The Collaborative International Dictionary of English v.0.48:

Regressive \Re*gress"ive\ (r?*gr?s"?v), a. [Cf. F.
   r['e]gressif.]
   [1913 Webster]
   1. Passing back; returning.
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   2. Characterized by retrogression; retrogressive.
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   Regressive metamorphism.
      (a) (Biol.) See Retrogression.
      (b) (Physiol.) See Katabolism.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Retrogression \Re`tro*gres"sion\, n. [Cf. F. r['e]trogression.
   See Retrograde, and cf. Digression.]
   1. The act of retrograding, or going backward;
      retrogradation.
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   2. (Biol.) Backward development; a passing from a higher to a
      lower state of organization or structure, as when an
      animal, approaching maturity, becomes less highly
      organized than would be expected from its earlier stages
      or known relationship. Called also retrograde
      development, and regressive metamorphism.
      [1913 Webster]