498 
Bulletin No. 162.—1915. 
hatching of A. The proportion of cases in which A and B hatch 
in the same half-day is less than half the number of cases in which 
A hatches before B; in only a small proportion of cases does B hatch 
before A. The actual numbers are as follows: 
• 
Number 
Proportion 
of cases. 
of total. 
A hatched before B. 
A hatched same half-day 
. 79 
07% 
as B. 
. 32 
27% 
B hatched before A. 
. / 
6% 
Total . 
. 118 
100% 
Attention has already been called to the longer period of incubation 
of A than of B. Leaving out the fractions, A is laid on the average 
44 hours before B, but hatches on the average only 11 hours before B. 
The difference — 33 hours, or almost a day and a half — is the mean 
time by which the incubation period of A exceeds that of B. Two 
possible explanations of this difference between A and B in the time 
from laying to hatching suggest themselves: (1) Egg A may be 
physiologically different from B and require a greater amount of 
incubation, or (2) although there is a longer interval between the 
deposition and hatching of A the birds may not sit on it so closely 
when it is first laid, so that it may really receive no more incubation 
than B. The first of these suppositions is a very unlikely one from 
a biological standpoint, for while the possibility of an intrinsic 
difference between A and B which might influence the time required 
for incubation, such as gross size, relation of size of yolk to amount of 
white, or what not , cannot be denied, there are no data to indicate such 
a relation. As for the second explanation, there are direct obser¬ 
vations in support of it. Nearly every writer on pigeons states that 
the birds do not incubate the first egg, but stand over it until the 
second is laid, when incubation properly begins. Thus going back 
to Moore (1735, p. 11) again, we find him stating that “When a 
