228 PRACTICAL COURSE IN BOTANY 
that manifest themselves are said to be vominant; those 
which do not, recessive. 
Fic. 337. — Effect of hybridization between related species in imparting superior 
vigor to offspring: M, Californian black walnut (Juglans californica), male parent 3 
F, Eastern black walnut (J. nigra), female parent; H, hybrid. 
258. Mendel’s Law.— So long ago as the middle of the last 
century it was discovered by Gregor Mendel, an Austrian 
investigator, that hybrids vary in certain cases according to 
a fixed law, by means of which the proportionate share of the 
characteristics of the two parent forms inherited by the off- 
spring can be foretold with almost mathematical precision. 
The controversy over Darwin’s “ Origin of Species,’ which 
was raging at the time, caused Mendel’s discoveries to be 
overlooked for a generation, and it is only within the last 
DXR 
2 D DL Impure dominants D’ R 2 
Diagram illustrating Mendel’s Law. 
few years that their importance has been realized. The 
principle of variation demonstrated by him in a series of 
experiments, and confirmed by later investigators is, briefly, 
