52 HELEN DEAN KING 



has diminished neither the average size nor the variability in 

 size." The reactions of the rat to close inbreeding are slightly 

 different from those of Drosophila. The closest form of inbreed- 

 ing possible, continued for many generations, has not caused a 

 diminution in the average body weight of inbred rats at any age. 

 On the contrary, through the selection of only the largest and 

 most vigorous animals for breeding, inbred animals, especially 

 the males, are superior in body size to the best stock animals 

 reared under similar environmental conditions. In the rat varia- 

 bility in body weight decreases after inbreeding, and in the 

 fifteenth inbred generation the variability was no greater than 

 fraternal variability in the body weights of stock albinos. 



The more general bearing of the results of these inbreeding 

 experiments will be discussed in a following paper dealing with 

 the fertility and constitutional vigor of inbred rats. 



SUMMARY 



1. The present paper gives an analysis of the data for the in- 

 crease in the weight of the body with age for 333 male and for 

 306 female albino rats. These animals belonged to two series 

 (A and B), both descended from the same ancestral stock, that 

 were inbred, brother and sister only, for fifteen successive 

 generations. 



2. The animals of the first six generations suffered from mal- 

 nutrition, and their body weights were much smaller than the 

 norms for stock animals of like age. Many of these animals 

 had defective teeth and the majority of the females were sterile. 

 When nutritive conditions were improved the animals quickly 

 regained their normal body size and the tendency to sterility 

 and to malformation was checked. 



3. The general course of the growth in body weight of inbred 

 rats is similar to that of stock animals as determined by the 

 investigations of Donaldson, Jackson and others. 



4. In both inbred series the average body weights of the males 

 was greater than that of the females at every age at which 

 weighings were taken. The males and females of the B series 



