EFFECTS OF IISTBEEEDHsTG AND CROSSBREEDING. 



47 



ceptionally heavy and vigorous at weaning. In this case, as in CL, 

 matings were made at random as far as ancestry was concerned. In 

 many cases the same family enters into the ancestry of both parents 

 of a given animal in these experiments. For this reason, as we have 

 just seen, a lower record is to be expected than in Experiment CC, 

 in which no family was used twice. 



SUMMARY AND GENERAL CONCLUSIONS. 



The Bureau of Animal Industry has conducted experiments tor 15 

 years on the effects of inbreeding on guinea pigs. About 34,000 

 animals have been recorded. These include the records of 23 sepa- 

 rate families, each descended from an original pair exclusively by 

 matings of brother with sister (over 25,000 animals), a control stock 



Fig. 27.— The vigor under crossbreeding (CO) and under renewed inbreeding (Cl, C2, 03, etc.) relative to 

 that in the inbred stock (A) and under continued crossbreeding (CC). Characters depending in various 

 degrees (I-V) on dam (or sire) and young. 



in which inbreeding has been carefully avoided (over 4,000 animals), 

 and crosses among the inbred families (nearly 5,000 animals). 



The fact that inbreeding of the closest possible kind has been car- 

 ried on for over 20 generations in several families, without any very 

 obvious degeneration, is a noteworthy result. 



There has been on the average, however, a decline in all elements of 

 vigor. The mortality at birth and between birth and weaning, the 

 weight at various ages, the regularity in producing litters, the size 

 ot litter and the resistance to tuberculosis are the principal character- 

 istics which have been studied in this connection. 



FuUy as important as the fact of an average decline in vigor, is the 

 conspicuous differentiation among the families, which has been 

 brought to light and increased by the inbreeding. This has been 



