The Sea-Wolf by Jack London: “I understood quite a good deal of _First Principles_, but his _Biology_ took the wind out of my sails, and his _Psychology_ left me butting around in the doldrums for many a day.”
The Descent of Man by Charles Darwin: “Herbert Spencer (27. 'Principles of Biology,' vol. i.”
Kokoro: Hints and Echoes of Japanese Inner Life by Lafcadio Hearn: “Now we find in Herbert Spencer's "Biology" a curious parallel for this idea. He explains the transmission of tendencies, and their variations, by a theory of polarities”