RETROGRESSION
regression, regress, reversion, retrogression, retroversion
(noun) returning to a former state
degeneration, retrogression
(noun) passing from a more complex to a simpler biological form
Source: WordNet® 3.1
Noun
retrogression (countable and uncountable, plural retrogressions)
A deterioration or decline to a previous state.
(biology) A return to a less complex condition.
Source: Wiktionary
Re`tro*gres"sion, n. Etym: [Cf. F. rétrogression. See Retrograde, and
cf. Digression.]
1. The act of retrograding, or going backward; retrogradation.
2. (Biol.)
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition