I. LITE PROCESSES 49 



ters, as a rule, being cast by females that were from ten to 

 twelve months old. 



The trend in litter size as the generations of gray rats ad- 

 vanced is shown graphically in chart 8. The graph was con- 

 structed from the data for average litter size given in table 7. 



The highest point of the graph in chart 8 is at its beginning, 

 since the litters cast by wild females were, on the average, 

 somewhat larger than those cast by females in the later gener- 

 ations, probably because wild females did not begin breeding 

 until they were relatively old. At the second generation the 



Gnerotion 



I 23456789 10 



Chart 8 Showing the average size of the litters in different generations of 

 captive Grays. 



graph drops to the level that it maintains, with slight fluctua- 

 tions, until its end. 



From the data given above, it is evident that litter produc- 

 tion and average litter size in gray rats had not changed ap- 

 preciably at the end of ten generations of life in captivity. 

 The average number of litters cast by gray females was 

 smaller than the norm for stock albino females, but litter 

 size, on the average, was about the same in the two races. 



STERILITY 



In commenting on the fact that many wild animals, when 

 brought into captivity, are completely barren or produce but 

 few offspring, Darwin (1875) states that the reproductive 



MEMOIR 14 



