The source is a commentary by Erhard Wetzel, the Racial Expert of Rosenberg's Ministry for the Occupied Eastern Territories, on the first draft of the Generalplan Ost prepared by the RSHA.
Wetzel's commentary was written in about April 1942, and from it we can reconstruct the basic thrust of the first draft of the Generalplan Ost, which has not been preserved.
The full text of Wetzel's commentary is published in this book:
"Vom Generalplan Ost zum Generalsiedlungsplan", herausgegeben von Czesław Madajczyk ; unter Mitarbeit von Stanisław Biernacki ... [et al.] (München ; New Providence : Saur, 1994)
The RSHA proposed to germanise certain parts of the Occupied Eastern Territories, including the Baltic States, Ingermanland(the area surrounding Leningrad), the Generalgouvernement, parts of Volhynia and the Zhitomir region, the Dnieper Bend, and the Crimea.
The germanisation was to be carried out by "ethnic reassignment" (Umvolkung) of a selected part of the existing population based on racial criteria, expulsion of the remainder across the Urals to Siberia, and the settlement of Germans from the Reich.
The RSHA estimated the total population to be expelled at 32 million. This was not the total native population of the areas to be germanised, but the majority that was assessed as not suitable for "ethnic reassignment".
They were to be replaced by some 10 million German settlers from the Reich (I am quoting this figure here from memory). Thus the population foreseen for the germanised areas, consisting of "ethnically reassigned" natives plus German settlers, was to be considerably lower than the existing level, resulting in what the RSHA planners considered to be a healthier population density.
The whole process was to take some considerable time, over 20 years.
Wetzel considered the plan unworkable, and made some trenchant criticisms of it. In the first place, he showed that the RSHA planners had greatly underestimated the size of the population to be expelled, since they had failed to take population growth into account (that assumption by Wetzel of future population growth in itself refutes the thesis that the German Government planned to reduce the native population by starvation).
In short, the number of persons to be expelled according to the criteria of the plan would be much greater than 32 million, making the whole thing much harder than the RSHA claimed.
Wetzel made another important comment at this point. He speculated whether the RSHA figure of 32 million projected expellees was too low because the planners had omitted the four million Jews of the regions selected for germanisation. He concluded that the Jews were not omitted from the total number, since the plan specifically stated that all the Jews were included among the expellees. That demonstrates that at the time the RSHA was drawing up its first draft of the Generalplan Ost, some time in late 1941 or early 1942, there was as yet no extermination plan in existence in relation to the Jews, only a plan to expel them across the Urals.
(Nevertheless, at the time Wetzel was writing his commentary, he was aware that killing of part of the Polish Jewish population had already begun, since he comments that Poles were afraid that what was being done to the Jews would also be done to them, and they needed to be reassured that that was not so).
In the second place, he showed that RSHA estimates of the number of Reich Germans available for resettlement in the East were wildly optimistic, and they would never be able to find enough settlers, making it improbable that the scheme could succeed.
In the third place, he rejects the notion of expelling the ungermanisable part of the native population across the Urals, on the grounds that that would just make them permananet enemies of Germany, and also reinforce the Slavic population already living in Siberia.
In place of the RSHA's expulsion scheme, Wetzel suggest forms of resettlement that will not create enemies of Germany. In the case of the Baltic peoples, he suggests that instead of pushing them over the Urals, they should be resettled all over German-occupied European Russia as a class of administrators, helping the Germans to rule the Russians.
Wetzel states that that role had been played by Baltics during the Tsarist period, and it could easily be reinstated. As administrators for the German rulers, enjoying a position of status and some power, the resettled Balts would become firmly attached to the new German domination of European Russia.
With regard to the ungermanisable part of the Polish population, Wetzel suggests that instead of expelling them to Siberia, they should be encouraged to emigrate to Brazil. Wetzels' rationale is that there had already been considerable emigration of Polish peasants to Brazil, and there was already a substantial Polish emigrant population there; furthermore, Brazil had vast amounts of unused land, more than enough to settle all the unwanted population of the Generalgouvernement.
Since Germany lost the war, it will never be known which variant of the Generalplan Ost would have been implemented if germany had not lost, The RSHA's or Wetzel's. However, Wetzel had shown that the RSHA plan was simply unworkable and based on faulty demographic assumptions.