Some critics believe that, in Heart of Darkness, Conrad illustrates how "the darkness of the landscape can lead to the darkness of social...

This statement suggests that the evil or corruption of one's particular location (its "darkness," in other words) corrupts the people who live in that particular location. It's a support of the idea that a human's personality (and morality) are "nurtured" by the society/place that he or she inhabits, and so it suggests that place has power over whether or not a human being becomes corrupted. There are several suggestions that Conrad feels this way, the most famous of which takes place toward the beginning of the book when a doctor measures Marlow's head. The doctor says, "'I always ask leave, in the interests of science, to measure the crania of those going out there,'" and he asserts that "'the changes take place inside, you know'" (24). The suggestion here is that it is the particular landscape of the African Congo that causes the personality of colonists to change. By extension, the doctor is suggesting that it is the place, the landscape of the ivory company's colonies, that causes the colonists to go insane and become corrupt.

To better understand how the environment could affect one's actions, feelings, and morals, consider the following example. While some people might claim that a drug dealer is inherently evil and corrupt, it's also possible that the environment that person was born into led him or her to make "corrupt" choices. What if, for example, the drug dealer was born into a life of extreme poverty? What if he or she grew up in a community that relied upon illegal transactions for income? In this context, dealing drugs becomes one of the primary ways to make a living and survive. It's still a corrupt and dark process, but it also becomes one of the only ways to make ends meet. As such, the person choosing to sell drugs is not inherently corrupt, but was affected by a corrupt environment. 

Within the context of the novel, there are a couple of ways this idea works. First, one could assume that Conrad is making an implicitly racist assumption. He could be arguing, for instance, that it is the primitive and barbaric aspects of the Congo that cause the corruption of the colonists. The overwhelming isolation of the African wilderness could drive the colonists to kill and enslave natives and to steal their natural resources. This is certainly part of what Conrad says about the effect of the environment, but it's not all of it.

For instance, one could also assume that the "landscape" that corrupts society is not actually African society, but rather the colonial landscape that employees of the ivory company inhabit. This landscape is devoted to making money at all costs, even if it means oppressing native peoples and pillaging the countryside. In this way, the darkness of this landscape could be seen as the primary corrupting force in the novel, and all who inhabit it become as corrupt as Kurtz himself. As such, perhaps Conrad's "heart" of darkness has little to do with Africa, and has more to do with the evils of the colonialist landscape.