The principal commodities of legitimate trade were palm oil and palm kernels, which were used in Europe to make soap and as lubricants for machinery before petroleum products were developed for that purpose. Although this trade grew to significant proportions—palm oil exports alone were worth £1 billion a year by 1840—it was concentrated near the coast, where palm trees grew in abundance. Gradually, however, the trade forced major economic and social changes in the interior, although it hardly undermined slavery and the slave trade. The incidence of slavery in local societies increased.
