A specific form (ṣūrah nawʿiyyah) accounts, in this philosophical scheme, for a body’s belonging to a determinate kind. Corporeal form explains the common feature of being bodily; specific form is invoked to account for differences between kinds of bodies.
Wood and iron are both extended bodies but have different physical properties. The classical theory attempts to explain kinds through distinct forms; this is not a substitute for modern chemical descriptions.
Even if one grants that different specific forms succeed one another in a substrate, this alone does not establish that the substrate, or every kind, has existed from eternity.