Henry David Thoreau, Laura Ross (2009). “Walden, Or, Life in the Woods: Bold-faced Ideas for Living a Truly Transcendent Life”, p.50, Sterling Publishing Company, Inc.
Philosophies (1910) "In Exile" pt. 7 (describing his part in discovering the life-cycle of the malaria parasite in 1897; cf. Oxford Dictionary of Quotations (1979) 77:1)