This has gotten to be too long for a comment. The asker asks in a comment:
How possibly can it be that in one sentence there is two parts containing complete structure? 'polis were supposed to' and ' the constraints are easily confused with' ? In my understanding, 'polis were supposed to' is supposed to be 'The constraints which a properly constituted polis based upon law supposed to put on'. Right?
The skeleton is 'the constraints are easily confused with...', I'm good with that part. But how to break down 'which a properly constituted polis based upon law were supposed to put on the more outlandish tendencies of human greed and ambition'? My way of breaking it down is: a properly constituted polis / based upon law / were supposed to put on / the more... But since it starts with which, then it can't be a complete sentence as it is now. – Arctic Tony 7 hours ago edit
Because you're reading it wrong. The connecting which is acting as an object here not asa subject. A simpler sentence illustrating this might be
The color which you wanted to paint the house is not available.
We could say that the essential skeleton is "the color is not available", but it isn't accurate to call "you wanted to paint the house" an embedded independent clause because there's an object missing here which you wouldn't be accounting for; namely, the color. You wanted to paint the house the color, and the which is linking that up to the main sentence.
That's exactly what's happening in McClelland's sentence, only it's harder to see there.
Perhaps you're thinking its using "to put on" like when someone puts their coat on, and this is leading you to imagine there is a complete independent clause embedded here. There is not, because this which here is also serving as an object. The subject of the main clause, "the constraints" is therefore the object of the embedded one. The people were supposed to put the constraints on the tendencies. That's a complete sentence. This isn't, because "the constraints" is pulled out of that otherwise complete sentence at the top to serve as the subject of the main clause even though it's the object of the subordinate one.
It's exactly like what happens with this simplified sentence:
The constraints that the people were supposed to put on the tendencies are confused with something else.
So the people were supposed to put the constraints on the tendencies. The essential constituents of my simplified sentence are parsed this way:
(S (NP (NP The constraints)
(SBAR (WHNP that)
(S (NP the people)
(S (VP to
(NP the tendencies))))))))))
(NP something else))))
I hope that when looked at this way it becomes clear to you. McClelland could have been kinder on your brain if he had said "a polis was" instead of "a polis were", which is also confusing. But he didn't.