A structural system is dynamic, constantly absorbing new surface features into its interpretive matrix, and constantly undergoing many tiny changes over time as its users imperceptibly modify it while using it.
Structuralists speak of the "structuring" power of a system, as in the "structuring of whiteness" or the "structuring of tragedy" or the "structuring of clan rituals." Transformations of symbol systems, like languages, are eloquently explained by Saussure as the inevitable result of the arbitrariness of the sign--because the signifier has no necessary relationship to the signifier, it is nearly impossible to make a rational case for reassigning the signified to some new signifier or some new signifier to the signified. Even if such a case could be made, the system is in constant use by vast numbers of people ignorant of its rules, operating by intuition alone, and their unconscious grasp of practice is constantly loosening, allowing infinite, tiny processes of change to occur in the aggregate system.
This is an enormously powerful concept. Think about the U.S. economy as a structuring system that transforms things into capital. In one era, copper pennies and gold pieces were structured as currency. Now, transformed by time, the copper penny might still be spent for a penny but would be more valuable for its copper, and the gold piece probably would not be accepted as legal tender, though it would be far more valuable than its face value declares. Each signifier's significance has been transformed by time, but in each time, the significance of each was stable and reliable. In a former era, human beings could be considered property, and could be bought, sold, or lent like any other moveable good. Now that would be illegal, but human beings still can be called valuable and the economy structures differently the worth of their bodies (sources of transplant organs) and lives (insurance compensation for injury, dismemberment, or death). We assume the "naturalness" of transplantable organs having monetary value, just as the earlier era assumed the "naturalness" of the inviolability of one's possession of one's body intact at death.
Language, too, is subject to transformation of its structuring rules over time. As Chaucer wrote, in the Proem of Book II of the Troilus, talking to his Middle English readers about classical Latin and Greek:
Ye knowe ek that in forme of speche is chaunge
Withinne a thousand yeer, and wordes tho
That hadden pris, now wonder nyce and straunge
Us thinketh hem, and yet thei spake hem so,
And spedde as wel in love as men now do;
Ek for to wynnen love in sondry ages,
In sondry londes, sondry ben usages. (II: 22-28)
[You know, too, that language's form has changed, within a thousand years,
and words then that were valued, we now think silly or odd, and yet they spoke
them, and had as much success in love as we now do; so to win love in various
eras, in various lands, there are various good words with which to go about it.]