"Why Adler Never Signed the Humanist Manifesto" -- Curt Collier, Leader of the Riverdale-Yonkers Society for Ethical Culture [1]

The Humanist Manifesto is one of the most comprehensive statements of the aspirations and values of the Humanist community in the 1930's. It was signed by a list of notable thinkers and clergy. However, one name was noticeably absent; Felix Adler's. Why didn't Adler sign the document, and what does this act teach us about what he considered the future direction of our movement?