“Maybe in order to understand mankind, we have to look at the word itself. Basically, it's made up of two separate words — "mank" and "ind." What do these words mean? It's a mystery, and that's why so is mankind.”

Speech in West Calder, Scotland (27 November 1879), quoted in The Times (28 November 1878), p. 10. The Conservative Prime Minister Benjamin Disraeli had proclaimed his policy as "Imperium et Libertas".
1870s

She set down her glass with a thump. “Or humankind. I’m afraid we’ve spent a lot of feminist energy on meaningless symbols rather than essential functions.”
Source: Gibbon's Decline & Fall (1996), Chapter 10, p. 170

Rescue (1945)
Context: Someone will read as moral
That the people of Rome or Warsaw
Haggle, laugh, make love
As they pass by martyrs' pyres.
Someone else will read
Of the passing of things human,
Of the oblivion
Born before the flames have died. But that day I thought only
Of the loneliness of the dying,
Of how, when Giordano
Climbed to his burning
There were no words
In any human tongue
To be left for mankind,
Mankind who live on.

Speech, quoted in The Times (February 15, 1923).
Other works
Variant: Words are, of course, the most powerful drug used by mankind.

Source: The Human Comedy : As Devised and Directed by Mankind Itself (1937), Ch. 2

Source: Zen and the Art of Motorcycle Maintenance (1974), Ch. 20
Context: Any philosophic explanation of Quality is going to be both false and true precisely because it is a philosophic explanation. The process of philosophic explanation is an analytic process, a process of breaking something down into subjects and predicates. What I mean (and everybody else means) by the word ‘quality’ cannot be broken down into subjects and predicates. This is not because Quality is so mysterious but because Quality is so simple, immediate and direct.

Literary Essays, vol. II (1870–1890), Rousseau and the Sentimentalists

Source: Down and out in Paris and London (1933), Ch. 32

1950s, Address at the Philadelphia Convention Hall (1956)