Perhaps you should try some spelling variations:
"middle layer / history" » "middle ages / history", "middle aged / history", "middle age / history"
"middle earth / history" » "middle east / history", "middle ear / history", "middle ears / history"
"middle class / history" » "middle east / history"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("middle layer / history") OR ("middle earth / history"))) OR ("middle class / history")).
• Removing quotes may allow a broader search: (((( middle layer / history ) or ( middle earth / history ))) or ( middle class / history )).
• Adding a wildcard symbol may retrieve word variants: (((("middle layer / history") or ("middle earth / history"))) or ("middle class / history"))*.