other respects were assumed to be due wholly to less 

 favorable environmental conditions. 



That there has been a real genetic decline in the inbred 

 stock in all elements of vigor is shown by comparison 

 with the control stock B, which has been superior in ever>^ 

 respect. Still better evidence has been obtained by com- 

 parison of the inbreds with the young from crosses be- 

 tween the different families raised at the same time and 

 under the same conditions. 



The Effects of Crossing 



In interpreting the effects of crossing, the character- 

 istics which depend on the hereditarj^ make-up of the 

 young must be distinguished from those which depend on 

 the dam or sire. In studying these questions, inbred 

 females have been mated with inbred males of another 

 family (experiment CO) and with crossbred males (ex- 

 periment CA ) . Crossbred females have been mated with 

 brothers (experiment CI), unrelated crossbred males 

 (experiment CC) and inbred males of an unrelated fam- 

 ily (experiment AC). 



Size of litter appears to depend wholly on the dam. 

 There is little or no improvement in the experiments in 

 which the dam is inbred (CO, CA). There is, however, 

 a marked increase, 10-30 per cent, depending on condi- 

 tions, in the litters produced bv crossbred females {CI, 

 CC, AC). 



The record of an experiment in frequency of litters 

 depends on the age of maturity as well as on the regu- 

 larity thereafter. Males mature considerably later than 

 females, so that the age of maturity of the male is the 

 controlling factor in this respect in matings, made as in 

 the present experiments, between immature animals. 

 The frequency of litters after maturity appears to de- 

 pend largely on the dam. There is no improvement in 

 the record of the first cross (CO) over the inbreds. There 

 is, however, marked improvement in the other experi- 

 ments in which either the sire or dam or both are cross- 



