CREATE TABLE table_18701 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

who is the the incumbent with candidates being percy e. quin (d) unopposed
SELECT "Incumbent" FROM table_18701 WHERE "Candidates" = 'Percy E. Quin (D) Unopposed'
SELECT "Incumbent" FROM "table_18701" WHERE "Candidates" = 'Percy E. Quin (D) Unopposed'