The Collaborative International Dictionary of English v.0.48:

Progressionist \Pro*gres"sion*ist\, n.
   [1913 Webster]
   1. One who holds to a belief in the progression of society
      toward perfection.
      [1913 Webster]

   2. One who maintains the doctrine of progression in organic
      forms; -- opposed to uniformitarian. --H. Spencer.
      [1913 Webster]