EXPERIMENTAL EVOLUTION. 



135 



"Mice were inoculated in ten age-groups: 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 

 or over days old, respectively. In calculating the amount of growth observed, 

 a single observation is the unit employed. 



"A short table will show the interesting difference between the albino stock 

 mice (N) and those of the back-cross (B. C.) generation. The sign + denoted 

 an observation showing a mass, the sign — one which is negative (table 11). 



T.^BLE 11. — Comparison of stock and back-cross mice. 



"The difference between the races is certainly significant, being almost 

 seven times its probable error. 



"If the mice in each series are di\'ided into a younger (2 to 10 days old) 

 and an older (12 to 20+ days old) group, according to their ages at inocula- 

 tion, an interesting fact is brought out (table 12). 



T-^JBLE 12. — Comparison of younger and older groups. 



"The differences between the two age-groups are in the opposite direction 

 in the two series. When the data are analyzed according to sex, another point 

 is brought out, namelj^, the fact that the females present certainly significant 

 differences between the age-groups, while the males do not (table 13) . 



Table 13. — Comparison of sexes and age-groups. 



"The suggested explanation is that the females of the series in the older 

 age-group reach sexual maturity earlier than the males, and in many cases at 

 least do so dming the period in which they are under observation. That the 

 ovary increases its activity as an organ of internal secretion at the onset of 

 sexual maturity is well known and affords an assurance that a more detailed 

 differentiation of the tissues is possible. The true physiological nature of the 

 individual expresses itself and the genetic factors, determining in the stock 

 mice elimination of the tumor and in certain of the back-cross mice its growth 

 come into full activity. 



