“You know, at times like this one feels, well, perhaps extinct animals should be left extinct.”

Meditations on Quixote (1914)
Source: as quoted in Susan Ratcliffe (ed.), Oxford Dictionary of Quotations by Subject, Oxford University Press, Oxford, 2010-03-11, page 223, ISBN 9780199567065

The Bread-Winner (1930)
Plays

Journal entry on the writing of her science-fiction novel The Last Man (14 May 1824)

Will that make you happy?"
Jace to Alec, pg. 10
Source: The Mortal Instruments, City of Ashes (2008)

Original: (it) ​Persone di valore sono in continua estinzione. Quando riesci a trovarne una, custodiscila nel cuore.
Source: prevale.net

Source: Less is More: How Degrowth Will Save the World (2021), Welcome to the Anthropocene, p. 8

"Dinosaur Renaissance", Scientific American 232, no. 4 (April 1975), 58—78
Dinosaur Renaissance (1975)

Address to the annual meeting of the Phoenix Chamber of Commerce (30 March 1961)
Later variant: Freedom is a fragile thing and is never more than one generation away from extinction. It is not ours by inheritance; it must be fought for and defended constantly by each generation, for it comes only once to a people. Those who have known freedom and then lost it have never known it again.
California Gubernatorial Inauguration Speech http://www.reagan.utexas.edu/archives/speeches/govspeech/01051967a.htm (5 January 1967)
1960s
Context: Freedom is never more than one generation away from extinction. We didn’t pass it on to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children's children what it was once like in the United States where men were free.