. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 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 origina
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 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 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,
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