One of the few exhibits which wasn't packed was a travelling exhibit with Persian art and sculpture. Up until about 800 a.d. it was as good as or better than anything the Greeks or Romans produced. After that, it suddenly took a nosedive and looked worse than something a moderately talented elementary school kid would produce now.
One of the little guys in our group asked "What happened that their art went downhill so fast." I replied that Persia was conquered by Islamic Armies about that time. You could have heard a pin drop as murmurs and gasps went up not only among our group but anyone in the exhibit within earshot. Yes, it was that dramatic of a difference.