CHAPTER XX 



Mutation 



Continuous variation and discontinuous variation. — Examples of 

 discontinuous variation. — Theories based upon it. — Korshin- 

 sky's heterogenesis. — De Vries' experiments; the theory of 

 mutation. Its theoretical importance according to De Vries. 

 — Formation of new varieties and species. — The mutations 

 of (Enothera. — The laws of mutation. — Individual selection 

 and interspecific selection. — The origin of mutations. — 

 Partisans and opponents of the mutation theory. — Its prac- 

 tical importance. 



THE orthodox Darwinian theory of natural se- 

 lection is based upon individual variations of 

 fluctuations which affect all the individuals, so that, as 

 far as one given character is concerned, mere differ- 

 ences of degree, sometimes insignificant, separate one 

 individual from another. If the degree of develop- 

 ment of that character in every individual of a genera- 

 tion were represented by various levels on the scale, 

 the line joining all those levels would describe a curve 

 without any sudden saltation. 



This mode of variation is designated as slow or con- 

 tinuous variation, slow because of the long accumula- 

 tion of hardly noticeable characters necessary to con- 



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