Natural kinds are secondary substances for Aristotle and so a fundamental feature of reality. Yet these natural kinds are most importantly (and as far as I know, exclusively!) biological species.
I think this is very strange on the face of it. To plagiarize from Jerry Fodor:
I suppose that sooner or later the physicists will complete the catalogue they've been compiling of the ultimate and irreducible properties of things. When they do, the likes of spin, charm, and charge will perhaps appear on their list. But catness surely won't; being a cat simply doesn't go that deep.
In the original quote it wasn't "catness" but "aboutness/intentionality". Yet, I am much more open to such mental phenomena being fundamental features of reality than the species felis catus!
And the problem doesn't stop with this prima facie implausibility:
It is an empirical fact that species evolved, species are not fixed and eternal, as Aristotle believed. We simply know that cat doesn't need to beget cat. So how can species be natural kinds? How can they be so fundamental?
And if we cannot answer this, can Aristotle's metaphysics still be salvaged? How can we disentangle it from its flawed assumptions about biology?