It's Alive!!

I was struck by a recent letter to the editor in the September 26, 2011 issue of The New Yorker.  In response to Jeffrey Toobin's profile of Virginia and Clarence Thomas, Albion M. Urdank of Los Angeles wrote in to criticize Thomas's originalist method of interpreting the Constitution:

In determining how many representatives Southern states might send to Congress, the [Constitution] provides that a black person, although disenfranchised, counts as three-fifths of a person.  To think that we are required to pay deference to the founders' views on this subject is absurd.  The Thirteenth Amendment, which abolished slavery, represents an updating of the Constitution in the light of history and experience. [Not to mention war. -- Mr. Torvik]  Amendments don't just complicate originalism; they refute it by establishing that the Constitution is a living document.

Perhaps you can explain this to me, Mr. Gillette, because I'm befuddled by it.  The idea of a "living Constitution," as I understand it, is that we can use interpretive methods to proactively update the meaning of the constitution to keep up with the times without the need for formal amendments.  I completely fail to understand how the 13th, 14th, and 15th Amendments "refute" originalism.  Indeed, if the Constitution were really a "living document," surely those Amendments would have been unnecessary, as the Constitution would have simply "grown" to leave slavery and the three-fifths clause behind after the searing national tragedy that was the Civil War.

Right?