They are sent periodically. "And when the hour at which Thy resistless Faith was to be made manifest did strike, Thou didst breathe a breath of Thy Spirit into Thy Pen, and lo, the entire creation shook to its very foundations, unveiling to mankind such mysteries as lay hidden within the treasures of Him... the Possessor of all created things." Gleanings Page 15 (18:5)

End of Quote

The Worlds of God
U.S.Baha'i National Ref Library Committee