The following chart illustrates line-breeding in a noted 

 Shorthorn strain; 



f Young Phoenix 



Phoenix 





[ Foljambe 



f Phoenix ] 



L Favourite I ( Foljambe 



Comet \ 



Favourite 



Phoenix 



^ Ballingbroke 



Ballingbroke 

 I Foljambe 



I 



i Foljambe 



( Foljambe 



Account for the extinction of the Bates- Duchess line of 

 Shorthorns. 



Observations on in-breeding with both plants and ani- 

 male frequently reveal the fact that fertility is reduced in 

 the first two or three inbred generations. On the other 

 hand, Castle states that he inbred Drosophila, the pomace 

 fly, for fifty-nine generations without any apparent effect 

 on either the vigor or the fecundity. Nevertheless when 

 he crossed two inbred strains the offspring were superior in 

 productiveness to either inbred strain (Compare ShuU & 

 East's Experiments with Corn, page 78). 



Bos inbred rats for six years; for the first three years no 

 ill effects were observed, but after that a rapid decline in 

 vigor and fertility occurred. 



Castle again states that "the heterozygous yellow 

 mouse is a vigorous lively animal, but the homozygous yel- 

 low mouse is so feeble that it perishes as soon as produced, 

 never attaining maturity. Cross-breeding has, then, the 

 same advantage over close-breeding that fertilization has 

 over parthenogenesis. It brings together differentiated 

 gametes, which, reacting on each other, produce greater 

 metabolic activity" (Castle, Heredity.) 



In relation to man most countries have laws preventing 

 the marriage of close relatives, but in others, notably Egypt 

 and Abyssinia, the custom permitted of close inter-marr- 

 iages. Some recent studies go to show that in certain com- 



149 



