Sex Ratios in Pigeons. 
509 
15. There is a very sensible positive correlation between the time 
of laying of the first and of the second egg. The equation of the 
regression curve is given, which enables one to calculate the most 
probable time of laying of the second egg when the time the first 
was laid is known (p. 493). 
Time of hatching. 
16. The mean time of hatching of the first egg is 16.5 days 
after the laying of the second (p. 496). 
17. The mean time of hatching of the second egg is 17 days after 
it is laid (p. 496). 
18. On the average, 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 
(p. 499). 
19. There is a high correlation between the times of hatching 
of the two eggs of a clutch (p. 497). 
11 Control of sex.” 
20. So far as the data presented go, they appear to indicate 
that sex in pigeons is determined according to the laws of chance 
(p. 501). 
'Time of sitting. 
21. 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 (p. 504). 
22. The length of time they will continue to incubate the eggs 
varies, but averages practically six days after the normal period, 
making the mean total time of incubation when the eggs do not 
hatch 23 days after the laying of the second egg (p. 504). 
23. This continuance of incubation beyond the normal time under 
such circumstances constitutes a “factor of safety” in the incubating 
instinct (p. 505). 
