duce delay in conceptions. Genetic variability 
of physiology, which is independent of the environ- 
ment and which produces delays in capability of 
conceiving, cannot be ruled out. However, the 
possibility of genetic variability alone causing a 
year’s difference in ability to conceive appears 
remote. 
The second reproductive statistic is the propor- 
tion of females pregnant. I chose data on 2,202 
Georgia rats (67) for this purpose. These rats 
were smaller than the Baltimore rats. The median 
length was 11 mm less, and a pregnancy rate of 
rate of 0.01 was reached at a 10 mm. smaller size. 
Therefore, I have assumed that the ages of these 
rats are equivalent to that of the next higher 
length class interval of the Baltimore rats. The 
pregnancy rates listed in column 5 of table 68 were 
taken from a smoothed curve drawn through these 
age adjusted values. 
Based upon the Towson study duration of 
pregnancy is 23 days and the minimum time 
between parturition and a following conception, 
when the first litter is successfully reared, is 28 days. 
Therefore, 51 days represents the minimim span 
between conceptions of successfully reared litters. 
I shall assume that no conceptions occur on the 
first post partum day if the litter survives, although 
in fact the first-day post partum conceptions do 
occur rather rarely. Pregnant individuals can be 
recognized by external examination of the exposed 
uterus after the first 6 days following insemination 
(22), that is during the last 17 days of pregnancy. 
Thus, we have several reproductive character- 
istics whose typical values may be considered as 
“knowns”. They are: 
r= Proportion of females parous by macro- 
scopic examination. Column 4, table 
68. These are the females which are 
either visibly pregnant or have placental 
scars on the uterus from prior pregnan- 
cies. It must be pointed out that the 
observed r is an underestimate because 
primiparous females cannot be recog- 
nized as pregnant during the first 6 of 
their 23 days of pregnancy. This 
amounts to an underestimate of 26 per- 
cent in the 74-day-old age class. This 
error rapidly diminishes in older rats 
for whom parity can be judged in the 
majority of cases by placental scars 
alone. For the present purpose of gain- 
ing insight into the maturation of repro- 
ductive performance this error has been 
ignored. 
p— Proportion of females in entire sample, 
including parous and nonparous rats, 
that are pregnant by macroscopic 
examination. 
q=Duration of pregnancy, 23 days. 
< 2 =The number of days, 17, of the 23 days 
of pregnancy during which pregnancy 
can be determined by macroscopic 
examination of the uterus. 
t 3 = Normal minimal number of days, 28, 
of lactation for successful rearing of 
litters before another conception can 
occur. 
/ 4 = Normal minimal span, 51 days, between 
successfully reared litters. Here the 
apparently rare event among wild rats 
of conception on the day of parturition 
is ignored. 
Let: 
T= Duration of time, 365 days, for which 
the incidence of reproductive events 
will be calculated. 
S— Average span in days between con- 
ceptions of parous females. 
7i = Incidence of pregnancies among parous 
rats only. This refers to the number of 
pregnancies per 365 days for particu- 
lar age classes. It is an instantaneous 
rate. 
h= Incidence among parous rats only of 
pregnancies where the litters are weaned 
successfully. 
P\ = Proportion of conceptions by the r 
parous females where the litters are 
weaned successfully. In other words 
this P\ defines success at rearing litters. 
7*2 = Proportion of females in the entire 
population who are characterized by 
the mature complex of behavior and 
physiology which enables them not 
only to conceive, but also to give birth 
and rear their young to the age of 
weaning. 
Then : 
h!S=Tht proportion of time parous rats are 
detectably pregnant by macroscopic 
examination of the uterus. 
Since : A sample of N rats at a particular mean 
age is equivalent to N days at that age. 
267 
