202 



1M1KUITANOE, FERTILITY, AND SKX IN 1'IGEONS. 



tluration of incubation one meets with the difficulty that the incubation for the 

 second egg is not the same as that for the first. All doves have the habit of "half- 

 sitting" on the first egg during the first day; sometimes they will even leave it 

 uncovered during the first night. As a rule the parents stand over the egg with 

 feathers just touching it, keeping it a little warm, but not as thoroughly warm as 

 if the bird sat closely. After the second egg is laid doves begin to sit steadily and 

 closely, leaving it rarely and for only very short periods. 



The incubation time for the first egg in the ring-dove (Streptopelia risoria) is 15 

 days, plus or minus a few hours; for the second egg the period is 14 days minus, or, 

 in rare cases, plus a few hours. The incubation periods of the common dove and 

 the ring-dove differ therefore by about 3 days. What happens if we take a male 

 with a longer incubation period and a female with a shorter period? Do the eggs 

 hatch earlier or later? My first impressions in regard to this matter were that the 

 egg always developed according to its normal rate, no matter what male united 

 with the female; the egg seemed to follow only its own regular period. But in order 

 to decide the matter the value of a few hours of difference must be made certain 

 and the various necessary conditions and crosses considered. I have looked at the 

 various difficulties, and it has seemed to me necessary not only to follow up the 

 common doves and ring-doves, making sure of the normal period of incubation of 

 the two species, but also to follow up very closely the crossed birds, noting the 

 nature and constancy of incubation. It is also further necessary to make the crosses 

 in both directions, and if possible at exactly the same season. I have had thus far 

 but two cases 3 from the crossing with the male ring-dove, so that there my obser- 

 vations can not be considered conclusive; they do seem, however, to be of interest. 



TABLE 179. Incubation lime of eggs of ring-dove fertilized by common pigeons. 



1 This egg was opened; it would have hatched next morning. 



In the cross of the ring-dove female and the common dove male, the incubation 

 period of the first egg is 15 days plus a number of hours; for the second egg it is 14 

 days plus a number of hours (sec tables 179, 180, 181, and 182). This rather closely 

 approaches the normal, the normal being 15 days plus or minus for the first and 14 

 days minus or plus for the second (see tables 183 and 185). The result all hangs on 



One further case was later added to these; see table 184. EDITOR. 



