turbance associated with handling would ensue, 
as in fact it did. Females less frequently ate their 
young, and cages could be opened without rats 
running about, hiding, or jumping out. Social 
stratification continued through succeeding gen- 
erations, but one would assume that social strife was 
reduced among cage mates, although King makes 
no precise statements in this connection. The only 
change in litter size was toward a slight increase 
in the frequency of litters under four or over seven 
[table 9, (63)]. Therefore, using only litters of 
size 4 to 7 for breeding did not reduce variability, 
nor was average size of the litter significantly 
altered. Therefore, it will be my contention that 
all changes observed by King and Donaldson 
were byproducts of selection for greater tolerance 
to disturbance. 
The major changes were as follows: 
1. Reduced sterility. Percent sterility for gen- 
erations 2 to 9: 70, 37, 30, 19, 11, 9, 6, 19. 
In all later generations all females were 
fertile. 
2. There was a gradual reduction in the frequency 
with which suckling young were killed by 
being eaten by the mother, or by being 
suffocated by other adult associates who lost 
their trait of crowding into the nest. 
3. From generations 1 through 10 there was a 
progressive decrease in size of the thyroid but 
no change in gonads, hypophysis, or adrenals. 
4. There was no change (at least for generations 
1 to 10) of mean length of either sex at 
approximately 600 days of age. 
5. For any given length the rat became fatter. 
6. There was a progressive increase in average 
weight and a progressive decrease in varia- 
bility in weight at any age. 
7. The average length of the reproductive span 
of females increased from 1 90 days in the 2d 
generation to 440 days in the 25th. 
8. There was a reduced mortality between 12 
and 20 months of age. 
King ( 63 ) concludes tentatively that these 
changes in weight were due to heredity rather than 
to environment. Although she never states this 
precisely, she implies at several points that there 
had been selections for genes which favor growth 
to a larger size, much as corn and many domestic 
animals have been selected for larger size by using 
the larger individuals of each generation for breed- 
ing stock. Since there was no evidence from the 
method of selecting breeding stock that larger 
animals were used, it is more logical to assume that 
selection was for temperament or emotionality, 
particularly toward a phlegmatic hypothyroid 
type. This assumption appears adequate for ex- 
plaining the increase in average weight, the 
decrease in variability in weight, reduction in 
sterility, and prolongation of the span of reproduc- 
tion. 
The maximum weight achieved by any rat in a 
sample may be assumed to represent the optimum 
results of both heredity and environment. Six- 
hundred and fifty grams for males was attained by 
King’s rats, mine housed in the 100-foot square pen, 
as well as feral rats in Baltimore ( 23 ). This sug- 
gests that all three groups possessed quite similar 
heredity in relation to size. Extremes of weight 
for any age are more revealing than are means. 
Such data for my rats and King’s are shown as 
log- log plots in figures 148 and 149. Straight 
lines were fitted by eye to those sections of the data 
having close approximations to a straight line. 
Certain conclusions may be drawn from these 
figures. 
MAXIMUM AND MINIMUM WEIGHTS 
30 100 300 600 
AGE IN DAYS 
Figure 148. — Maximum and minimum weights of feral (Cal- 
houn’s) and captive (King’s) male Norway rats. Here 
maximum and minimum refer to the actual extremes at 
each age. 
263 
