44 HELEN DEAN KING AND HENRY H. DONALDSON 



age, as the downward trend of the graph indicates. In fe- 

 males of the ninth generation, reproduction began, on the 

 average, two months earlier than in females of the first gen- 

 eration. The onset of reproductive life in females of the last 

 generation, however, was still considerably later than that in 

 albino females, which, cast their first litters, as a rule, when 

 they are about three months of age. It would appear, from 

 the data obtained, either that gray females became more 

 adapted to their changed environment as the generations ad- 

 vanced and so began breeding at or near the time normal for 

 the beginning of reproduction in wild Norways, or, as seems 

 more probable, that life in captivity hastened the beginning 

 of reproductive life, as has been the case in many domesti- 

 cated animals (Buff on, 1832; Darwin, 1875). 



There were marked individual differences in. the time at 

 which the menopause appeared in these rats. Many females 

 ceased to bear young after they had cast two or three litters, 

 although they were seemingly in good physical condition; 

 other females continued to breed until they were nearly two 

 years old. While the total length of the reproductive period 

 varied considerably in different generations, as shown in 

 table 6, the variations showed no definite trend as the genera- 

 tions advanced. The average age of the females when the 

 'menopause appeared is shown by the upper graph in chart 6. 

 The course of this graph, in general, is much like that of the 

 lower graph on the same chart. 



There are, apparently, no definite limits to the reproductive 

 period in female rats. Its length depends, as do growth and 

 many other life processes, on the interaction of a number of 

 different factors. Not only environment, but also the physi- 

 cal condition of the animals, influences reproduction. One fac- 

 tor of paramount importance is the lung infection which fre- 

 quently attacks rats of both sexes after they are a year old. 

 This disease is not immediately fatal, but it soon puts an end 

 to breeding and so materially shortens the reproductive life 

 of its victims. 



