ous grades of reduction, indicating the probable result of
several distinct causes, sometimes acting separately, sometimes in
combination, such as those we have already pointed out.

And if we find no positive evidence of _disuse_, acting by its direct
effect on the individual, being transmitted to the offspring, still less
can we find such evidence in the case of the _use_ of organs. For here
the very fact of _use_, in a wild state, implies _utility_, and utility
is the constant subject for the action of natural selection; while among
domestic animals those parts which are exceptionally used are so used in
the service of man, and have thus become the subjects of artificial
selection. Thus "the great and inherited development of the udders in
cows and goats," quoted by Spencer from Darwin, really affords no proof
of inheritance of the increase due to use, because, from the earliest
period of the domestication of these animals, abundant milk-production
has been highly esteemed, and has thus been the subject of selection;
while there are no cases among wild animals that may not be better
explained by variation and natural selection.

_Difficulty as to Co-adaptation of Parts by Variation and Selection._

Mr. Spencer again brings forward this difficulty, as he did in his
_Principles of Biology_ twenty-five years ago, and urges that all the
adjustments of bones, muscles, blood-vessels, and nerves which would be
required during, for example, the development of the neck and fore-limbs
of the giraffe, could not have been effected by "simultaneous fortunate
spontaneous variations." But this difficulty is fully disposed of by the
facts of simultaneous variation adduced in our third chapter, and has
also been specially considered in Chapter VI, p. 127. The best answer to
this objection may, perhaps, be found in the fact that the very thing
said to be impossible by variation and natural selection has been again
and again effected by variation and artificial selection. During the
process of formation of such breeds as the greyhound or the bulldog, of
the race-horse and carthorse, of the fantail pigeon or the otter-sheep,
many co-ordinate adjustments have been produced; and no difficulty has
occurred, whether the change has been effected by a single variation--as
in the last case named--or by slow steps, as in all the others. It seems
to be forgotten that most animals have such a surplus of vitality and
strength for all the ordina