The reason for this preponderance, according to East and 

 Shull, is that the vigor of "a plant is influenced not only by 

 the characters it possesses but also according to whether 

 these characteristics are present in the pure or in the hybrid 

 condition. If, therefore, self- fertilized strains or varieties 

 have survived competition in the struggle for existence they 

 are inherently stronger than cross-fertilized strains or var- 

 ieties" (East, Bui. 243. U.S. Bureau of Plant Industry). 



Miss H. D. King inbred, brother and sister, two series of 

 albino rats for 25 generations. The productiveness did not 

 decrease, nor did the constitutional vigor. The space of 

 life in both sexes was increased (Journ. Exp. Zool; 1918, 



26). 



Exercise. — Show by Mendel's formula that when a 

 heterozygous pair is self- fertilized, the progeny becomes after 

 several generations more and more homozygous. Make a 

 graph showing the reduction of heterozygous individuals in 

 successive generations. 



The following chart illustrates line-breeding in a 

 noted Clydesdale strain; 



Lodeski 



Hiawatha 

 IV 



Hiawatha 

 HI 



Elva. 



Alda. 



Halo. 



Daisy . 



Hiawatha 

 H 



Selina. . . 



Hiawatha 

 Luda 

 Hiawatha 

 ( Selina 



J 



I Hiawatha 

 I Bossie. . . 



I Hiawatha 

 I Jada 



r Hiawatha 

 ( Rudie 



Hiawatha 



H 

 Notie 



(from Journal of Heredity) 



Lodeski is line bred to Hiawatha. She is not bred in 

 Ime to Jada but rather to her grandsire Hiawatha whose 

 blood has been perpetuated and intensified. 



148 



