occupied by Birds in Incubation. 55 



celerated or retarded without injuring the embryo is an 

 interesting question, which I have not, so far, been able to 

 investigate ; but I imagine that, as regards the great majority 

 of species, it is probably confined within comparatively narrow 

 limits, say from one to two days either way. It is but 

 natural to suppose that the limits of variation in this respect 

 will be, to some extent at least, proportional to the whole 

 period of incubation, so that in the case of species having a 

 very long period — the Swan or the Ostrich, for instance — 

 the variation in either direction may be expected to amount 

 to several days. Newly laid Hens' eggs hatched some hours, 

 or even a day, before those kept, say, a week. 



Another point well worthy of investigation is the length 

 of time eggs at different stages of incubation may be exposed 

 to an insufficient temperature without destroying their vi- 

 tality. Out of curiosity, more than anything else, I had a 

 number of Sparrows', Blackbirds', Rooks', and Lapwings' 

 eggs taken from the nests at different stages of development, 

 and placed in the incubator after the lapse of various intervals 

 up to fourteen or fifteen hours, and was surprised to find 

 how many of them produced young birds in due course. The 

 experiments were not sufficiently accurate or extensive to 

 afford a basis for any precise conclusions, but in a general 

 way they seem to point to this — that the eggs are least suscep- 

 tible to harm before the blood-vessels have been formed and 

 circulation set in ; that the most critical time is immediately 

 after circulation has been established ; and that thereafter 

 the power to resist the effects of exposure gradually increases 

 as incubation progresses. I have placed in the incubator, 

 after haviug carried tliem in my pocket for over twelve hours, 

 eggs that had not been sat on more than a couple of days, 

 and also others in which incubation was far advanced, and 

 have found them hatch as if they had never been disturbed. 



The literature of this subject is very scattered and frag- 

 mentary. The most comprehensive tabular statement I 

 know of is that given by Tiedemann in his ' Anatomic und 

 Naturgeschichte der Vogel ' (vol. ii. pp. 136-139), published 

 so long ago as 1814. For the knowledge of this work, as 

 well as of those of Thienemann, I am indebted to Professor 



