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 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
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. {460}

It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially amongst broken and
failing groups of organic beings; 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 being, could not have arrived at its present state
by many graduated steps. There are, it must be admitted, cases of special
difficulty on the theory of natural selection; and one of the most curious
of these is the existence of two or three defined castes of workers or
sterile females in the same community of ants; but I have attempted to show
how this difficulty 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 eighth chapter, which
seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two trees to be grafted together; but
that it is incidental on constitutional differences in the reproductive
systems o