The expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient.

One general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die.

If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find no such case.

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 confess, absurd in the highest degree...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 subversive of the theory.

A grain in the balance will determine which individual shall live and which shall die - which variety or species shall increase in number, and which shall decrease, or finally become extinct.

The moral sense perhaps affords the best and highest distinction between man and the lower animals

It is difficult to believe in the dreadful but quiet war lurking just below the serene facade of nature.

In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history.

I fully agree with all that you say on the advantages of H. Spencer's excellent expression of 'the survival of the fittest.' This, however, had not occurred to me till reading your letter. It is, however, a great objection to this term that it cannot be used as a substantive governing a verb; and that this is a real objection I infer from H. Spencer continually using the words, natural selection.(Letter to A. R. Wallace July 1866)