I. LIFE PROCESSES 39 



variability in body weight due to the action of genetic factors, 

 as the individuals would tend to become more homozygous 

 as the generations advanced. Close inbreeding does not 

 reduce variability in body weight very rapidly, as is shown 

 by the fact that albino rats, inbred brother and sister from 

 the same litter for twenty-five consecutive generations, 

 showed at the end of this period a variability in body weight 

 at different age periods that was but little less than that in 

 outbred albino controls. In these inbred Albinos, however, 

 changed conditions of nutrition undoubtedly acted to increase 

 the variability in body weight in individuals of the later gen- 

 erations (King, '18, '19). 



In a given litter of rats the birth weights of members of 

 the same sex often show considerable diversity, and they may 

 differ by more than 100 per cent. Individuals in the litter 

 tend to maintain the order relation in their weights at birth 

 throughout life, as Dunn ( '08) determined for albino rats and 

 as records for a large number of litters in different series 

 of rats in our colony show. There are, of course, exceptions 

 to this rule, but they seem to be due, in most cases, to a change 

 in the physical condition of the individuals that is caused 

 by disease or by an excessive accumulation of fat. When rats 

 are very small at birth, their growth seems to be inhibited 

 from the beginning of postnatal life by causes that are con- 

 stitutional. This is shown in the case of the so-called 'runts' 

 that are found frequently in litters of rats. In these indi- 

 viduals growth continues at a slow rate for some time, but 

 no matter how much care is given, they never exhibit normal 

 vigor, nor do they ever attain the body size normal for a 

 given age (King, '16). 



Female rats of the same age, living under similar condi- 

 tions of environment and of nutrition and breeding at the 

 same season of the year, may cast litters of like size in which 

 the individuals of the same sex show marked differences in 

 their birth weights. On comparing the later growth of such 

 individuals, it has been found that the weight differences at 

 birth are indicative of unlike growth capacities, and that the 



