Nothing at first can appear more difficult to believe than that the more complex organs and instincts should have been perfected, not by means superior to, though analogous with, human reason, but 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, which we may consider, either do now exist or could have existed, each good of its kind,— 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.
Charles Darwin, The Origin of Species
evolution, natural-selection