100 HELEN DEAN KING 



and sister matings have not, seemingly, altered the relative 

 variability in the strain, whether the total amount of variability 

 has been influenced by inbreeding cannot be determined until 

 it is possible to rear a number of generations of these animals 

 under uniform conditions of environment and of nutrition. 



GENERAL CONCLUSIONS 



As a whole, this experiment has shown that the closest form 

 of inbreeding possible in mammals, the mating of brother and 

 sister from the same litter, is not necessarily inimical either to 

 body growth, to fertility, or to constitutional vigor, provided 

 that only the best animals from a relatively large number are 

 used for breeding purposes. Selection, seemingly, is able to 

 hold in check any tendency that inbreeding may have to bring 

 out the undesirable, latent traits inherent in the strain. 



In the course of this investigation it has been shown that 

 adverse conditions of environment and of nutrition produce far 

 more detrimental effects on growth and fertility in the albino 

 rat than does inbreeding. These factors, apparently, do not 

 alter the genetic constitution of the individual, since the animals 

 soon resume their normal growth and fertility when environ- 

 mental and nutritive conditions are again favorable. 



The sex ratio in the rat is seemingly a character that is amen- 

 able to selection, since through this process the inbred strain 

 has been separated into two lines: one line (A) showing a high 

 sex ratio, the other line (B) showing a low sex ratio. The effects 

 of selection on the sex ratio seem to be limited, however, since 

 there has been no cumulative effects of the selection, although 

 the two lines have been kept distinct for eighteen successive 

 generations. Whether it will be possible to change the sex 

 ratio in the two lines by reversing the selection is the chief prob- 

 lem in view in the continuation of this work. 



Throughout the entire course of this investigation there has 

 been a great similarity between the two inbred series as regards 

 the variability in the body weights of the animals at different 

 age periods. In the earher generations the variability in body 



