Sentence:  It is natural from the marksman 's viewpoint to call a bull's-eye a success , but in the mice example it is arbitrary which category corresponds to straight <WSD>hair</WSD> in a mouse . 
Meaning of the word marked with <WSD> tags:  a covering for the body (or parts of it) consisting of a dense growth of threadlike structures (as on the human head); helps to prevent heat loss