Alexander's death occurred in the absence of any designed heir, except for a yet unborn son and an imbecile stepbrother. It was decided that these two would be kept under the custody of Perdiccas, while Alexander's other generals (the so-called Diadochi, Successors) divided between themselves the government of his empire.
Bouchard, Elsa. "Death of Alexander the Great". The Literary Encyclopedia. First published 01 March 2010
[http://www.litencyc.com/php/stopics.php?rec=true&UID=11292, accessed 17 March 2014.]