“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibilites.”

As quoted in Words from the Wise : Over 6,000 of the Smartest Things Ever Said (2007) by Rosemarie Jarski, p. 312
Context: I doubt if a single individual could be found from the whole of mankind free from some form of insanity. The only difference is one of degree. A man who sees a gourd and takes it for his wife is called insane because this happens to very few people.

Source: Equisse d'une Théorie de la Pratique (1977), p. 164; as cited in: Jan E. M. Houben (1996) Ideology and Status of Sanskrit, p. 190

after 1920, The Epic, From immobile form to mobile form (1925)

My Reviewers Reviewed (lecture from June 27, 1877, San Francisco, CA)

pg 45.
Science in a Free Society (1978)

Source: History and Class Consciousness (1968), p. 64

Source: Mendel's Principles of Heredity (1913), p. 3

The Notebooks of Leonardo da Vinci (1883), XIX Philosophical Maxims. Morals. Polemics and Speculations.