EFFECTS OF INBREEDING 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 for 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 
YOUNG / 
LY7 O 
BLE CAT 
XAOULT EF 
L244 
D4 EQN ALIVE 
X LITTERS PEP 
YEAR" 
+5/ZE OF 
| L/77ER o 
Fic. 27.—The vigor under crossbreeding (CO) and under renewed inbreeding (C1, C2, C3, 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. 
Fully 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 
