John -- The essay appeared in the July 1906 issue of HARPER'S MONTHLY.  It
is digitized online at:

https://babel.hathitrust.org/cgi/pt?id=uc1.c087222515;view=1up;seq=255

Scroll to p. 221.

The essay was also collected in the 1917 edition of WHAT IS MAN? AND OTHER
ESSAYS.

Barb