There is a gap in this history, and it has been open since Part 1.
A bacterium holds two models of the world on two clocks. The first is chemical: the methylation state of its receptors, a memory about two seconds long, written by experience and lost when the cell dies. The second is the genome, a record of what worked across billions of years, written by selection, carried by the lineage, and unavailable for editing by any individual.
Two seconds, or an aeon. Nothing in between.
That gap is a catastrophe for anything that lives a long time. An animal with a two-year lifespan inhabits a world full of regularities that unfold over hours, seasons, and years: where the water is this summer, which of the local predators hunts at dusk, what the sound of that particular branch cracking means. None of it can be learned by a two-second memory, and none of it can be written into a genome, because the genome cannot be edited by experience and would be a generation too late even if it could.
Part 2 did not close this gap. It widened it. The nervous system Sherrington described is fast, integrated, and exquisitely coordinated, and every strength of it is a strength of wiring that evolution laid down in advance. A reflex is a policy written by natural selection and shipped, finished, in the animal. It is a model of a world the animal’s ancestors lived in. It cannot be revised by the animal living now.
So the animal in Part 2 can outrun the world’s consequences and cannot learn a thing from them.
What is missing is a memory on the timescale of a life. And the thing that provides it is not a new organ, or a new kind of cell, or a new principle of physics. It is a modification to a component we already met: the synapse, which turns out not to be a fixed connection at all.
