126 EVOLUTION 



sudden though minute change. It is perhaps 

 appropriate to compare such a change with 

 a chemical substitution." 



T prrypQfl^ tr> givfi to this 



jfe 



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 

 flip qft-pflllftfl .^prmtnn^iff ynrintirm^ 

 i' nf 



fl.nr| (]]fft.inpt Y/vn'ptm fl^p They are also 

 termed, fitly enough, sports. In spite of the 

 fact that they occur lairly 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. Tjiese jnew forms can be 



na .in Nntiirr 



But as yet we have no power of inducing them 

 at will. It_is mybelief that- a1| frfre simple 



th^ ggyferaljterm variation, then, 

 are included two distinct phenomena: muta,- 

 bility, and ffuctufotion or orc^inayy variation^" 

 "The methods of artificial selection cor- 

 respond to these two types of variability. 



