48 HELEN DEAN KING AND HENRY H. DONALDSON 



cord with those for gray females of the later generations, and 

 they doubtless cover the greater part, if not all, of the off- 

 spring produced. 



For the entire series of 417 breeding females in the first 

 nine generations of captive Grays, the average number of 

 litters per female was 3.69, and the deviation from this mean 

 in any generation was relatively slight. The largest number 

 of litters cast by any of the females was ten; the smallest 

 number was one. Conditions of captivity, although affording 

 abundant food and protection from enemies, did not increase 

 the average number of litters cast as the generations 

 advanced. 



It may be assumed, perhaps, that breeding was restricted 

 in these gray females by the fact that the animals were not 

 completely adapted to conditions of captivity, and therefore 

 did not reproduce at the rate normal for the race in its natural 

 habitat. Since no adequate data for wild Grays are available 

 for comparison, definite conclusions regarding this point can- 

 not be drawn. The average number of litters cast by captive 

 gray females was considerably less than that in a large series 

 of stock Albinos, reared under similar environmental condi- 

 tions, where the females produced an average of 5.5 litters 

 (King, '24). 



The estimation of eight to ten young as the litter average 

 in wild gray rats is undoubtedly much too high, if applied to 

 the entire litter output of any single female or of any group 

 of females. In both the gray and the albino race of Norway 

 rats litter size changes with the age of the female, and litters 

 cast at the beginning and at the end of the reproductive period 

 are much smaller than those cast during the height of breed- 

 ing activity. 



For the entire series of 1539 litters in the first ten genera- 

 tions of captive Grays the average size of the litter was 6.17, 

 and the deviation from this mean in any generation was not 

 significant (table 7). This average accords with the finding 

 of 6.3 young per litter in stock Albinos (King, '24). The 

 range in litter size was from one to fourteen, the largest lit- 



