In Joseph Conrad's Heart of Darkness, what is the most important thing in each station?

In Joseph Conrad's Heart of Darkness,
each station at which the protagonist, Marlow, stops has something important or
puzzling.

In the first station, the Outer Station, when
Marlow arrives, he sees Africa as it has become with the interference of the white man
and his ferocious desire for more ivory. He sees the “deathlike indifference of unhappy
savages.” A cliff is being blown-up for no good reason. It is not in the way. Machinery
lies about unused and rusting away. The whites are particularly vicious with their
"slaves." Many are diseased and starving to death.

Marlow
meets the company's accountant at this location.

In less
than two weeks, Marlow departs with about sixty men, all headed for the Central Station,
their next stop. Here Marlow finds that someone has foolishly driven his boat (which is
to carry him to the Inner Station) onto the rock bottom of the river that has ripped out
its hull; this will probably take two months to fix. (It is underwater.) Marlow tries to
get supplies (rivets), but for many weeks, the supplies mysteriously do not arrive even
though they have been repeatedly ordered.

It is here that
Marlow meets the manager and his uncle (who feel threatened by Kurtz), as well as a
brickmaker who seems to make no bricks, and an engineer who is willing to help Marlow.
Other groups visit the location, pilgrims pass through, and
also:

...[a]n
exploring party, the Eldorado Exploring Expedition...For the next six weeks, they appear
in sections. A white man on a donkey leads each group, followed by a band of blacks.
They are reckless, greedy, and cruel. They will “tear treasure out of the bowels of the
land.” (e-Notes
Summary/Analysis)

The last
station, the Inner Station, is where Marlow will find Kurtz. But this place is full of
insanity and death. Civilization has reached here even though Kurtz, a Company man, has
lived with the natives for sometime. Kurtz is unwell, and seemingly mad. Around his hut
there are stakes with human heads on them. It seems there have been instances of human
sacrifice. The natives worship Kurtz and do now want him to leave. This then, is the
heart of darkness, not only found within Kurtz, but in the center of the jungle which is
a place of mystery and the unknown.

Each station has
something different to teach Marlow, and secrets that are closely
kept.