Nevertheless, it was recognized that the stage during which developed socialism would be built would be a lengthy one and would not be achieved by the USSR on its own. According to the official textbooks, the first stage of the transition period from capitalism to socialism had been completed by the 1970s in the European socialist countries (except Poland and Yugoslavia), and in Mongolia and Cuba. The next stage of developed socialism would not be reached until "the economic integration of the socialist states becomes a major factor of their economic progress" and social relations had been reconstructed on "collectivist principles".[73] Communist writers accepted that during these earlier stages in constructing socialism, the exchange of commodities on the basis of the average socially necessary labour embodied within them occurred and involved the mediation of money.