Historical Rules · 1 February 2013
Rule 702.98a
Official source text from the 1 February 2013 Rules release
Evolve is a triggered ability. "Evolve" means "Whenever a creature enters the battlefield under your control, if that creature's power is greater than this creature's power and/or that creature's toughness is greater than this creature's toughness, put a +1/+1 counter on this creature."