Darwin, however, considered such hypotheses as hardly belonging to the
domain of science; they belong, he said, to the realm of miracles. That
species have a capacity for change is admitted by all evolutionists; but
there is no need to invoke modifications other than those represented by
ordinary variability. It is well known that in artificial selection this
tendency to vary has given rise to numerous distinct races, and there is no
reason for denying that it can do the same in nature, by the aid of natural
selection. On both lines an advance may be expected with equal
probability.

His main argument, however, is that the most striking and most highly
adapted modifications may be acquired by successive variations. Each of
these may be slight, and they may affect different organs, gradually
adapting them to the same purpose. The direction of the adaptations will
be determined by the needs in the struggle for life, and natural selection
will simply exclude all such changes as occur on opposite or deviating
lines. In this way, it is not variability itself which is called upon to
explain beautiful adaptations, but it is quite sufficient to suppose that
natural selection has operated during long periods in the same way.
Eventually, all the acquired characters, being transmitted together, would
appear to us, as if they had all been simultaneously developed.

Correlations must play a large part in such special evolutions: when one
part is modified, so will be other parts. The distribution of nourishment
will come in as one of the causes, the reactions of different organs to the
same external influences as another. But no doubt the more effective cause
is that of the internal correlations, which, however, are still but dimly
understood. Darwin repeatedly laid great stress on this view, although a
definite proof of its correctness could not be given in his time. Such
proof requires the direct observation of a mutation, and it should be