Origin, Chapter 14: Recapitulation and Conclusion

Darwin's last chapter is probably his most accessible. In his own words:

That many and grave objections may be advanced against the theory of descent with modification through natural selection, I do not deny…

Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected…by the accumulation of innumerable slight variations, each good for the individual possessor.

Nevertheless, this difficulty, though appearing to our imagination insuperably great, cannot be considered real if we admit the following propositions, namely, --that gradations in the perfection of any organ or instinct…either do now exist or could have existed…-that all organs and instincts are, in ever so slight a degree, variable,-and, lastly, that there is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct.

The truth of these propositions cannot, I think, be disputed.

Shown here: Stages in the evolution of the eye, from http://www.pnas.org/content/104/suppl.1/8567/F1.large.jpg