1010] ANIMAL PRODTTOTIOlSr. 865 



last few generations, since table scraps became scarce and of low grade and 

 no satisfactory substitute was found. 



The animals produced in the first six generations were so few that practically 

 all that lived were bred from. Beginning with the seventh, however, it was 

 possible t(» inaugurate a system of selection which is described as follows: 

 " In the A series of inbreds, which is called the ' male line,' all litters contain- 

 ing an excess of female young were always discarded ; in the B series, the 



* female line,' litters with an excess of male young were never reared. Unless 

 the individuals in the litter were of normal size and vigorous at birth they 

 were killed at once." Additional eliminations took place at the age of one 

 month and at sexual maturity (3 months), all animals not meeting the re- 

 quirements of normal weight, as given by Donaldson (E. S. R., 40, p. 546), 

 and normal vigor being discarded. 



The subtitles of the individual papers and the main topics treated therein 

 follow : 



I. The effects in inhyeedvng on the yyoicth and variability in the body weight 

 of the albino rat. — This paper summarizes (1) the adult body weights of 177 

 male and 137 female rats of generations 1 to 6, and (2) the systematic monthly 

 weighings of 15G males and 169 females of generations 7 to 15 during the first 

 15 months of their lives. For this second set of measurements from 8 to 5 

 litters of each series were selected for study in each generation, the litters 

 chosen being of medium size (5 to 8 young) and composed of individuals 

 normal in size at birth. " There was no culling of the less desirable individuals, 

 however, and all members of evei-y litter were reared and weighed at the ages 

 noted." 



Inbred males belonging in generations 7 to 15 were heavier at all ages than 

 stock albinos. In the adult state they were on the average, 18 per cent heavier 

 than the general run of stock albinos and about 12 per cent heavier than males 

 from a selected stock series reared under tlie same environmental conditions. 

 Inbred females were, as a rule, slightly heavier at any given age than the fe- 

 males of the control series, but the difference between the two groups was much 

 less marked. At the age of one year the average body weight of the inbred 

 females was 3.7 per cent greater than that of the stock females. Some males 

 of the seventh to ninth generation were the largest specimens of the albino 

 rat on record. 



• The animals of the B series were somewhat heavier at all ages than those of 

 corresponding sex in the A series. In both series the variability of the body 

 weights — as measured by the coefficient of variation — was lower in the later 

 than in the earlier generations. 



II. The effects of inbreeding on the fertiliiy and on the constitutional vigor 

 of the albino rat. — Litter size, age at sexual maturity, fertility, mortality, and 

 behavior are the topics considered in this paper. 



The average number of young in a litter in the A series (1,752 litters) was 

 7.5 and in the B series (1,656 litters) 7.4. The average in a stock of nonin- 

 bred albinos (424 litters) kept under the same environmental conditions was 6.7. 

 These averages include the first four litters cast by each female or a lesser 

 number in those cases where four were not secured. There were some changes 

 in litter size in the inbred series as the experiment progressed. The average 

 was 6.8 for generations 1 to 6, 7.2 for generations 7 to 10, 7.5 for generations 

 11 to 14, 7.7 for generations 15 to 18 and 19 to 22, and 7.4 for generations 23 

 to 25. Inbred females in giving birth to their first litters produced 6.9 young 

 on the average. The second litters averaged 7.9, the third 7.8, and the fourth 

 7.3. The averages for the successive litters of the stock females were 6.2, 

 7.3, 6.5, and 6.7 young, respectively. 



