I've seen various names used to refer to constitutions and to other such documents.  Some people call them organic law.  I believe that's a silly name for them.  Things that are organic are things that eat, breathe, and so forth.  Some people have called constitutions fundamental law.  I don't believe that constitutions are fundamental.  They aren't even law.  See Milam's Dictionary of Distinctions Differences, and Other Odds and Ends, at Law vs. Legislation.  Constitution is equivalent to legislation in that both are enacted by men.  I believe that the principles upon which constitutions are based, or upon which they ought to be based, are fundamental, although those principles are not necessarily law.  However, even if I don't regard constitutions as fundamental, they are the sources of legitimate governments.  See my essay The Long and Winding Doctrine:  Social Contract.

A legitimate government doesn't exist by it's own authority.  It exists by the authority of a constitution.  The constitution, not the government, is the source of the government's existence.  The constitution, not the government, declares the government's powers, limits, departments, officers, and so forth.  The government isn't the source of the constitution.  The constitution is the source of the government.  Again, see The Long and Winding Doctrine:  Social Contract. Thus, I'll refer to constitutions and to other such documents as source documents.  If somebody thinks of a better term, then I'm open to suggestions.

The idea that a government is a creature of its constitution has a practical consequence that isn't usually acknowledged.  That consequence is that, when a constitution is terminated and a new constitution is enacted in its place, then the government doesn't exist continuously through the transition.  Rather, the previous government, under the previous constitution, is utterly destroyed when its constitution is terminated or superseded.  A new government, under the new constitution, is erected in its place.  The name of the new government might be the same as the name of the previous government.  That's irrelevant.  My grandfather, my father, and I all shared the same name but we were three completely different men.  Similarly, one government isn't the same institution as another government simply because they have the same names.¹    Some of the forms of the new government might be the same as some of the forms of the previous government.  That's also irrelevant.  My grandfather, my father, and I all had the same form but we were three completely different men.  The territory occupied by the new government might be the same as the territory occupied by the previous government.  Even that is irrelevant.  Over a period of time, my grandfather, my father, and I each occupied the same chair in the kitchen.  We were still different men.  A culture might have continuity during a change in constitutions but a state, a government, is discontinuous when one constitution is replaced by another.  Thus, the State of Georgia, for example, isn't the same State of Georgia that previously existed on this continent.  It's convenient to speak of it that way but it isn't accurate.  The previous states named Georgia were completely different states.  They're completely gone.  They were unconnected to the present State of Georgia except in a way that's somewhat analogous to the way in which my grandfather and my father are connected to me.  So, there have been on this continent several completely different states named Georgia, each occupying roughly the same territory as the present State of Georgia.  They don't have any more connection to the present State of Georgia than England has to The United States of America.