retrogression

n. [ Cf. F. rétrogression. See Retrograde, and cf. Digression. ] 1. The act of retrograding, or going backward; retrogradation. [ 1913 Webster ]

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 ]