This gradual increase of (a more or less fixed) Identity, and its stability with respect to small chemical changes, can be visualized by considering a small crystal of, say, Alum : Substitution of ions (electrically charged atoms or groups of atoms), for example a replacement of Aluminum by Chromium, could, during the initial growth phase of the crystal, affect all relevant places in the crystal lattice. But especially in the case of larger crystals the replacement could well be incomplete (for example, not all Aluminum ions are replaced by Chromium ions). And if only a very small proportion of Aluminum ions has been replaced, then the original Identity of that crystal is conserved. So crystals are in this respect large Wholes.
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