CROSS-BREEDING 247 



quickly correct any tendency to inferior size or weakened 

 constitution. Not only are in-bred animals benefited 

 in certain definite qualities by crossing, but breeds and 

 families which have not suffered in any way from in-breed- 

 ing are sometimes improved in size, vigor and fertility 

 by crossing. 



231. Crossing and heredity. As in-breeding tends 

 to simplify the germ-plasm and strengthen the powers of 

 transmission, so cross-breeding tends to weaken the 

 prepotency and complicate the elemental constitution 

 of the hereditary substance. Crossing has a tendency 

 to break up established characters. It destroys com- 

 binations of characters which have long existed in the 

 strain and which under systems of pure breeding have 

 behaved in a manner like unit characters in trans- 

 mission. The result of crossing pure-bred animals is 

 often to destroy the results of generations of careful 

 breeding and selection. 



232. First cross an improvement. The cross-bred 

 offspring of pure-bred parents often show an improvement 

 over either of the parents. This superiority may be 

 exhibited not alone in increased fertility and more vigorous 

 constitution, but also in the very qualities which char- 

 acterize the parents. A cross between animals belong- 

 ing to distinct breeds may be a better beef animal than 

 either parent. The Scotch farmer breeds the Aberdeen 

 Angus cow to a white Shorthorn bull. The offspring is 

 the well known " blue gray " which is highly prized by 

 the feeder and in the fat cattle market commands a 

 premium over the pure-bred animals of either breed. 

 The fat cattle exhibitions of the world have not infre- 

 quently given the highest prizes of the show to cross- 

 bred animals. (See Plate XVI.) - 



