> 2. A single tree which returns multiple values > > For the second case, you'd need GPNodes which return not one but two numbers -- just modify the GPData to have two instance variables as appropriate. This may or may not make sense for your problem. Just a suggestion -- this could be a job for Cartesian GP or similar graph-based GP. The ECJ homepage has a link to an ECJ implementation of Cartesian GP. James -- Dr. James McDermott Natural Computing Research & Applications Group, Complex and Adaptive Systems Laboratory, University College Dublin, Ireland. http://www.skynet.ie/~jmmcd