314 Cc! CO. LITTLE 
higher percentage of growth among observations on the rela- 
tively younger mice. The back-cross generation (B.C.) shows 
the opposite relationships. ‘The small numbers of observations 
in the fourteen- to sixteen-day classes in the B.C. series vitiate 
somewhat the value of the percentage of 43.1 obtained there. 
If, however, the data in table 4 are divided into two general 
age groups comprising the upper and lower halves, from twenty 
Sot 
So 
30 
(AS; 
20 
Weeks AFTER y) 3 4 5 z 
INOCULATION 
Chart 1 
plus to twelve and from ten to two days, respectively, we shall 
find a fair basis for comparison and some most interesting facts. 
For example, in the common stock the lower age group averages 
12.87+0.6 per cent observations showing growth, while the 
higher group shows 9.49+0.6 per cent. The difference between 
them is 3.38+0.85, or about four times its probable error. 
There is then in the common (N) series apparently a real differ- 
ence due to, or correlated with, age at inoculation. 
