CREATE TABLE table_26437 ("No. by series" FLOAT, "No. by season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production number" FLOAT, "U.S. viewers  ( in millions ) " TEXT)

Name the number of series for 19
SELECT "No. by series" FROM table_26437 WHERE "No. by season" = '19'
SELECT "No. by series" FROM "table_26437" WHERE "No. by season" = '19'