40 BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 



which is good by accident, not by heredity, may at any time undo 

 all previous work. It is only by inbreeding a number of lines and 

 comparing them that the real hereditary differences can be recognized. 

 This was done in the inbreeding experiment with guinea pigs referred 

 to above. Certain inbred lines averaged 15 per cent white, others 

 85 per cent, while others were intermediate. It is very doubtful 

 whether the extreme types could ever have been obtained from the 

 original stock by straight selection without close breeding. Simi- 

 larly, differences in growth, fertility, and vitality were brought to 

 light among the different families of guinea pigs. These are discussed 

 under the next heading. 



It would be difficult to overemphasize the importance of close 

 breeding in the past history of livestock breeding, as the agent in 

 bringing out the real hereditary differences between different stocks, 

 and so leading to improvement in characteristics which could not 

 have been improved by selection alone. 



Fig. 9.— Four generations of inbred guinea pigs. The young pair at the right end of the line is descended 

 from 19 generations of matings of brother with sister. Three of these generations, the parents, grand- 

 parents, and great-grandparents are in the picture. Color and other characteristics have become fixed 

 automatically in this family because of the inbreeding. The exact coat pattern, however, as is generally 

 the case, is not wholly determined by heredity, and is therefore unfixable. 



THE EFFECT OF INBREEDING ON VIGOR. 



Along with the advantages of inbreeding, certain unfortunate 

 effects have long been known. A general reduction in vigor, espe- 

 cially in fertility, has long been ascribed to inbreeding, and there 

 can be no doubt that these are common effects. It is not, however, 

 so certain that they are invariable effects. Dr. Helen D. King, for 

 example, has inbred rats, brother with sister, for 25 generations with- 

 out any decline in size, constitutional vigor, or fertility, but rather 

 the reverse. 



The Bureau of Animal Industry has made experiments on the 

 subject involving more than 26,000 guinea pigs. A number of dis- 

 tinct famihes have been maintained wholly by matings of brother 

 with sister. The fact that one of these has reached the twentieth 

 generation without any conspicuous decline in vigor in any respect 

 is further evidence that the evils of inbreeding are by no means as 

 great as often pictured. Other families, however, suffered a rapid 



