Obvious printer's errors have been repaired, other inconsistent spellings have been kept, including inconsistent use of hyphen (e.g. "far reaching" and "far-reaching").

Some illustrations were relocated to correspond to their references in the text.

Pg. 816, word "of" added to sentence "...acceleration of social evolution."

Entries in Index refer to all five issues of Popular Science Monthly Vol. LV:

• May (pp. 1-136) /ebooks/44880
• June (pp. 137-288) /ebooks/45115
• July (pp. 289-432) /ebooks/45361
• August (pp. 433-576) /ebooks/45938
• September (pp. 577-720) /ebooks/46383
• October (pp. 721-864), this issue.

Page number links have been created for index entries refering to the October issue.