356 
GENETICS: COLE AND KIRKPATRICK 
second day following. (2) The mean interval between the la3dng of 
the two eggs is practically 44 hours. (3) The mean time between the 
laying of the first and second eggs decreases progressively in the months 
from February to July inclusive. (4) There is a very sensible positive 
correlation (0.38 =t 0.057) between the time of laying of the first and of 
the second egg. The equation of the regression straight line is 3/ = — 0.48 
+ 0.378a:, where x is the time p.m. of laying of the first egg, and y that 
of the second. This enables one to calculate the most probable time 
of laying of the second egg when the time the first was laid is known. 
Time of hatching. — (1) The mean time of hatching of the first egg 
is 16.5 days after the laying of the second. (2) The mean time of 
hatching of the second egg is 17 days after it is laid. (3) On the aver- 
age, therefore, the time from laying to hatching of the first egg is nearly 
a day and a half longer than it is for the second egg. This is probably 
to be accounted for by the fact that the first egg receives very little 
incubation until the second is laid. (4) There is a high correlation be- 
tween the times of hatching of the two' eggs of a clutch. 
Control of sex. — So far as the data obtained go, they appear to indi- 
cate that sex in pigeons is determined according to the laws of chance. 
Time of sitting. — (1) In case the eggs do not hatch they are seldom 
abandoned at the end of the normal period of incubation, but the birds 
continue to sit on them for a time longer. (2) The length of time they 
will continue to incubate the eggs varies, but averages practically six 
days beyond the normal period, making the mean total time of incub- 
ation, when the eggs do not hatch, 23 days after the laying of the second 
6gg- (^) This continuance of incubation beyond the normal time under 
such circumstances constitutes a 'factor of safety' in the incubating 
instinct. (4) A pair of Mourning Doves continued to sit on substituted 
eggs for four days after their own had hatched in an incubator, thus 
disproving Raspial's assertion that wild birds have an 'exact notion of 
the time required for the eggs to hatch.' 
^ Summary of the results of a report bearing the same title and published as Bulletin 
No. 162 of the Rhode Island Agricultural Experiment Station. 
2 GROUP A. Hatching period. Embryos which were fully developed but which failed 
to hatch ('dead in shell') and birds which died within five days after hatching. 
GROUP B. Nestling period. Squabs dying at from 6 to 28 days, inclusive. 
GROUP C. Immature or juvenile period. Squabs living over 28 days but dying before 
six months of age. 
GROUP D. Adult period. All birds living to the age of six months or more. 
