Nature exhibits organisms with one-chambered hearts, two-chambered hearts, three-chambered hearts, and four-chambered hearts.
A certain kind of thinker regards this as proof of Darwinism. See? First came the one-chambered heart, then the two, then the three, then the four.
But try to imagine a transition. How could a species make a gradual transition from, say, a two- to a three-chambered heart? No halfway house could function.
This is not an argument against descent with modification. But it does count as a strong objection to the gradualist, adaptationist explanation of how the modifications happened. Other things may evolve; basic forms are conserved.
I first came across the argument years ago in biochemist Michael Denton’s Evolution: A Theory in Crisis. It was the first work I had read which showed me that there might be purely scientific grounds to reject a theory which up to then I had considered unassailable.
What spurs the recollection is that Denton has just published a new book, Evolution: Still a Theory in Crisis. I look forward to reading it.