38 HELEN DEAN KING AND HENRY H. DONALDSON 



This same correlation is shown, also, by the data for the 

 gray rats. In males of the first generation variability in 

 body weight was highest at the time that the growth rate 

 was most accelerated (60 to 151 days), and it showed no 

 abrupt drop during early adult life when the rate of growth 

 was decreasing but slowly. In these rats variability in body 

 weight closely followed changes in the growth rate through- 

 out life (table 4). In the females of this generation there 

 was only a slight increase in body-weight variability at the 

 sixty-day period when the rats were growing rapidly, and 

 variability increased during adult life when the growth rate 

 was decreasing. In both sexes of gray rats of the tenth gen- 

 eration the correlation between growth rate and body-weight 

 variability was as marked as in the Albinos. 



Darwin (1875) was of the opinion that all domesticated 

 animals, with rare exceptions, vary far more than animals 

 in a wild state. He states : 



Variability of every kind is probably directly or indirectly caused 

 by changed conditions of life, or put another way, if it were possible 

 to expose all the individuals of a species during many generations to 

 absolutely uniform conditions of life there would be no variability. 



Had the science of genetics been developed during Darwin's 

 time, he would doubtless have modified many of his views, 

 including that of the cause of variability. 



In gray rats that have lived for a number of generations 

 in captivity under fairly uniform environmental conditions 

 variability in body weight has tended to decrease. Such a 

 result was to be expected if, as seems probable, some part of 

 this variability was due to the action of factors inherent in 

 the individuals themselves. 



Our colony of gray rats is, in a sense, an inbred colony, 

 since no new stock has ever been added and all of the rats 

 have descended from six wild females. These rats have not 

 been very closely inbred, however, since it has always been 

 the rule to select litters for weighing and for breeding stock 

 that were the offspring of parents not closely related. Such 

 a system of breeding would, no doubt, gradually lessen the 



