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On the Origin of Species

Recapitulation And Conclusion]
Recapitulation of the objections to the theory of Natural Selection -- Recapitulation of the
general and special circumstances in its favour -- Causes of the general belief in the
immutability of species -- How far the theory of Natural Selection may be extended --
Effects of its adoption on the study of Natural History -- Concluding remarks.
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. There are, it
must be admitted, cases of special difficulty opposed to the theory of natural selection;
and one of the most curious of these is the existence in the same community of two or
three defined castes of workers or sterile female ants; but I have attempted to show how
these difficulties can be mastered.
With respect to the almost universal sterility of species when first crossed, which forms
so remarkable a contrast with the almost universal fertility of varieties when crossed, I
must refer the reader to the recapitulation of the facts given at the end of the ninth
chapter, which seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two distinct kinds of trees to be grafted together; but
that it is incidental on differences confined to the reproductive systems of the intercrossed
species. We see the truth of this conclusion in the vast difference in the results of crossing
the same two species reciprocally--that is, when one species is first used as the father and
then as the mother. Analogy from the consideration of dimorphic and trimorphic plants
clearly leads to the same conclusion, for when the forms are illegitimately united, they
yield few or no seed, and their offspring are more or less sterile; and these forms belong
 
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