Use the same size for all worlds. Right now, there are these scroll bars and it is not really cool that you have to use them instead of seeing everything ;)

In the original, you can move a tetromino when it hits the bottom for a short time. If you still didn't turn it in yet, you could try to implement that.

The box popping up when clicking on a creature should be displaced on top of everything else. If you need a new idea of what you could do next... you could display the number of creatures of each type over time in a diagram and show it after the simulation is over.

Then you should try out Traffic Simulation 2 (http://www.greenfoot.org/scenarios/13356) and Traffic Simulation 3 (http://www.greenfoot.org/scenarios/15117). They are much better than this. ;)

Every act-cycle represents a time step. In each time step, a living cell dies if it has 0, 1, 4, 5, 6, 7 or 8 living neighbour cells. A new cell is created if it has exactly 3 living neighbours. You can read more about it here: https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life

Cool thing, it reminds me of one of my simulations :) Did you do it on purpose that the reproduced creatures spawn randomly and not at the parent's location?