17 However, here it must be carefully observed that whatever be the form of civil power in a nation, it cannot be considered so definitive as to have the right to remain immutable, even though such were the intention of those who, in the beginning, determined it.... Only the Church of Jesus Christ has been able to preserve, and surely will preserve unto the consummation of time, her form of government. Founded by Him who was, who is, and who will be forever,(8) she has received from Him, since her very origin, all that she requires for the pursuing of her divine mission across the changeable ocean of human affairs. And, far from wishing to transform her essential constitution, she has not the power even to relinquish the conditions of true liberty and sovereign independence with which Providence has endowed her in the general interest of souls . . . But, in regard to purely human societies, it is an oft-repeated historical fact that time, that great transformer of all things here below, operates great changes in their political institutions. On some occasions it limits itself to modifying something in the form of the established government; or, again, it will go so far as to substitute other forms for the primitive ones-forms totally different, even as regards the mode of transmitting sovereign power.
Source: Au Milieu Des Sollicitudes (Vatican.va)