“The fittest survive.
What is meant by the fittest?
Not the strongest; not the cleverest —
Weakness and stupidity everywhere survive.
There is no way of determining fitness except in that a thing does survive.
"Fitness," then, is only another name for "survival."”

Source: Manhood of Humanity (1921), p. 136. Chapter: Capitalistic Era.
Context: Such as contribute most to human progress and human enlightenment — men like Gutenberg, Copernicus, Newton, Leibnitz, Watts, Franklin, Mendeleieff, Pasteur, Sklodowska-Curie, Edison, Steinmetz, Loeb, Dewey, Keyser, Whitehead, Russell, Poincaré, William Benjamin Smith, Gibbs, Einstein, and many others — consume no more bread than the simplest of their fellow mortals. Indeed such men are often in want. How many a genius has perished inarticulate because unable to stand the strain of social conditions where animal standards prevail and "survival of the fittest" means, not survival of the "fittest in time-binding capacity," but survival of the strongest in ruthlessness and guile — in space-binding competition!

Source: The Selfish Gene (1976, 1989), Ch. 2. The replicators

Source: What Is This Thing Called Science? (Third Edition; 1999), Chapter 5, Introducing falsification, p. 60.

As "Trudy"
Contributions of Jane Wagner, The Search for Signs of Intelligent Life in the Universe (1985)

The Great Illusion (1910)

Source: I Know This Much Is True

The Desert of Wheat (1919).

Diary (12 December 1890)
Diary and Letters of Rutherford Birchard Hayes (1922 - 1926)

From 1980s onwards, Buckminster Fuller Talks Politics (1982)