EFFECTS OF INBREEDING AND CROSSBREEDING. 11 



The average age under ordinary conditions seems to be about 3 

 months, bringing the first litter between 5 and 6 months. 



The birth of a litter is followed at once by an oestrus period. In 50 

 to 60 per cent of the matings in a vigorous stock, fertilization takes 

 place at this time, and one litter follows another after an interval of 

 65 to 74 days. If fertilization does not take place, there is a period 

 of about 17 days before the next oestrus, and recurrence thereafter at 

 about this period. The average interval from one litter to another, 

 if there is no delay, is about 69 days (68. 93 ± 0.04 in 1,332 cases among 

 inbreds and controls in which the interval was between 65 and 74 

 days) . The true gestation period would of course be slightly shorter. 

 The gestation period is subject to much variation, its standard devia- 

 tion, judging by that of the intervals between litters, being almost 

 two days (1.91 ±0.03 in the above data). The most important cause 

 of variation is the size of litter. Large litters are born earlier than 

 small ones. The correlation between size of litter and interval in 

 the data mentioned above was —0.457 ±0.015. Under unfavorable 

 conditions the average gestation period is slightly shorter than under 

 favorable conditions. Young born before 65 days are seldom raised, 

 or even born alive. 



FERTILITY. 



The number of litters produced per year depends in the main on 

 whether many of the litters succeed one another without delay, which 

 doubtless depends in part on whether or not ovulation takes place 

 immediately after the birth of the preceding litter. Evidence 

 which will be presented later, however, shows that the sire is more apt 

 to be responsible than the dam for irregularity in this respect. The 

 most important factors are associated with the conditions at this 

 time. If the preceding litter is small, if environmental conditions are 

 improving (as in April and May as a rule) , or if the female is above the 

 average weight for her age, there is considerably more likelihood 

 that a second litter will start on its career at once than if, for example, 

 a large litter is born in December leaving the dam much under 

 weight. The age of the dam, at least up to 3 years, does not seem to 

 be an important factor. There is, however, a trifle more regularity 

 between 1 and 2 years of age than before or after. Regularity or ir- 

 regularity is not characteristic of particular matings to any marked 

 extent. The correlation between successive intervals, classified as 

 more or less than 77 days, came out virtually zero in the control 

 stock ( — 0.01 ±0.03). This excludes both heredity and condition of 

 health over long periods of time as important factors. Other results 

 show the small importance of heredity in particular cases, the corre- 

 lations between parent and offspring matings, in litters per year, being 

 insignificant in both controls and inbreds. On the other hand, as we 



