EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 351 



In the A series of inbreds the range in litter size was from one to 

 seventeen. The Htter of one was cast b}^ a female of the nine- 

 teenth generation that was suffering from pnemnonia and had to 

 be killed three days after parturition. This is undoubtedly a 

 case where the phj^sical condition of the female prevented the 

 normal development of all of the embryos except one; the other 

 embryos probably became atrophic and were absorbed. The 

 litter containing seventeen members occurred in the fifteenth 

 generation. All of the individuals were born alive, but they were 

 all very small, weighing not more than three grams each: the 

 average weight of the albino rat at birth is about four grams 

 (King, '15b). 



In the B series, as table 8 shows, the range of variation in litter 

 size was not as great as that in the A series: no litters smaller 

 than two or larger than fifteen were obtained. In both series 

 litters containing seven j^'oung were the most frequent, while those 

 with eight young were only slightly less in number. 



Figure 2 shows graphs for litter frequencies in the two series 

 of inbreds that were constructed from the data given in table 8. 



In figure 2 each graph rises quickly to the modal point at seven, 

 falls slowly at first and then rapidly. The drop in graph A at 

 the point of 6 has apparently no significance, since there is no 

 similar drop in the B graph. Each graph is a simple frequency 

 curve with one modal point, and is exactly the sort of graph that 

 one would expect to obtain from the data for litter frequencies in 

 a large series of animals belonging to a pure race. 



C. Puberty 



Under normal conditions puberty tends to appear at approxi- 

 mately the same age in the different individuals of a given race, 

 but the time of its appearance is seemingly more dependent on 

 the growth changes incident to age than it is on age itself. 



In the albino rat both males and females attain sexual maturity 

 when they are about two months of age (Donaldson, '15), but the 

 environmental and nutritive factors that hasten or retard growth 

 have considerable influence on the reproductive activity of the 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 26, NO . 2 



