348 HELEN DEAN KING 



muiXL effect of malnutrition in lowering the fertility of the females 

 was reached at the third generations. 



Cyclic changes in productiveness were noted by Castle et al. 

 ('06) in an inbred strain of Drosophila, in which, for three suc- 

 cessive years, there was a gradual rise in fertility followed by an 

 abrupt decline. These changes in productiveness were likewise 

 ascribed to the variations in temperature at different seasons of 

 the year. 



The data given in table 1 to table 6 and the graph in figure 1 

 show clearly that, despite all theories to the contrary, it is possible 

 to maintain a high degree of fertility in a mammal for at least 

 twenty-five generations of the closest possible form of inbreeding, 

 by a careful selection of breeding stock and by keeping the ani- 

 mals under enviromnental conditions that are favorable for their 

 growth and reproduction. 



While, in general, the size of the litter varies according to the 

 age of the mother, individual females differ greatly regarding the 

 number of offspring that they produce in an}^ litter of the litter 

 series. Sisters from the same litter, mated to the same male, 

 will show marked variations in their fertility at the same age. 

 One female may never have a litter that contains more than five 

 young; the other may always throw litters in which there are 

 nine or more young. Some females, regardless of their age, tend 

 to cast the same number of offspring in every litter. One female 

 so noted had ten young in each of her four litters. Marked indi- 

 vidual differences in fertility are also found among female 

 guinea-pigs, according to Minot ('92). 



The average number of young in a litter of albino rats is 6.3, 

 according to the data for 394 litters collected by Crampe ('84) ; 

 Cuenot ("99) found an average of 8.5 young in the 30 litters that 

 he examined. Records for 1089 litters of stock Albinos born in 

 the Wistar Institute animal colony during the years 1911 to 1914 

 give 7.0 as the average number of young per litter (King and 

 Stotsenburg, '15). When this last series of data was collected 

 it was not realized that litter size in the rat depends to such a 

 marked degree upon the age of the mother, and that in this species 

 the maximum fertility comes at a relatively early age, as it does 



