Slowly this feeling of opposition grew. The first definite mark of the growth was the struggle over the admission of Missouri, in 1820. This was settled by the famous "Compromise," embodied in the Act of March 6, 1820, whereby the people of the Territory of Missouri were allowed to frame a state government with no restriction against slavery; but a clause also enacted that slavery should never be permitted in any part of the remainder of the public territory lying north of the parallel of 36° 30'. By its efficiency during thirty-four years of constantly increasing strain this legislation was proved to be a remarkable political achievement; and as the people saw it perform so long and so well a service so vital they came to regard it as only less sacred than the Constitution itself. Even Douglas, who afterward led in repealing it, declared that it had an "origin akin to the Constitution," and that it was "canonized in the hearts of the American people as a sacred thing." Yet during the long quietude which it brought, each section kept a jealous eye upon the other; and especially was the scrutiny of the South uneasy, for she saw ever more and more plainly the disturbing truth that her institution needed protection. Being in derogation of natural right, it was peculiarly dependent upon artificial sustention; the South would not express the condition in this language, but acted upon the idea none the less. It was true that the North was not aggressive towards slavery, but was observing it with much laxity and indifference; that the crusading spirit was sleeping soundly, and even the proselyting temper was feeble. But this state of Northern feeling could not relieve the South from the harassing consciousness that slavery needed not only toleration, but positive _protection_ at the hands of a population whose institutions were naturally antagonistic to the slave idea. This being the case, she must be alarmed at seeing that population steadily outstripping her own in numbers and wealth.[61] Since she could not possibly even hold this disproportion stationary, her best resource seemed to be to endeavor to keep it practically harmless by maintaining a balance of power in the government. Thus it became unwritten law that slave States and free States must be equal in number, so that the South could not be outvoted in the Senate. This system was practicable for a while, yet not a very long while; for the North was filling up that great northwestern region, which was eternally dedicated to freedom, and full-grown communities could not forever be kept outside the pale of statehood. On the other hand, apart from any question of numbers, the South could make no counter-expansion, because she lay against a foreign country. After a time, however, Texas opportunely rebelled against Mexico, and then the opportunity for removing this obstruction was too obvious and too tempting to be lost. A brief period of so-called independence on the part of Texas was followed