hanges would be
necessary to explain the origin of species. Some authors had propounded
the idea that highly adapted organs, e.g. the wings of a bird, could
not have been developed in any other way than by a comparatively sudden
modification of a well defined and important kind. Such a conception
would allow of great breaks or discontinuity in the evolution of highly
differentiated animals and plants, shortening the time for the evolution
of the whole organic kingdom and getting over numerous difficulties
inherent in the theory of slow and gradual progress. It would, moreover,
account for the genetic relation of the larger groups of both animals
and plants. It would, in a word, undoubtedly afford an easy means of
simplifying the problem of descent with modification.

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