Poleshuks & Ruthenians - what has happened to them?

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In the census of 1921 in Polesia there were 375,220 Belarusians, 156,142 Ruthenians and 38,565 Poleshuks:

https://statlibr.stat.gov.pl/exlibris/al...SXMS17.pdf - data by county starts on page 12/39

https://upload.wikimedia.org/wikipedia/c...y_1921.pdf - Tablica XI. on page 73/264

In the census of 1931 in Polesia there were 75,338 Belarusians, 54,047 Ukrainians and 707,088 Poleshuks:

https://upload.wikimedia.org/wikipedia/c...y_1931.pdf - Tabl. 12. on page 57/314 data by county

Table based on the 1931 census:



The census of 1921 counted only "Ruthenians" (no "Ukrainians" category), while the census of 1931 counted "Ukrainians" and "Ruthenians" separately.

Here is a map of Ruthenians according to the 1931 census:



And here is a map of Ukrainians according to the 1931 census:



In Volhynia there were only Ukrainians, in Galicia both Ukrainians and Ruthenians, in Krakowskie Voivodeship (e.g. Lemko territory) only Ruthenians.