viduals comprising successive class intervals from 
low to high Maturity Index; that is from stout to 
thin body types? For the first group of rats there 
was a negative exponential indicated for the distri- 
bution; whereas for the second group there was a 
positive exponential. 
I was puzzled as to why these data did not re- 
semble a normal distribution, as most frequently 
happens with data on size. Mr. James U. Casby 
of the Neuropsychiatry Division, of the Army 
Medical Services Graduate School, suggested that 
the absence of an indication of normality might be 
due simply to the fact that the tails of the distribu- 
tion of the B/W ratio were posted with more 
medium type data in my development of a mean 
life span Maturity Index from the basic B/W ratio 
data. Therefore, the B/W ratio was calculated 
for 200 days of age for all those rats which had 
sufficient data to plot growth curves of body and 
weight (see table 62). These data do indicate that 
the B/W ratio at this age resembles a normal 
distribution. 
However, there is little in this table that helps in 
understanding the apparent exponential distribu- 
tion of mean Maturity Indices shown in figure 142. 
I believe that the data in this figure is actually 
more representative of the stresses arising from 
social action than is the more simple data for 200 
days of age. In the first place the former includes 
Maturity Indices for a number of individuals who 
died between 50 and 200 days of age. Such 
individuals were in general the thinner more 
stunted individuals. The former data also is 
modified by the inclusion of B/W ratios in later life 
when individuals subjected to stress lost weight. 
That the entire life span requires evaluation, 
when the B/W ratio is utilized as an index of growth 
and stress, is indicated by the following analysis. 
In table 62 there are 9 males in the first group and 
16 in the second whose B/W ratio at 200 days of 
age ranged between .452 and .529. All of these 
would have been assigned to Maturity Index 
growth channel I. Because of less favorable 
maintenance of weight at other times, both groups 
exhibited mean Maturity Indices less than 1.00. 
Later changes in growth and maintenance of 
weight are shown in table 63 for those males who 
lived into the breeding season of the following year. 
The important aspect of this table is that, whereas 
the later-bom males exhibited as nearly favorable 
TRENDS IN FREQUENCY OF MATURITY 
INDEX WITH TIME OF BIRTH 
Approximate Trends 
Angle of social 
and environmental 
pressure on rate 
of growth 
o° 
MIDPOINT OF MATURITY INDEX 
CLASS INTERVAL 
Figure 142. — Trends in the frequency of Maturity Index 
ratings as they are related to the time of birth, that is to the 
size and complexity of the population. The lower is the 
Maturity Index, the larger is the rat for any given age. 
The change in the slope of the curves from the earlier born 
to the later born rats suggested a manner in which they 
might schematically reflect the total intensity of those 
conditions which inhibit growth. A line drawn perpen- 
dicular from one of these curves and extending to the 
abscissa, will form an angle with it. The greater the angle 
the less the intensity of conditions inhibiting growth. 
growth status at 200 days of age as did the earlier- 
born males, their ability to maintain weight 
declined with the onset of the breeding season. 
All these same later-born males exhibited much 
poorer B/W ratios prior to 200 days of age. The 
number of times records of these males fell into the 
three growth channels were as follows: 
Growth channel 
Time born 
i 
II 
III 
Early born 
17 
5 
0 
Later born 
17 
13 
8 
231 
