MODIFICATION OF THE GERM-CELLS IN MAMMALS 149 



ing comparisons between young guinea-pigs it is important to 

 know whetlier the individuals concerned occurred in litters of 

 equal size. 



In the normal lines all individuals born singly survived, and, 

 as the seventh space shows, 30 per cent of them were unusually 

 large or over size when three months old. Normal animals 

 born in litters of two or three survive in about 84 per cent of the 

 cases and are often of large size. The members of litters of 

 four survive in only 62.5 per cent of the cases and are not gener- 

 ally vigorous animals. The records show that 80 per cent of 

 the young in litters of five survived, but this is very unusual 

 and is due probably to the small number involved, and possibly 

 to a slight extent to the extreme care with which the pregnant 

 females with the larger number of young were handled. This 

 extreme care, however, only saved 13.33 per cent from the same 

 number of alcoholic-line young born five in a litter. 



The second column indicates that over 81 per cent of alcoholic 

 animals born in litters of one or two are capable of survival. 

 Such a record is almost as good as the control, showing how very 

 strong the members of small litters are and indicates again that 

 an early individual selection may have played some part, since 

 no doubt there has been a prenatal mortality among the weaker 

 individuals which originally existed in some of these litters. 

 This is emphasized further by the fact that the members of 

 litters of three survive in only 60.93 per cent of the cases. Here 

 the prenatal mortality has not played so severe a role and many 

 weaker individuals are born. The power of survival of animals 

 born three in a litter from the control is about 23 per cent better 

 than from the alcoholic lines. Only 48 per cent of the alcoholic- 

 line individuals from litters of four were able to live three months- 

 Recognizing the small numbers involved, only 13.33 per cent of 

 the alcoholic guinea-pigs born in litters of five were viable. It 

 thus appears that when the alcoholic animals produce large 

 litters the quality of the young is very poor, whereas their 

 small litters contain animals with good survival records. There 

 is little doubt that this apparent difference in quality is in part 

 due to a prenatal selection which, in the case of the small litters, 

 has eliminated most of the weaker individuals and left only the 



