868 
J. FRANK DANIEL 
TABLE 2 
EXAMPI-E 
DESIGNATION OF 
FEMALE 
NUMBER OF YOUNG 
SUCKLED 
PERIOD OF GESTATION 
NUMBER BORN 
1 
H 
3 
22 days 
5 
2 
L 
3 
25 days 
5 
3 
H 
5 
25 days 
1 
4 
J 
5 
24 days 14 hours 
7 
5 
M 
6 
24 days 
6 
6 
M 
6 
24 days 19 hours 
11 
7 
J 
8 
27 days 
7 
8 
F 
8 
27 days 15 hours 
8 
9 
F 
10 
30 days 
9 
10 
M 
10 
30 days 3 hours 
9 
Indeed, the variation in such cases has been found to follow 
so definite an order that by knowing the time of copulation and the 
number suckled, I have been able to predict within a day or so 
when the litter would be born. This, as seen in the table, can 
be done by adding to an initial period of 20 days a number equal 
to the number suckled. The number resulting, barring certain 
irregularities, which seem to be individual in nature, is usually 
slightly greater than the number of days in the period of gesta- 
tion. Thus, 20 + 5, the number suckled by J, will give her 
period of gestation in case 4, and 20 + 8 will give her period in 
case 7. 
In concluding the discussion of this variability, a law may be 
formulated which for practical purposes is as follows: 
The period of gestation, in lactating mothers, varies directly with 
the number of young suckled. 
The cause, and time of occurrence, of the lengthening in the 
period of gestation is a problem in itself, a more complete investi- 
gation of which I shall reserve for a future discussion, but a word 
concerning it may not be out of place at this time. 
If it could be shown, for example, that ovulation were in any 
way retarded, an effective explanation of the cause of lengthened 
period would be forthcoming. In this case, however, the length- 
ening would be previous to the true period of gestation. But 
