Experiment One

Because damage remains marked on a creature until it's removed as the turn ends, nonlethal damage dealt to Experiment One may become lethal if you remove +1/+1 counters from it during that turn. In this case, it dies before the activated ability that would regenerate it resolves.

If multiple creatures enter the battlefield at the same time, evolve may trigger multiple times, although the stat comparison will take place each time one of those abilities tries to resolve. For example, if you control a 2/2 creature with evolve and two 3/3 creatures enter the battlefield, evolve will trigger twice. The first ability will resolve and put a +1/+1 counter on the creature with evolve. When the second ability tries to resolve, neither the power nor the toughness of the new creature is greater than that of the creature with evolve, so that ability does nothing.

Whenever a creature enters the battlefield under your control, check its power and toughness against the power and toughness of the creature with evolve. If neither stat of the new creature is greater, evolve won't trigger at all.