a small capacity cartridge and a large capacity cartridge containing the developer more than the small capacity cartridge is selectively installable (see Japanese Patent Application Laid-open No.  2011-186880, for example).  Further, in such an image forming apparatus, each of the cartridges includes a casing containing the developer, a developing roller supported by the casing, an agitator configured to transport the developer in the casing toward the developing roller, and a supply roller configured to supply the developing roller with the developer transported by the agitator.