To demonstrate what would happen if singers were to follow "natural" temperament, in his first example Planck has one of the four voices sustain its pitch into the next (different) harmony. In this synthesized rendition, the voices readjust to just ("natural") intonation around each successive sustained note. Note that, by the end, the overall pitch has sunk about one semitone, as you can hear by replaying the example.
A live performance of Planck's first example; note that the singers do not sink down in pitch by a semitone, as they should if governed purely by "natural temperament." Instead, they remember and return to their starting pitch.