Mutations 



itself the block does not return to its old position, 

 but tips over and comes to rest on another facet, 

 we have a representation of the kind of change 

 indicated by a mutation. 



The analogy is far from perfect, for it makes 

 it appear that the smallest mutation must of 

 necessity involve a departure from the normal 

 type more considerable than that of the largest 

 fluctuating variation. Now, although mutations 

 ordinarily consist in considerable deviations from 

 the mean or mode of the type, while continuous 

 variations are usually minute deviations, it some- 

 times happens that the extreme fluctuations are 

 more considerable than some mutations. Hence 

 " fluctuating " describes this latter kind of 

 variation more accurately than " continuous " 

 does. 



The test, then, of a mutation is not so much 

 the amount of deviation as the degree in which 

 it is inherited. Mutations show no tendency to 

 a gradual return to the mean of the parent 

 species ; fluctuating variations do display such a 

 tendency. A mutation consists, as M. E. East 

 says, in the production of a new mode or centre 

 for linear fluctuation ; it is, as it were, a shifting 

 of the centre of gravity ; the centre about which 

 those fluctuations which we call continuous varia- 

 tions occur. 



As it is of considerable importance thoroughly 

 to grasp the true nature of mutations or discon- 



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