In this wonderful universe,God has created billions (84 lakhs according to Hindu shastra calculation) of creatures (including Gods,demi-gods,insects,beasts,and men) inhabiting heaven,hell,earth,ocean,sky and other intermediate regions.Of these,those creatures or souls whose merits preponderate,go to heaven and live there till they enjoy the fruits of their actions and when this is done,they are cast down;while those souls,whose sins or demerits preponderate,go down to hell and suffer the consequences of their misdeeds for as long as they deserve it.When their merits and demerits balance each other,they are born on earth as human beings and are given a chance to work out their salvation.Ultimately,when their merits and demerits both are worked out completely,they get their deliverance and become free.To put the matter in a nutshell,souls get their birth or transmigration according to their deeds and evolvement.