That many and grave 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.
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, as is proclaimed by the canon, Natura non facit
saltum, 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 of the intercrossed species. We see the truth
of this conclusion in the vast difference in the result, when the same
two species are crossed reciprocally; that is, when one species is
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