228 



PRACTICAL COURSE IN BOTANY 



that manifest themselves are said to be dominant; those 

 which do not, recessive. 



r-fiwr^. H 



Fig. 337. — Effect of hybridization between related species in imparting superior 

 vigor to offspring : M, Californian black wsdnut (Juglans call for nica), male parent; 

 F, Eastern black walnut {J. riigra), female parent ; //, 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 



DX R 



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, 



