Cumulative Selection
Complexity can accumulate without a leap to a finished design when each small improvement is preserved because it already works. The relevant question is whether an intermediate form is useful in its own context, not whether it resembles the eventual result.
A sophisticated eye need not begin as an eye. It can begin as a light-sensitive patch, then become a shallow depression, a deeper cup, a pinhole-like opening, and only later acquire a transparent covering and lens.
E1Improvement needs a ratchet, not a blueprint
Cumulative selection has two moving parts: variation produces a slightly different form, and selection preserves changes that improve performance now. Preservation turns each workable stage into the starting point for the next round, so improvements stack instead of repeatedly vanishing. In the eye sequence, a depression need not deliver modern vision; it only has to extract more useful information about light than a flat patch. This is Intermediate Functionality: incomplete relative to the endpoint can still mean effective relative to the alternative.
E1Where it shows up
From light patch to lens
The successive patch, cup, opening, covering, and lens show how a structure can change jobs by degrees while remaining useful at every preserved stage. The apparent impossibility arises only when the finished eye is treated as the first required step.
E1A possible staircase is not a complete history
A sequence of useful intermediates answers the objection that complexity must arrive all at once. By itself, however, it does not establish that every proposed stage occurred in exactly that order, or supply the full historical evidence for the path.
E1Search for the first useful rung
When a finished system looks too complex to emerge gradually, write down its simplest component that could perform any useful part of the job. Then ask what one small modification would improve that component without requiring the final design.
E1Episodes that teach this
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God vs Evolution - Who Created Humans? The Blind Watchmaker - FutureIQ
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The eye is explained as a sequence: light-sensitive patch, saucer-shaped depression, deeper cup, pinhole-like opening, transparent covering, and lens.