WHAT EVOLUTION IS 125 



appeared some thirty years previ- 

 ously. The principles, contained in 

 Mendel's writings, were at once 

 made the basis of an extensive and 

 thorough-going experimental pro- 

 gramme and served, at the same time, 

 as ground on which de Vries erected 

 his mutation theory. 



Those portions of Mendel's work 

 that are directly concerned with 

 the mutation theory are easily and 

 quickly grasped. They can be illus- 

 trated by what occurs in animals as 

 well as in plants. If we breed a pure 

 black guinea pig to a pure white one, 

 the offspring are always black and if 

 these offspring are bred amongst 

 themselves, they produce young one- 

 fourth of which are pure white and 

 three-fourths are black. On testing 

 the black individuals, one-third of 

 them, or one-fourth of the total, can 

 be shown to be pure black and the 



