“Social Darwinism and eugenics are unscientific in two aspects. First, they commit the naturalistic fallacy, which is the assumption that what is, must be. The fact that the natural environment selects for reproductive advantage does not mean that we, as humans, should be forcibly selecting people according to some preconceived notions. The second nonscientific aspect of these movements was their narrow interpretation of the meaning of fitness. In evolution, this simply means fitness to survive and reproduce, not fitness according to some externally imposed criteria.
Darwin himself never engaged in these speculations, nor did he support these perversions of his theory.”

Introduction, p. 2.
The Causes of Evolution (1932)

As We May Think (1945)

Source: A social information processing approach to job attitudes and task design. 1978, p. 231

Source: The Red Queen (1993), Ch. 1. Human Nature

p 11; this was originally listed here in a somewhat misleading form combining it with another statement on the interpretations of dreams on p. 14: No language exists that cannot be misused … Every Interpretation is hypothetical, for it is a mere attempt to read an unfamiliar text.
Modern Man in Search of a Soul (1933)
Context: It is imperative that we should not pare down the meaning of a dream to fit some narrow doctrine. … No language exists that cannot be misused. It is hard to realize how badly we are fooled by the abuse of ideas, it even seems as if the unconscious had a way of strangling the physician in the coils of his own theory.

Source: 1980s and later, Models of my life, 1991, p. 166; As cited in Ronald J. Baker (2010, p. 122).

"The Tallest Tale", p. 312
Leonardo's Mountain of Clams and the Diet of Worms (1998)

Source: What On Earth Is About To Happen… For Heaven’s Sake? (2013), pp. 34-35

There is a special providence in the fall of a sparrow. Even Drought bears fruit. Even death is a seed.

Chapter 5, A Special Providence (p. 82)
The Beak of the Finch: A Story of Evolution in Our Time (1994)