The Next Generation button will cause the genetic engine to breed a new population of individuals and have the evaluative agents rank them in order of fitness. You can guage how long the process is taking by watching the progress bar at the bottom of the dialog. If you have selected a large number of starting curves, it may take a very long time to produce a new generation. Sadly, the process is not interruptable, so if it is taking longer than you want to wait, your only option is to close Maya from the Task Manager. Therefore, it is recommended that you begin using AgencyGP with very few starting curves and add more as you get a feel for how long the runs take.