only loosely related to the lengths of incubation, and both 

 are effects of another single underlying character. 



The more consideration I have given to this phase of 

 our problem, the more incredible it seems to me that condi- 

 tions as variable and elastic as the size and weight of eggs 

 liave been proven to be, can have much, if any, influence on 

 the true duration of incubation, an epoch in the individual's 

 life of the utmost importance to it and its race. 



Age of the Egg 



Any effect that the age of an egg may have on its in- 

 cubation duration is probably correlated with the length of 

 viability of such egg. by which term the writer would have 

 understood the possibility of the embryo's remaining alive 

 without the egg being incubated. Viability undoubtedly 

 varies with the species, and too, it must be related to the 

 number of eggs in a set, and to the time interval between 

 the laying of the eggs in such a set. It is self-evident that, 

 often, with the ruffed grouse, the first eggs must remain 

 viable at least fourteen days. It appears from Crandall's 

 observations (94) on an emu in captivity that this particular 

 bird laid six eggs at intervals of five days each ; if this were 

 to occur in nature and all the eggs were to hatch it would 

 show an amazingly tenacious viability for this species. Pick- 

 nell (160) states that in captivity the ostrich lays twelve 

 to sixteen eggs during a period of thirty days, and at the 

 end of that time begins to incubate, which is another ex- 

 ample of long viability. 



I believe that eggs of the lower birds will be found to 

 remain viable a much longer period than those of the higher 

 birds ; I doubt very much that a robin's or a hummingbird's 

 egg would remain viable, even with extraordinary care, three 

 or four weeks after being laid. The grounds for this belief 

 are that long viability is probably a reptilian character, 

 and is related to a particular phase of a bird's physiology, to 

 be discussed later on. It is highly desirable to have this 

 question of the duration of viability of birds' eggs thor- 

 oughly investigated. All that I could find in literature on 

 this question was that hens' eggs remained viable up to the 

 end of the eighth or ninth day, and those of pheasants, up 

 to the fifteenth or twenty-first day. Inside of the limits of 

 viability, the age of an egg (at least a hen's egg) unques- 

 tionably seems to influence the apparent length of incuba- 

 tion, for if fresh and old eggs be placed together in an 

 incubator (or under a hen, it is said) (162), the fresher 

 eggs hatch first. It also appears that the nearer the egg is 

 to its limit of viability when placed under incubation con- 

 ditions, the longer it takes to hatch. 



Heinroth (162) says that some ducks sit upon their 

 eggs at night during severe cold, to protect them until the 



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