degenerationist

from The Century Dictionary.

• noun One who advocates the theory of degeneration; one who believes that the general tendency of organized beings, especially of man in his mental and moral life, is to degenerate; one who maintains that the natural course of civilization is downward rather than upward.
• Pertaining to the theory of degeneration.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.) A believer in the theory of degeneration, or hereditary degradation of type.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology A believer in the theory of degeneration, or hereditary degradation of type.