1. Biology. a. the reappearance in an individual of characteristics of some remote ancestor that have been absent in intervening generations.
b. an individual embodying such a reversion.
2. A revision to an earlier type; a throwback.
3. A step, or several steps, backwards in evolution.
EX. It appears that secondary mutations are not always useful in the X-Men universe. Sure, Angel gets metal skin and Beast becomes a cat guy, but sometimes you get your total f***-ups. Shooting concussive laser beams out of your knuckles would seem to be a bit harder to control, but shooting claws out of your eyes would be just about useless, if not agonizingly painful.
In such situations, could an atavism come in handy to reverse the effects?