on which is acted on essentially governs the result.
Distinct organisms, when placed under similar conditions, vary in different
manners, whilst closely-allied organisms under dissimilar conditions often
vary in nearly the same manner. We see this in the same modification
frequently reappearing at long intervals of time in the same variety, and
likewise in the several striking cases given of analogous or parallel
varieties. Although some of these latter cases are simply due to reversion,
others cannot thus be accounted for.

From the indirect action of changed conditions on the organisation, through
the impaired state of the reproductive organs--from the direct action of
such conditions (and this will cause the individuals of the same species
either to vary in the same manner, or differently in accordance with slight
differences in their constitution)--from the effects of the increased or
decreased use of parts,--and from correlation,--the variability of our
domesticated productions is complicated in an extreme degree. The whole
organisation becomes slightly plastic. Although each modification must have
its proper exciting cause, and though each is subjected to law, yet we can
so rarely trace the precise relation between cause and effect, that we are
tempted to speak of variations as if they spontaneously arose. We may even
call them accidental, but this must be only in the sense in which we say
that a fragment of rock dropped from a height owes its shape to accident.

* * * * *

It may be worth while briefly to consider the results of the exposure to
unnatural conditions of a large number of animals of the same species,
allowed to cross freely, with no selection of any {421} kind; and
afterwards to consider the results when selection is brought into play. Let
us suppose that 500 wild rock-pigeons were confined in their native land in
an aviary, and fed in the same manner as pigeons usually are; and that they
were not allowed to increase in number. As pigeons propagate so rapidly, I
suppose that a thousand or fifteen hundred birds would have to be annually
killed by mere chance. After several generations had been thus reared, we
may feel sure that some of the young birds would vary, and the variations
would tend to be inherited; for at the present day slight deviations of
structure often occur, but, as most breeds are already well established,
these modifications are rejected as blemishes.