This is something of a broad question because "human geography" can encompass a huge range of topics. However, we can generally say that these parts of the book argue that human geography can be explained by physical geography.
In the Prologue and the first two chapters, for example, we see many ways in which physical geography determines human geography. We see that the size and climates of Polynesian islands have a huge impact on the sorts of societies that are on those islands and on the ability of their people to conquer others. We later see that things like the physical geography of continents can determine the degree to which human technologies spread across those continents. In all of these cases, we are seeing physical geography determine human geography.