LIFE PROCESSES IX CAPTIVE GRAY RATS 61 



vigorous than other rats in the F 2 generation, that many of 

 them were reared and crossed with progeny from other wild 

 rats. Selection of the best litters in each generation to propa- 

 gat'e the race tended to preserve the descendants of female 1 

 and her mate, and at the ninth generation they comprised 

 nearly 88 per cent of all individuals in the strain. On tracing 

 the pedigrees of the various mutant types that appeared, it 

 was found that in each case the line of descent went back, 

 unbroken on the maternal side, to the first pair of wild rats. 

 Our present concept of the mechanism of inheritance does not 

 permit of an assumption that this pair of apparently normal 

 gray rats could have carried genes for all the recessive color 

 mutations found. The dominant curly gene would have mani- 

 fested its presence in the FX generation if it had been carried 

 either by female 1 or by her mate. Considering the fact that, 

 aside from the hooded rats, captive grays bred true to type 

 until the eleventh generation, I am of the opinion that these 

 various mutations had their origin in gene changes that 

 occurred after the strain had been in captivity for several 

 years. 



Recent work in genetics has shown that gene mutations can 

 be induced by x-rays, radium and other media, but as yet the 

 agencies are unknown that cause such mutations in animals 

 living under natural conditions, or in those maintained in 

 laboratories under fairly uniform conditions of environment 

 and nutrition. The suggestion, advanced years ago, that age 

 changes in the variability of the germplasm may have an 

 important role in the production of such mutations did not 

 receive much support until Little ('33) cited various series 

 of data indicating that there is an age-variability relation not 

 only in the reactions of somatic tissues, but in the germ cells 

 as well. Thus young and old mice exhibit an increased toler- 

 ance of tumor transplants as compared with that of young 

 adults (Little, '20; Strong, '22). Variability in the litter size 

 of inbred mice, as shown by coefficients of variation, changes 

 with the age of the mother, being very high among the early 

 litters cast, decreasing to a base level and subsequently rising 

 in litters produced toward the end of the reproductive period. 



