INCUBATION. 
267 
quarters from the foot to the knee. In all the above varieties of this 
species, the conformation of the bill and legs, in shape and length, is a 
strong proof that they are the same : another strong characteristic dis- 
tinction, is the back toe, which is long, and a continuation of the heel, 
or plant of the foot. 
The Ibis is adopted as a part of the arms of the town of Liverpool, 
and formerly, if not at present, stood conspicuous upon the Guildhall 
in truly golden array. This is termed a liver, from which that flou- 
rishing town derived its name, and is now standing on the spot where 
the pool was, on the verge of which the liver was killed. * 
ICE-BIRD. — A name for the Rotche or little Auk {Uria Alle, 
Temminck.) 
ICELAND GULL. — * Temminck and Captain Sabine give this as 
a variety of the Silvery Gull (Larus argentatus ) ; Dr. Fleming makes it 
a distinct species (X. Islandicus.^ 
IMBER DIVER. — A name for the Loon. 
INCUBATION. — It is probable birds are endowed with an in- 
stinctive power of regulating the necessary heat for this purpose ; of 
course, should the heat of the air, together with the natural warmth of 
the body, on the close contact of the bird to the eggs, be too great, her 
feelings would dictate the necessity of leaving them for a time to cool. 
At the early period of incubation birds quit their eggs more frequently 
than at the time the foetus is more perfect. Yet, in the advanced state, 
the embryo young is not in more danger of being destroyed, if so 
much ; for we have frequently found a \Wm^ foetus in an egg that has 
been taken from the nest two days. If, however, the young is within 
a few hours of being excluded, and the egg is suifered to be some time 
cold, it either dies, or becomes so weak, as not to be able to extricate 
itself from the shell. Various degrees of heat will enlarge the embryo 
young, but regular heat seems necessary to its production ; and yet 
artificial heat, regulated by the brooding of a bird, will not produce 
young with such certainty. In Egypt, a vast quantity of eggs are 
hatched by artificial heat in stoves. It is probable, however, one third 
or one fourth miscarry. The necessary heat for this purpose is about 
ninety-six degrees of Fahrenheit’s thermometer, or thirty-two of Reau- 
mur’s scale. Birds frequently turn and change the situation of their 
eggs in the nest ; besides this, it is possible the moisture of the bird’s 
body may assist the natural growth of the foetus and production of the 
young. 
The male birds of some species supply the place of the female on the 
nest ; but then it is of short duration, and rarely, if ever, when the eggs 
