▁the ▁Tre aty ▁of ▁R iga ▁negoti ations ▁and ▁the ▁commission ▁for ▁the ▁del im itation ▁of ▁Poland ' s ▁eastern ▁borders . ▁ ▁Sch olar ▁After wards ▁he ▁concentr ated ▁on ▁historical ▁research . ▁He ▁research ed ▁lingu istics ▁( part icular ly ▁S lav ic ▁languages ), ▁eth n ography ▁of ▁the ▁Central ▁and ▁Eastern ▁European ▁lands , ▁and ▁history ▁of ▁literature . ▁He ▁would ▁serve ▁as ▁the ▁director ▁of ▁two ▁instit utes ▁( Inst yt ut ▁Bad ania ▁Naj n ows zej ▁Histor ii ▁Polski ▁( Inst itute ▁of ▁Studies ▁of ▁Modern ▁Polish ▁History ) ▁in ▁the ▁ 1 9 2 0 s ▁and ▁Inst yt ut ▁B ada ń ▁Narod owo ści owych ▁( Inst itute ▁of ▁National ity ▁Studies ) ▁in ▁the ▁ 1 9 3 0 s ) ▁and ▁editor ▁of ▁the ▁journal ▁Nie pod leg ło śc . ▁Supp orter ▁of ▁Mi ędz ym or ze ▁feder ation ▁idea , ▁as ▁well ▁as ▁Prom ethe ism , ▁he ▁was ▁also ▁a ▁vocal ▁oppon ent ▁of ▁pol on ization , ▁arg uing ▁that ▁Uk ra ini ans ▁and ▁Bel arus ians ▁living ▁in ▁Poland ▁should ▁be ▁allowed ▁to ▁assim il ate ▁into ▁Polish ▁society ▁at ▁their ▁will ▁and ▁speed . ▁Author ▁of ▁many ▁works , ▁among ▁them ▁Lit wa ▁i ▁B iał oru ś ▁(" L ith u ania ▁and ▁Bel arus ", ▁ 1 9 1 2 ), ▁Uk rai ńska ▁spraw a ▁narod owa ▁w ▁jej ▁roz wo ju ▁history cz nym ▁(" The ▁Ukrain