In Scotland at the time, there wasn't primogeniture at all. You got to be king by general acclimation -- you were king when everybody said you were king. The p

In Scotland at the time, there wasn't primogeniture at all. You got to be king by general acclimation -- you were king when everybody said you were king. The previous king might nominate a successor, and it might be his son -- any son. And if the thanes didn't like it, they'd fight about it. ("Thane" was an Anglo-Saxon context that's not really appropriate to Scotland at the time, but they had something kinda parallel.) Shakespeare hints at that, in an important scene were Mac expects to be named the heir. Shakespeare's audience would have understood that, because it had been roughly the same in England under the Saxons. (That was more than a half-millennium ago, but the history was well understood by the educated, using the same sources Shakespeare did.) There was a problem with offspring in general. Shakespeare actually discussed that one, too, in King Lear. Properties could be divided, but not indefinitely, and it often caused fighting. It's what led to the clan system, which really evolved after the Norman invasion but hints of it lay earlier.