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