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 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 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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