The law of the conservation of history was a theory in temporal mechanics which stated that "once an event has occurred, it will always tend to occur, despite outside interference, proportional to its significance." This theory proposed that in cases of time travel, any attempt to change historical events would meet resistance as the original timeline would attempt to reassert itself. The greater the event, the greater the resistance, and the greater the energy required to effect any substantive change.
In the 24th century, this theory would be favored by several agents of the Department of Temporal Investigations as a reason why most attempts to change history fail. (Star Trek: Pendragon: "This Solemn Starlight")
A corollary to this theory proposed that when temporal incursions occurred, some changes were inevitable (except in the case of predestination paradoxes), but that, in the case of minor changes, these "ripples" would smooth out over a short span of time and the timeline would march on virtually unchanged. In the event of a larger change in history, a new quantum reality would diverge from the parent timeline, as a sort of "temporal lobotomy." (Star Trek: Pendragon: "Prodigal Realities")