EFFECTS OF INBREEDING AND CROSSBREEDING. 15 



these percentages was high, as 1910 and 1914, the other was also 

 high, the same years showing, moreover, high records in size and 

 frequency of litter and rate of growth. 



It will be shown later that there are significant differences among 

 the inbred families, differences which must be hereditary. It will be 

 shown also that success in bearing living young is not correlated 

 with success in rearing them. Genetically the constitutional vigor 

 of the young themselves and the qualities of the dam which favor 

 their successful rearing seem to be largely independent of the qualities 

 which insure against stillbirth. 



The size of litter naturally makes a difference in the percentages 

 born alive and reared. The effect, however, is different in different 

 stocks. In the vigorous control stock there was not much difference 

 even up to litters of 6. Litters of 3, however, were most successful 

 in both respects, with litters of 2, 4, and 5 following closely. In the 

 inbred families the most favorable size of litter has shifted to 2 or 

 even 1, and litters of 3 or more are at a disadvantage. Apparently 

 in a vigorous stock a litter of 1 is itself so much of an indication of 

 lack of vigor that smaller percentages are born alive and reared. 

 This effect is not a seasonal one, since litters of 1 and 2 were hardly 

 more numerous in winter than in summer among the controls, while 

 litters of 3 were much more numerous in winter. 



There is no appreciable difference between males and females in 

 the percentages born alive and reared. Among the controls (112 

 matings) 87.1 per cent of the males and 87.5 per cent of the females 

 were born alive. Among those born alive, 84.9 per cent of the males 

 and 85.4 per cent of the females were reared. Equally insignificant 

 differences, reversing, however, the slight advantage of the females, 

 were found among the inbred families. 



The age of the dam has little bearing on the percentage born alive 

 or reared, except as it is correlated with seasonal changes. The 

 cyclical tendency toward maxima at 12 and 24 months and minima 

 at 6 and 18 months, noted in the case of size of litter, is also pro- 

 nounced in the mortality percentages. 



There is naturally a strong tendency for the young in the same 

 litter to share the same fate. 



RATE OF GROWTH. 



The birth weight of guinea pigs varies greatly. Animals which 

 ultimately reach maturity may have weighed anywhere from 40 to 

 150 grams at birth. The average is about 80 grams. About half 

 fall between 64 and 91 grams. There is little difference between the 

 sexes. An extensive tabulation among the controls gave a difference 

 of only 2.7 ± 0.8 grams in favor of the males. There was a difference 

 of 11.1 ± 2.6 grams in favor of the males at 33 days. The males con- 



