It is true that the chacteristics differentiating one species from another are clearer than those separating one sex from another, which are clearer than those separating one race from another. However, I think the shakiness of arguments to the effect "species don't really exist" is also shared in kind (though not nearly to the same degree) by arguments that conclude that "races don't really exist."
^^^^^^
Charles: What are some of your arguments that human, biololgical races do exist ? What is the unclarity in differentiting one sex from another other ? Isn't the difference being able to have fertile sex with the different sex ?