Full Idea
The reason Aristotle gives for calling species and genera substances is that of what is predicated only they reveal what the primary substance is.
Gist of Idea
Genus and species are substances, because only they reveal the primary substance
Source
report of Aristotle (Categories [c.331 BCE], 02b29-37) by Michael V. Wedin - Aristotle's Theory of Substance III.6
Book Reference
Wedin,Michael V.: 'Aristotle's Theory of Substance' [OUP 2000], p.94
A Reaction
Thus we should not be misled into thinking that the genus and species ARE the essence. We edge our way towards the essence of an individual by subdividing its categories.
Related Idea
Idea 16141 In 'Met.' he says genera can't be substances or qualities, so aren't in the ontology [Aristotle, by Frede,M]