My own work with digital evolution follows (conceptually if not chronologically) in the progression of allowing increasing freedom to the evolutionary process. I have set up a system of self-replicating computer programs, where the only clearly defined ``fitness'' criteria imposed on the system is the same as what is found in organic evolution: the ability to replicate, or transmit genetic material to future generations. My system, called Tierra (Spanish for Earth) creates the basic Darwinian scenario: self-replicating entities in a finite environment with heritable genetic variation, inside the computer (http://www.hip.atr.co.jp/ ray/tierra/tierra.html) [22,24,25,26,27,32,6].