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THE AMERICAN NATURALIST [Vol. LV 



bred. Crossbred males mature earlier and the cross- 

 bred females have litters more regularly than inbreds. 



The percentage of the young born alive depends almost 

 wholly on the dam. There is little or no improvement in 

 experiments CO and CA, but an increase of 6 to 8 per 

 cent, where the dam is crossbred. The percentage which 

 are raised to 33 days of age of the young born alive de- 

 pends both on the dam and on the heredity of the young. 

 There is a marked increase, 9 to 12 per cent., in all of the 

 crossbreeding experiments mentioned above. 



Somewhat similarly, birthweight depends largely on 

 the dam, while the gains between birth and 33 days de- 

 pend to a considerable extent, though far from wholly, 

 on the heredity of the young. Guinea pigs become inde- 

 pendent of the dam at a very early age. There is an in- 

 crease of 2 or 3 per cent, in experiments CO and CA, but 

 one of about 7 to 10 per cent, where the dam as well as 

 the young are crossbred. In the gain between birth and 

 33 days, there is an improvement of about 16 per cent, in 

 the first cross, which is somewhat increased in the young 

 produced by crossbred dams. There is an increase of 

 15 to 20 per cent, in the adult weight in the first cross 

 (CO). This is maintained in the young produced by 

 matings of unrelated cro^sbreds (CC), but half of it is 

 lost where the parents, though crossbred, are brother 

 and sister (07). The influence of the dam does not ap- 

 pear to extend to the adult weight. 



A loss in the improvement brought about by the cross- 

 ing becomes apparent in the second generation of in- 

 breeding following a cross (experiment (72) in those 

 cases in which it is not apparent in the first generation. 



Comparison of Different Families 

 A comparison of the different inbred families with 

 each other has revealed persistent differences in color, 

 pattern, tendency toward polydactylism, tendency toward 

 production of monsters, mortality among the young, 

 weight and both elements of fertility. It was found that 



