Mendel Norn

The Mendel Norn is a genetic breed for Creatures 2 made by jcarrcwalk of Close Encounters of the Creatures Kind. They are based on the Canny Norn genome, and 'were made in an effort to remove unfit norns from the genepool'. They accomplish this by having a chemical called 'evolutionary meter' which starts at max and decays to 0 through their life, and when it hits 0, the creature dies, similarly to the Life chemical. There is also another chemical which reduces the 'evolutionary meter' chemical which is emitted when a norn constantly has high drives.

They were available here.