Traditional custom, taken the world over, is a mass of detailed behaviour more astonishing than what any one person can ever evolve in individual actions, no matter how aberrant.
I think "taken the world over" means "considered globally".
But I don't know why it is written like this.
Does it equal "which is taken the world over"?
What does "take over" mean in this expression?
You're right, it does mean "considered globally". Specifically, this is "taken" in the sense of "considered" or "viewed", and "the world over" means, of course, across the whole world, globally.
So the phrasal verb "take over" (dominate, assume control) is irrelevant to this context.