362 HELEN DEAN KING 



authors ('15) state: ''Fully half of our stock rats have died before 

 the age of 600 days." Unfortunately, the mortality data given 

 by Osborne, et al. are not in a form which makes it possible to 

 compare them directly with the records for these inbred rats. 

 It would seem, however, from the results as given, that their 

 animals tended to live longer than the rats in my inbred strain. 

 The mortality data for the series of stock Albinos, given in table 

 12, can be directly compared v/ith those for the inbred rats given 

 in the same table, since both series of animals were reared under 

 similar environmental conditions and the records were taken at 

 the same age intervals. Relatively more of the stock than of the 

 inbred males w^ere living at six, nine, and twelve months of age, 

 but only 28 per cent of the stock males lived to be fifteen months 

 old, while 37 per cent of the inbred males attained this age. 

 The records for the female groups show that relatively as many 

 inbred as stock females lived to the age of six months, but that 

 more of the stock than of the inbreds were living at nine, twelve, 

 and fifteen months of age. Taken as a whole, therefore, lon- 

 gevity in the inbred strain seemed to be somewhat less than that 

 in the stock controls. 



Some of the inbreeding data for animals which Darwin ('75) 

 collected' were so at variance with his own results on plants that 

 he was forced to admit that; "manifest evil does not usually 

 follow from pairing the nearest relations for two, three, or even 

 four generations." In a long-continued series of inbreeding 

 experiments, therefore, the deleterious effects of inbreeding 

 would supposedly be more accentuated in the later than in the 

 earlier generations. A comparison between the mortality rec- 

 ords for stock animals and those for the inbred group comprising 

 the animals in the nineteenth to the twenty-third generation 

 should show the effects of inbreeding on longevity much better 

 than the comparison between the groups as previously made. 

 Such a procedure is the more justifiable, perhaps, because these 

 two groups of animals were reared in the colony simultaneously. 

 While in the two male groups only about 1 per cent of the animals 

 failed to reach the age of six months, relatively more of the inbred 

 than of the stock males were living at all other age periods noted: 



