Evolve (Whenever a creature enters the battlefield under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)
Whenever a +1/+1 counter is put on Fathom Mage, you may draw a card.
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.
When comparing the stats as the evolve ability resolves, it’s possible that the stat that’s greater changes from power to toughness or vice versa. If this happens, the ability will still resolve and you’ll put a +1/+1 counter on the creature with evolve. For example, if you control a 2/2 creature with evolve and a 1/3 creature enters the battlefield under your control, it toughness is greater so evolve will trigger. In response, the 1/3 creature gets +2/-2. When the evolve trigger tries to resolve, its power is greater. You’ll put a +1/+1 counter on the creature with evolve.
If a creature enters the battlefield with +1/+1 counters on it, consider those counters when determining if evolve will trigger. For example, a 1/1 creature that enters the battlefield with two +1/+1 counters on it will cause the evolve ability of a 2/2 creature to trigger.
If Fathom Mage enters the battlefield with a +1/+1 counter on it (perhaps due to Master Biomancer), its last ability will trigger once for each +1/+1 counter it entered the battlefield with.
Fathom Mage’s last ability will trigger whenever any +1/+1 counter is placed on it, not just ones due to the evolve ability.