in order to more successfully incubate their eggs, saying 

 "the bare spaces of the body of a bird are adaptive * * * 

 (the) belly (is) bare in most birds because of incubation, 

 and in ducks, penguins and auks becomes bare during incu- 

 bation." Pycraft draws attention to a further step in this 

 process, and says that the bare abdominal area becomes 

 seemingly inflamed, with its blood vessels more distinct than 

 normal, a condition not likely to be a true inflammation, 

 but a functional hyperaemia only, resulting in a more plenti- 

 ful and more frequently renewed blood supply to the parts, 

 in the end maintaining more easily the optimum incubation 

 temperature. 



Before entering upon a larger consideration of bird 

 temperatures and their relation to the length of incubation, 

 it seems desirable to consider briefly a few facts concerning 

 the effects of heat applied to viable eggs. 



There is a small amount of development in a fertile egg 

 before it leaves the body of the parent, but it is very slight, 

 and never reaches any advanced stage as is the case with 

 many near relatives of birds, the reptiles, with which ani- 

 mals incubation of the egg frequently goes on to completion 

 within the body of the female, and the young are born alive, 

 resulting in the fact that while many reptiles are ovovi- 

 parous, birds are never anything but oviparous. The slight 

 development in an egg which starts and goes on while it is 

 still within the female's body continues for a while at a 

 very slow rate after the egg is extruded, even under com- 

 paratively low temperatures, viz., 86 F. (38). Prolonged 

 or excessive chilling or over-heating promptly kills the 

 embryo. And both of these effects are persistently avoided 

 by incubating parents; thus ducks, etc., cover their eggs 

 with down or feathers when leaving the nest in cold weather, 

 and the ostrich shades its eggs with its body and wings from 

 the sun's excessive heat (160). 



It is demonstrated that there is an optimum incubation 

 temperature, and it is almost demonstrated that each species 

 of domesticated bird has an optimum of its own, an opti- 

 mum which hatches the eggs in the shortest time possible, 

 and which probably varies little, if at all, with each species. 



Nevertheless, it is extraordinary how, for example, a 

 hen's egg will hatch under what seem anything but optimum 

 conditions; thus, such eggs have been known to hatch in a 

 barn-yard manure pile (33). 



H. Milne-Edwards gives the optimum (without desig- 

 nating for what species) as 104 F. Various other figures 

 have been given for this incubation optimum, a conflict 

 arising, most likely, because the optimum differs with differ- 

 ing species. 



47 



