Simplify the following text for a Grade 14 native English-speaking person.
Original: Over time , Longfellow 's popularity rapidly declined , beginning shortly after his death and into the twentieth century as academics began to appreciate poets like Walt Whitman , Edwin Arlington Robinson , and Robert Frost .
Simplified: Over time , Longfellow 's popularity rapidly declined , beginning shortly after his death and into the 20th century as academics began to appreciate poets like Walt Whitman , Edwin Arlington Robinson , and Robert Frost .