From his (Adam's) progeny Allah chose prophets and took their pledge for his revelation and for carrying His message as their trust. In course of time many people perverted Allah's trust with them and ignored His position and took compeers along with Him. Satan turned them away from knowing Him and kept them aloof from His worship. Then Allah sent His Messengers and series of His prophets towards them to get them to fulfil the pledges of His creation, to recall to them His bounties, to exhort them by preaching, to unveil before them the hidden virtues of wisdom and show them the signs of His Omnipotence namely the sky which is raised over them, the earth that is placed beneath them, means of living that sustain them, deaths that make them die, ailments that turn them old and incidents that successively betake them.
Allah never allowed His creation to remain without a Prophet deputised by Him, or a book sent down from Him or a binding argument or a standing plea. These Messengers were such that they did not feel little because of smallness of their number or of largeness of the number of their falsifiers. Among them was either a predecessor who would name the one to follow or the follower who had been introduced by the predecessor.
source : http://www.al-islam.org
