specialization: meaning narrows to refer to something specific
The original "d or" meant "any four-footed wild animal". This could include lions, giraffes, pandas, raccoons, frogs, squirrels,ladybug, etc. The word deer now refers to a specific animal, a deer. They do have four feet, but they are a specific species and live in only specific areas. The word has changed from being more general "d or" meaning any four-footed animal to "deer" a specific species.
