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 first
used as the father and then as the mother.

The fertility of varieties when intercrossed and of their mongrel
offspring cannot be considered as universal; nor is their very general
fertility surprising when we remember that it is not likely that either
their constitutions or their reproductive systems should have been
profoundly modified. Moreover, most of the varieties which have been
experimentised on have been produced under domestication; and as
domestication apparently tends to eliminate sterility, we ought not to
expect it also to produce sterility.

The sterility of hybrids is a