EFFECTS OF INBREEDING AND CROSSBREEDING. 



13 



sizes has been strikingly different in these cases. The average size 

 of litter among the controls born in the years 1911 to 1916 under 

 the unfavorable conditions from January to June was 2.77. The 

 average for the eight poorest inbred families during the months 

 July to December in 1906 to 1910 was nearly the same, 2.74. The 

 difference in distribution may be seen in Table 3. It appears that 

 inferior heredity reduces the size of litter in a different way from 

 inferior environmental conditions. 



Table 3. — Percentage of litters of each size in two stocks of guinea pigs — a vigorous 

 stock under poor conditions and a weak stock under good conditions. 



Kind of stock. 



Number 

 of litters. 



Average 

 size of 

 litters. 



Percentage in litters of 1 to 6. 



1 



2 



3 



4 



5 



6 





386 

 372 



2.77 

 2.74 



11.4 

 16.9 



27.5 

 31.7 



40.7 

 24.5 



15.0 

 16.4 



4.4 

 8.6 



1.0 



8 inbred families (conditions favorable) 



1.9 



The age of the dam has an influence on the size of the litter, but 

 not an important one. First litters are smaller than later ones on 

 the average (2.77 compared with 3.05 in the first 112 ma tings of the 

 controls). This difference, however, exists mainly because first 

 litters are especially apt to be born in winter and spring. Most.mat- 

 ings have been made in summer and fall, when conditions are favor- 

 able, and the first litter, born when the dam is about 6 months old, 

 has thus been smaller on the average than litters born at 12 months 

 or 24 months, but not much smaller than litters born in the neigh- 

 borhood of 18 months. Females from 1 to 2\ years old, however, 

 produce slightly larger litters than younger or older females, apart 

 from the seasonal complication. 



Among the controls, litters which follow others without delay are 

 slightly larger than those born after a long interval (3.16 compared 

 with 2.91 in the tabulation referred to above). Presumably the 

 same causes which were favorable to immediate fertilization were 

 favorable also to a large size of litter. Curiously enough, a tabula- 

 tion among the inbreds born in 1916 gave a contrary result. The 

 average was 2.22 after a short interval and 2.46 after a long one. 

 In this case only 36 per cent of the litters were born after an interval 

 of less than 75 days as compared with 56 per cent in the controls in 

 the former tabulation. Apparently in this case the advantage of 

 the recuperation furnished by the delay outweighed ^he unfavorable- 

 ness of the conditions which was indicated by the mere fact of a 

 delay. 



Taking the record of each mating as a whole, there is no significant 

 correlation between the size and frequency of litters. 



