I finally took the plunge and got this tattoo that I have been wanting for 4 years.
This diagram is found in one of Charles Darwin's notebooks from 1837 (more than 20 years before The Origin was published). This is what is written beneath it: "Thus between A & B immense gap of relation. C & B the finest gradation, B & D rather greater distinction. Thus genera would be formed. — bearing relation"
*EDIT (Jan 7 2012) - Over 5000 views! I'm glad you all like the ink :)