Mars had been a dead red planet for many a millennium when pioneers from Earth descended to start a rehabilitation program. The pep pill developed to liven up the landscape was in the form of a ""gas plant"" which gave off the missing atmospheric ingredient oxygen. The plants flourished into forests but, unknown to the pioneers, things were becoming more ""lively"" than supposed. Sixteen-year-old Rick Rawn commits himself to a Martian future (he can't return to Earth) and arrives to find the place buzzing with a deadly life form. Then all scales are rung off the evolutionary ladder and new life forms are accelerating themselves into the reptilian era. The battered Earthmen are busy trying to stay alive and ahead of Nature's next move. Rick, though a ""shrimp,"" proves himself mansized enough in any predicament. Fast-paced, undemanding adventure.