As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and serious objections may be advanced against the theory of
descent with modification through variation and natural selection, I do not
deny. I have endeavoured to give to them their full force. 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.
It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially among broken and
failing groups of organic beings, which have suffered much extinction; but
we see so many strange gradations in nature, that we ought to be extremely
cautious in saying that any organ or instinct, or any whole structure,
could not have arrived at its present state by many graduated steps.
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