250 HEREDITY AND ENVIRONMENT 



are duplex the progeny, which are simplex, 

 bear pink flowers; in this case red flowers are 

 produced only when the factor for red is de- 

 rived from both parents, pink flowers when it 

 is derived from one parent, white flowers when 

 it is derived from neither parent ( Fig. 50 ) . 



5. Summary of Mendelian Principles. 

 Since the rediscovery in 1900 of Mendel's work 

 many investigators have carried out similar ex- 

 periments on many species of animals and 

 plants and have greatly extended our knowl- 

 edge of the principles of inheritance discovered 

 by Mendel, but in the main Mendel's conclu- 

 sions have been confirmed again and again, so 

 that there is no doubt that they constitute an 

 important rule of inheritance among all 

 organisms. 



In brief the "Mendelian Law of Alternative 

 Inheritance" or of hereditary "splitting" con- 

 sists of the following principles: 



(a) The principle of unit characters. The 

 total heritage of an organism may bq analyzed 

 into a number of characters which are inherited 

 as a whole and are not further divisible ; these 

 are the so-called "unit characters" (deVries). 



