While angels and demons alike are generally regarded as invisible to human sight, they are frequently depicted as human-like creatures with wings, though many theologians have argued that they have no physical existence, but can take on human form. Most theologians agree that angels have no gender. Therefore, they usually appear as androgynous, although guardian angels appear more feminine and maternal. The angels of the Bible generally appear in the role of God's messengers to mankind. They are His instruments by whom He communicates His will to men, and in Jacob's vision they are depicted as ascending and descending the ladder which stretches from earth to heaven while the Eternal Father gazes upon the wanderer below. Such appearances of angels generally last only so long as the delivery of their message requires, but frequently their mission is prolonged, and they are represented as the constituted guardians of the nations at some particular crisis, e.g. during the Exodus. Surreal Artist Gallery