Nothing at first can appear more difficult to believe than that the more complex organs and instincts 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 all parts of the organisation and instincts offer, at least individual differences — that there is a struggle for existence leading to the preservation of profitable deviations of structure or instinct — and, lastly, that gradations in the state of perfection of each organ may have existed, each good of its kind.

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