At the beginning of the book, the doctor makes a peculiar request: to measure Marlow's skull "in the name of science ". His significance in this act changes from literal at that point in the novel to figurative when taken in context with the later events in Heart of Darkness.
When the doctor asks in Marlow has a family history of mental illness, there is immediate foreshadowing about the madness that is to come in the novel. Characters such as Fresleven and Kurtz entered Africa as rational, decent civilians underwent significant psychological distress during their time there. When the doctor says that "those men never come back ", he means it in both a literal and a figurative sense "they physically never return to his office, but also that they don't come home in the same state of mind as they had left in. The doctor himself admits that these men never come back, so he's never able to re-measure skulls in order to track a change or do anything "in the name of science ". He is measuring internal, psychological changes ("the changes take place inside ") through a physical, seemingly unrelated method "again foreshadowing the irrational and seemingly illogical practices that Marlow would see in Africa. .
Within the context of the entire novel, the doctor's bizarre behavior seems to be an analogy for what Marlow thinks is the essence of Imperialism: a pointless method of judging an unknown, a ˜heart of darkness', by its exterior physical features (Europeans go into Africa looking for resources, emerge psychologically unstable after seeing the ˜heart of darkness'). Conrad provides several examples of this theme throughout the novel. Many characters, such as Kurtz, are judged by their exterior successes when in fact they have succumbed to the madness prevalent in Africa. Even Africa as a whole follows this: Marlow decision to voyage to Africa was based on his presumptions of the physical landscape and features; his actual expedition, however, proved to expose a much darker and richer side.