"Wake up, David. This is the country. This is Africa."

What I think I was sort of getting to (in a kind of roundabout way) in the last blog was the evidence of Achebe's idea that we (Westerners) see Africa as it is projected in Heart of Darkness, as "'the other world,' the antithesis of Europe and therefore of civilization, a place where man's vaunted intelligence and refinement are finally mocked by triumphant bestiality." I think Lucy's repetition of the statement that "This is Africa," fortifies that image of the bestial other, even compared to David's civilized life in the city of Cape Town (which at the end of the book is being engulfed by the shanties that will return it to Africa's state of otherness).
The first part of the book certainly plays into this perception. All we see is the austerity of the university, David Lurie's house, and Melanie's apartment, all in the civilized world of academic Cape Town. Right away, upon Lurie's arrival on Lucy's farm, that image is contrasted starkly with the realities of the country, and the insecurity of its inhabitants. Ettinger's farm-fortress, Lucy's need for a gun and her sense of security in her dogs, and the silent tension between white and African characters all exist in Lurie's experience on Lucy's farm, an experience very foreign to his city life.
Not only is that contrast evident in the differing lifestyles of Cape Town and Grahamstown, it's also evident in Lucy's attempts at reconciliation with that past. When David is confronted with the slaughter of Petrus' two sheep, and is disgusted, Lucy's only reply is, "Wake up, David. This is the country. This is Africa." This land is the opposite of everything you believe in. It is the counterbalance to your academic, "civilized" life. This is Africa.