Tendency to Vary 67 
useless. On the other hand, he strongly combated the belief, that 
great changes 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 varia- 
tions. 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 varia- 
bility itself which is called upon to explain beautiful adaptations, 
but itis 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 distri- 
bution 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 
stated here that even the first observations made in this direction 
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