Known sourcecanonical
"The expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient."
Charles Darwin / The Origin of Species. Chap. iii.
Bartlett's Familiar Quotations, 9th ed. (Little, Brown, 1905), public domain
Bartlett's Familiar Quotations, 9th ed. (Little, Brown, 1905), public domain