12 



INHERITANCE IN GUINEA-PIGS. 



cates that laboratory conditions of close captivity are not as favorable 

 for full growth and vigor as the freer life and better air of the original 

 habitat. The 4 wild-caught females produced 33 young in 14 litters, an 

 average of 2.36 young to a litter. Their daughters or granddaughters, 

 reared in captivity, when of like age, have produced 27 young in 13 

 litters, an average of 2.07 young to a litter. 



Too much emphasis must not be laid, however, on this difference, 

 because productiveness depends largely on food, care, and weather con- 

 ditions, and it is not certain that these were equally favorable for the 

 original females and for their descendants, respectively. 



Table 1 shows for each mother how many litters of young she has 

 borne, at what age she bore them, and how many young were contained 

 in each litter. In the case of the 4 females caught wild, the age given 

 for the mother is of course not known ; the age recorded is an estimate 

 based on the size of the mother when captured. 



Table 3 shows in what month each litter of young was born and what 

 its size was. This table brings out rather strikingly the effect of the 

 seasons and consequent character of food available upon the size and 

 number of the litters. 



TABLE 4. Variation in period of gestation (interval since previous litter) 

 in Cavia culleri. Average, 63.3 days. 



In the 6 months from July to December inclusive, litters were born 

 which were conceived under summer conditions, with an abundance of 

 green food available. It will be observed that in this half of the year 

 the litters are numerous (38) and large (average 2.05 young to a litter). 

 The young born in the 6 months January to June inclusive were con- 

 ceived under winter conditions, when the mothers were subsisting 

 largely on a diet of dried or concentrated foods, with a limited amount 

 of green food available. In this half of the year the litters are less 

 numerous (15) and smaller (average size 1.47 young). 



Temperature probably does not directly affect the result, as the 

 animals were kept in a heated house, but purity of the air may possibly 

 do so, as the house is much better ventilated in the warmer months. 

 But food is probably the most important factor, as the condition of the 

 animals changes promptly with change of food, even when other condi- 

 tions show no change. 



