It is held that an inquiry into human nature is important, not only because it can provide us with a theoretical knowledge of what man is, but also because it has far-reaching implications for what we ought to do in our lives. However, Hume's thesis that from ”is” cannot be derived ”ought” seems to undermine such implications. In this paper, I argue that Hume's thesis is not as well-founded as one might think it is. Then I analyze how Darwin's evolutionary theory challenges the traditional conception of human beings. Finally, a portrait of man will be made in the light of Darwinism, before I conclude that Darwinism helps to shed light on some hotly debated ethical issues.