I. LIFE PROCESSES 41 



amount of exercise, etc. Unfortunately, no way has been 

 found, as yet, to determine how much of the variability shown 

 is due to genetic factors and how much of it depends upon 

 environmental, nutritive, and physiological causes. 



With environmental and nutritive conditions as uniform 

 and as favorable as it was possible to make them, with tem- 

 perature changes in the colony relatively small, and the hous- 

 ing conditions such that the rats could obtain considerable 

 exercise, it would seem that factors such as these could have 

 had but little effect on the growth rate and on the variability 

 in body weight of the captive gray rats. Physiological factors 

 acting during prenatal life could not, of course, be controlled, 

 and they undoubtedly were responsible for much of the vari- 

 ability found in the birth weights. The variability shown 

 at all age periods during postnatal life, however, was prob- 

 ably due, in part at least, to differences in the genetic con- 

 stitution of the individuals. 



THE REPRODUCTIVE PERIOD 



Little is known regarding the reproductive life of gray 

 Norways living in their natural habitat. According to Miller 

 ( '11), offspring of wild parents, born and reared in captivity, 

 attain sexual maturity at least by the end of the fourth month. 

 Eaton and Stirrett ('23), in agreement with Lantz ('10), 

 maintain that wild gray rats mature at about three months 

 of age and that breeding continues until the animals are about 

 twenty months old. These investigators, how-\. i. -ive no 

 data in support of their assertions. The youngest of the wild 

 rats used as foundation stock for the captive Cray- in our 

 colony must have been at least six months old before they 

 began to breed. Possibly the changed conditions incident 

 to captivity retarded the onset of puberty and so delay. -.1 

 reproduction. 



In this study the reproductive life of any gray female is 

 considered to have begun with the birth of the first litter for 

 which a record was obtained. Since females of the onrlier 

 generations often destroyed their litters at or soon att.-r 



