92 HELEN DEAN KING 



cause a decrease in body size, as Darwin (75, 78), Crampe ('83), 

 Ritzema-Bos ('93, '94), and others have asserted. Inadequate 

 nutrition, seemingly, is far more detrimental to body growth than 

 is close inbreeding, even when continued over many generations. 



VARIABILITY IN THE BODY WEIGHTS OF INBRED RATS 



At the end of fifteen generations of brother and sis,ter matings 

 the rats in the inbred strain were over 96 per cent homozygous, 

 according to the calculations of Fish ('14). Animals of the later 

 generations, which had attained a degree of homozygosity prob- 

 ably greater than that ever before reached by any group of labora- 

 tory mammals, might be expected, perhaps, to show a very 

 great uniformity in body weight at different age periods, if the 

 body weight increase with age in the rat is entirely dependent 

 on the action of genetic growth factors. But just as the rate 

 and extent of body growth in this animal seems to be largely a 

 matter of environment and of nutrition, so also the variations 

 in body weights at different age periods are apparently greatly 

 influenced by these conditions. As it is impossible, at present, 

 to distinguish the variability due to environmental and nutritive 

 action from that resulting from a difference in the genetic factors 

 for body growth, one can only calculate the total amount of 

 variabiUty in given groups of animals and then, by comparison, 

 determine the relative variabihty of the groups. No very defi- 

 nite conclusions can be drawn regarding the effects of close 

 inbreeding on the variability in the body weight of the rat until 

 the animals can be kept under environmental and nutritive 

 conditions that are so uniform that their effect is practically 

 constant and therefore neghgible. 



In order to obtain some idea regarding the relative extent of 

 variability in the body weights of the animals in various genera- 

 tions of the inbred strain, coefficients of variability, with their 

 probable error, were determined for the body weights of the 

 individuals in the sixteenth to the twenty-fifth generations of 

 each of the two inbred series and for the weights of the animals 

 in the two series combined (A, B). These coefficients, with 



