:) I think that only people with the roots coming from Mazovia peasants are pure Polish...
It is quite funy but in my opinion it is not serious talking about nationalities like Bielarusian or Lituanian in XVI century,
In the beginning of XX century a lot of people form Polesie (part of todays Bielarus and Ukraine) asked about nationality answered "we are form here" I think that during XVI century they would answer the same.
But to show how multicultural our country was:
Poles: 65,5 %
Ukrainians 16%
Jews 9,4 %
Bielarusians 5,5 % ("we are from here" are added here as well about 1,5%)
Germans 2,6 %
Others 1%
what you should know, on east Poles and Jews lived in towns and Others in vilages (thats why there are not to green on the map ;) )
What can be intersting, Poles are still wining local elections in the grounds surrounding Wilno (in Wilno town we are minority because most of them were expeled from there after the war to towns like Wroclaw or Szczecin)
I m Pole but my mothers name is definetely lithuenian, when I asked my grandmother about her nationality she answered me that Polish, and that in the past Lituenia was just like Texas in USA now, and Poland was something like other state in USA. And being Pole was only problem of choice, if you wanted to be Pole you could be one ;) So Mickiewicz "writting Lithuania my fatherland", could mean something like singing in USA "my lovely Arizona" With lithuanians we had the same government, the same culture, there there were no anty Polish rebels ... What can be interesting, now in Lithuania there is group of people calling themselfs "Zmudzini" and they dont consider themselves Lithuanians, few years ago they would answer they are Lithuanians form region "Zmudz"
We should notice that in our history we have a lot of great people coming to our country and choicing to be Pole just because it was good place to live. It was good place to live because we were tolerant, in comparison to other countries we were giving a lot of freedom and it worked quite good.
Suddenly after II WW we were expeled by Red Army form eastern lands (in some regions we could stay) to terytories form where Germans were expeled. And now we live in country with 96,7% of native Poles. But we dont need to learn how to live together with other nations, we just have some good expirences.