there be a law which controls the length of incubation, or 

 at least to detect indications of such a law, or to point out 

 lines of investigation which give promise of being helpful 

 in reaching a final solution of the problem. 



Definitions 



In this discussion the term incubation is held to mean 

 the period of time during which heat is applied more or 

 less continuously to a set of eggs, a period varying within 

 a wide range according to the species; by "incubation 

 period" is meant the whole time so involved, regardless of 

 its duration; and by "length of incubation" is meant the 

 number of days or weeks necessary to completely hatch the 

 young. 



The records of incubation as given in the literature on 

 the subject embrace two varieties of lengths: (A) the true 

 length (or specific length), and (B) the false length (or 

 apparent length) ; the first, or true length, being the mini- 

 mum number of days, under opti?num conditions, necessary 

 to hatch a normal bird, while the second is the true length, 

 plus or minus the time added to, or subtracted from, it by 

 errors in observation, or through errors caused by the dif- 

 ferent types of hatching, types to be defined later on, or 

 plus the time added to it by such conditions as retard or 

 temporarily suspend embryonic development. With birds 

 which lay a considerable number of eggs in each set, and 

 only begin to incubate when the set is completed, it is not 

 difficult to fix the beginning of the incubation period, as 

 with eiders (137) ; when, however, several eggs are laid in 

 a set, and the female warms the first eggs more or less while 

 the other eggs of the set are being laid, it is impossible to 

 say exactly when the period of incubation begins, and the 

 incubation duration has to be determined for each egg by 

 marking it when laid. 



It is almost impossible at times to decide when the 

 parent actually begins steadily to apply the heat necessary 

 to successful hatching. It is also extremely difficult to esti- 

 mate how much heating the first eggs receive while a whole 

 set is being laid, a fact necessitating one's defining the dif- 

 ferent types of hatching, so as to keep in mind the effects 

 of a parent's partially heating the first laid eggs. It has 

 been shown (92) that the domesticated pigeon's average in- 

 cubation period is seventeen days, the eggs (two in a set) 

 being laid on alternate days; the second egg usually hatches 

 in (almost) exactly seventeen days, while the first takes 

 eighteen and one-half days, measured from the day it is 

 laid. If it be assumed that the real incubation begins with 

 the laying of the second egg, it becomes manifest that the 

 first egg receives the equivalent of one-half a day incuba- 



