vii] HEREDITY 89 



Moth' (Amphidasys betularia), it may spread rapidly 

 until it becomes common, and if recessive it will 

 equally often be represented in the germ-cells of 

 many individuals and will appear when two which 

 bear it mate together. In either case if the mutation 

 be advantageous it may be preserved at the expense 

 of the type by natural selection, until it obtains a 

 firm footing. But the difficulty has naturally been 

 felt that the marvellously perfect adaptations which 

 are so frequent in nature cannot be imagined to 

 have arisen by large steps, but must have been ac- 

 quired gradually, and therefore many naturalists 

 reject the suggestion that mutations can have been 

 largely operative in evolution. But the fallacy here 

 is the assumption that all discontinuous variations 

 must be large ; the case of Johannsen's beans shows 

 that essentially stable variations occur, which pro- 

 bably differ from mutations only in their small extent, 

 and by the selection of such ' minute mutations' 

 the wonderfully perfect structures of living things 

 might be produced. It may perhaps be regarded as 

 hair-splitting to distinguish between minute mutations 

 and fluctuating variability, but the distinction lies in 

 the nature of their inheritance, which is the essential 

 thing in evolution. It has been seen that no selection 

 within the 'pure line' in the case of the beans has 

 any effect ; for progress to be made, a new mutation, 

 small though it might be, is necessary. 



