seen, and has shown no indications of dimin- 

 ished fertility. 



In the production of pure breeds of sheep, 

 cattle, hogs, and horses inbreeding has fre- 

 quently been practiced extensively, and where 

 in such cases selection has been made of the 

 more vigorous offspring as parents, it is doubt- 

 ful whether any diminution in size, vigor, or 

 fertility has resulted. Nevertheless it very 

 frequently happens that when two pure breeds 

 are crossed, the offspring surpass either pure 

 race in size and vigor. This is the reason for 

 much cross-breeding in economic practice, the 

 object of which is not the production of a new 

 breed, but the production for the market of 

 an animal maturing quickly or of superior size 

 and vigor. The inbreeding practiced in form- 

 ing a pure breed has not of necessity dimin- 

 ished vigor, but a cross does temporarily (that 

 is in the Fj generation) increase vigor above 

 the normal. Now why should inbreeding un- 

 attended by selection decrease vigor, and cross- 

 breeding increase it? We know that inbreed- 

 ing tends to the production of homozygous 

 conditions, whereas cross-breeding tends to 

 11 149 



