It is the opponent of acceleration, so to speak, or the opposite or negative of that mode of development.

Thus series may occur in which, either in size or characteristics, they return to former characteristics; but a better discussion of this point you will find in the little treatise which I send by the same mail as this letter, "On Reversions among the Ammonites." LETTER 254.

TO A.HYATT.Down, December 4th, 1872.
I thank you sincerely for your most interesting letter.

You refer much too modestly to your own knowledge and judgment, as you are much better fitted to throw light on your own difficult problems than I am.
It has quite annoyed me that I do not clearly understand yours and Prof.
Cope's views (254/1.

Prof.Cope's views may be gathered from his "Origin of the Fittest" 1887; in this book (page 41) is reprinted his "Origin of Genera" from the "Proc.