EFFECTS OF INBREEDING ON BODY WEIGHT 15 



Tables 4 to 7 are inserted chiefly for reference, although they 

 bring out two important facts more clearly, perhaps, than do any 

 of the other tables. The data, as given in these tables, show that 

 rats belonging to the earlier generations of the inbred series did 

 not live as long, as a rule, as did the individuals in the later gen- 

 erations. This was, particularly noticeable in the individuals of 

 the B series. Up to the twelfth generation only three rats in 

 the B series (two females and one male) lived to the age of 455 

 days; in subsequent generations many individuals lived for the 

 entire weighing period of fifteen months, and some of them 

 were kept until they were nearly two years old. One who be- 

 lieves with Crampe and Ritzema-Bos that continued inbreeding 

 necessarily lessens vitality and so shortens the life of the indi- 

 vidual meets here with the seemingly paradoxical fact that the 

 animals that belonged to the later inbred generations outlived 

 those that belonged to the earlier generations. In this experiment 

 the use of only the most vigorous animals for breeding purposes 

 has seemingly overcome any tendency that inbreeding might 

 have to shorten the life of the individuals. 



The second point of interest brought out by tables 4 to 7 is 

 that in each generation of the two inbred series the average 

 body weight of the males exceeded that of the females at every 

 age for which records were taken. At birth the male albino 

 rat is slightly heavier than the female, whether the animals 

 belong to a stock or to an inbred strain (King, '15 b). Data 

 for the growth in body weight of the albino rat, as recorded by 

 Donaldson ('06), show that as early as the seventh day after 

 birth the growth of the female is more vigorous than that of 

 the male, and that the female is, as a rule, a relatively heavier 

 animal than the male up to about fifty-five days of age. Ferry's 

 ('13) growth data for the albino rat (Donaldson, '15; table 65) 

 confirm Donaldson's findings. In Jackson's ('13) data for the 

 albino rat, ''the excess of average weight was invariably in favor 

 of the male at birth, and also in the majority of cases at all 

 succeeding ages;" while the records obtained by Hoskins ('16) 

 show that the albino female is a heavier animal than the male 

 only at the age of about six weeks. In a series of stock albino 



