The syrinx or lower larynx is the most interesting and absolutely avine modification, although absent as a voice-producing organ (probably due to retrogression) in most Ratitae, storks, turkey buzzards (Cathartes) and Steganopodes.
Politically, indeed, the whole period was one of retrogression and stagnation.
He had no profound insight into the problem of Judaism, and there was no lasting validity in his view that the problem - the thousands of years' old mystery - could be solved by a retrogression to local nationality.
In virtue of this defect, due largely to the failure to enter into the Apostolic experience of mystic union with Christ, he can rightly speak of "an immense retrogression" in theology visible "at the end of the century, and in circles where it might have been least expected" (ii.
This theological "retrogression" is of much significance for the history of dogma.