40 
BULLETIN 905, U. S. DEPARTMENT OE 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 ofthelineis 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 bv hereditv, 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 (Bulletins 1090 and 1-121, U. S. 
Department of Agriculture) has made experiments on the subject 
involving about 34,000 guinea pigs. A number of distinct families 
have been maintained wholly by matings of brother with sister. 
The fact that one of these has reached the twenty-fifth generation 
without any conspicuous decline in vigor in any respect is further 
evidence that the evils of inbreeding are bv no means as great as 
