why is Pudd'nhead Wilson a tragedy?

When writing this book, Twain found himself with two stories uncomfortably meshed together. One was a comical farce focusing on Siamese twins, while the other was a tragedy about racial identity and slavery. He concluded that the two storylines could not appropriately coexist, and that one needed to go. He decided to extract and discard the former, as it had long since been eclipsed by the newer characters and plot. After some reworking of the story, Pudd'nhead Wilson emerged. Twain finds tragedy in the sense that classifications on the basis of race are completely arbitrary. You can check this out in more detail in the GradeSaver link below. Scroll down to "The Arbitrary Nature of Racial Distinctions".

http://www.gradesaver.com/puddnhead-wilson/study-guide/major-themes/