Each of Aristotle's early works, the Topics and the Categories, provides important evidence for Aristotle's holding two accounts of the nature of genus and differentia. In one account genus and differentia are radically distinct in nature. In the other they are much the same. In this paper I make a case for Aristotle holding each account, suggest why he adopts them, and consider which of the two is the earlier one.
Granger, Herbert, "Aristotle on Genus and Differentia in the Topics and Categories" (1983). The Society for Ancient Greek Philosophy Newsletter. 106.