Q. I cannot finder via Widener THE DAWN OF RELIGIOUS PLURALISM a Harvard PhD Diss by R H Seager, 1986.

Seager’s 1987 dissertation is available on microfiche at Andover-Harvard Theological Library.  The shelf number is Mfiche no. 593

Seager, Richard Hughes.

The World's Parliament of Religions, Chicago, Illinois, 1893 : America's religious coming of age / by Richard Hughes Seager.

1986.

vi, 286 leaves ; 29 cm.

Includes abstract.

Thesis (Ph. D.)--Harvard University, 1987.

Includes bibliographical references.

Andover-Harv. Theol | Mfiche | no. 593

Harvard University Archives | HU90.12553 Harvard Depository

Microforms (Lamont) | Ref | Microfiche HT 87784

The author's original typescript is at the University Archives.

I hope this helps.