126 EVOLUTION 



sudden though minute change. It is perhaps 

 appropriate to compare such a change with 

 a chemical substitution." 



'The name I propose to give to this 

 'species-forming' variability is Mutability 

 - a term in general use before Darwin's time. 

 The changes brought about by it, the Muta- 

 tions, are phenomena as to the exact nature 

 of which we understand very little so far. 

 The best known examples of such mutations 

 are the so-called spontaneous variations (the 

 'single variations' of Darwin) by which new 

 and distinct varieties arise. They are also 

 termed, fitly enough, sports. In spite of the 

 fact that they occur fairly often, they are 

 usually not noticed until the new form has 

 already appeared, when of course it is too late 

 to study the phenomenon of its origin ex- 

 perimentally. These new forms can be 

 sought for in cultivated species, which are 

 seldom of pure origin; as well as in Nature. 

 But as yet we have no power of inducing them 

 at will. It is my belief that all the simple 

 characters of animals and plants arise in this 

 way.' 3 



'Under the general term variation, then, 

 are included two distinct phenomena: muta- 

 bility, and fluctuation or ordinary variation." 



'The methods of artificial selection cor- 

 respond to these two types of variability. 



