Sowell says neither geography nor demography is egalitarian. Let's look at just geography. Africa is more than twice the size of Europe, but it has a shorter coastline. The European coastline constantly twists and turns, creating innumerable natural harbors, while the African coastline is smooth, with few harbors. Sowell asks how surprising it should be that international trade has played a smaller role in the economic history of Africa than of Europe, especially Western Europe.
Less trade produces more isolation.
No great civilization developed in isolation. When the British crossed the Atlantic, they were able to do so because they used rudders invented in China, they navigated with trigonometry invented in Egypt, their calculations were done with numbers invented in India, and their general knowledge was preserved in letters invented by the Romans. The resulting clash between them and the native population wasn't a clash between a British culture and an Indian culture. It was a clash between cultural developments from vast regions of the world, in the case of the British, vs. a more restricted cultural development. The cultural opportunities were unequal, and the outcomes were unequal.