26 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTURE. 



virtually identical in the cases of inbred females mated respectively 

 with brothers (young inbred) and with unrelated inbred males (CO). 



When the crossbred young themselves become parents there is 

 a marked increase in the regularity with which litters appear. The 

 result is practically the same whether these crossbred parents are 

 unrelated (CC) or brother and sister (Cl), again indicating that 

 the vigor or weakness of the young is not a factor. A falling off' 

 appears when the parents are from the first generation of renewed 

 inbreeding (C2). In these respects there is considerable similarity 

 in the results shown in Figures 8 and 12, dealing with percentage 

 born alive and birth weight, respectively, characters which we con- 

 cluded were largely dependent on the dam. 



There is, however, a striking contrast with those cases in the 

 standing of Experiments CA and AC. The great increase in fre- 

 quency of litter when a crossbred male instead of an inbred is mated 

 with an inbred female (CA) seems to mean that the sire is most apt 

 to be responsible for irregularity in producing litters. The mating of 

 inbred males with crossbred females (AC) however gives a better record 

 than where both parents are inbred. This indicates that the female 

 is also responsible to some extent. The still greater improvement 

 where both parents are crossbred (CC, Cl) substantiates further the 

 responsibility of both parents. 



It should be recalled here, however, that Experiments CA and AG 

 are compared with inbreds during a period when the latter were 

 producing litters more frequently than the inbreds breeding simul- 

 taneously with CC and Cl. The actual records of AC and CA 

 were only slightly below CC and Cl and were so near the upper 

 limit possible for a guinea pig (which is about 5.3 litters per year) 

 that the superiority of CC and Cl over CA and AC is probably 

 somewhat exaggerated in Figure 16. The same considerations 

 apply to Experiments CG and CL, which would probably have made 

 records more nearly like CC and Cl under strictly comparable 

 conditions. The main conclusion that frequency of litter depends 

 primarily on the sire and secondarily on the dam is not weakened. 



Another important result is the great superiority of crossbreds 

 derived from only two inbred families over the random-bred stock. 

 In the other characters with which we are dealing there is merely a 

 recovery of the condition of the latter. It is probably not a coin- 

 cidence that frequency of litter is the only one of our characters in 

 which the miscellaneous inbred families (01) are inferior to the five 

 largest families (2, 13, 32, 35, and 39). It seems probable that 

 regularity in producing litters has been the most important factor 

 in the unconscious natural selection among the inbred families and 

 that the crossbreds are in this case derived from a selected ancestry. 

 The possibility of improvement in all characters through conscious 



