EFFECTS OF USTBEEEDIN'G AISTD CEOSSBREEDTNG. 11 



rhe average age under ordinary conditions seems to be about 3 

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



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

 }0 60 per cent of the matings in a vigorous stock, fertilization takes 

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

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

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

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

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

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

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

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

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

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

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

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

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

 jonditions the average gestation period is slightly shorter than under 

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

 iir even born alive. 



FERTILITY. 



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

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

 loubtless depends in part on whether or not ovulation takes place 

 mmediately after the birth of the preceding litter. Evidence 

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

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

 iaost important factors are associated with the conditions at this 

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

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

 (.verage weight for her age, there is considerably more likelihood 

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

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

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

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

 '(letween 1 and 2 years of age than before or after. Regularity or ir- 

 egularity is not characteristic of particular matings to any marked 

 xtent. The correlation between successive intervals, classified as 

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

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

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

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

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

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



