This “one species can’t evolve into another” argument that Rush alludes to is certainly an argument used by people who are uncomfortable with the implications of Darwin’s application to humans of the theory of evolution by means of natural selection.

But logically, once you accept that evolution occurs, even within a species, what’s to prevent a group splitting off from that species from changing so radically that we eventually call its descendants a new species?

You can’t Zeno’s Paradox your way out of it.