Diamond

According to Diamond, "race" should not be defined in one way, that is to say, it possibly has a lot of definitions, depending on different perspectives. Moreover, Diamond concludes that "the racial classification is arbitrary"(11-3). Therefore, the definition of race should be determined by people's arbitrary viewpoints.
Next, Diamond states that human beings really have no clear ideas about races (sub-species). This is because if we try to classify humans into races, we will face a lot of contradictions, for example, we cannot categorize humans in "one" specific way, because each human has various characteristics so the classification of humans varies depending on which trait we choose as a standard of the category.
In conclusion, as mentioned above, Diamond emphasizes that the racial classification should be determined by the arbitrary judgement of individuals. By the way, he thinks that the variation of skin color plays a role in humans' sexual and social choices.