100 THE STORY OF THE BIRDS. 
as the result of permanently leaving the shores for re- 
gions where sand for burial was not convenient or the 
sun’s heat was not sufficient, and the egg being laid 
upon the bare ground or rock, incubation was necessary. 
This would naturally result also from migration into a 
cooler region or from a permanent change of climate. 
In the varying amount of incubating heat required 
to hatch eggs of the same size but from different spe- 
cies there is a hint that the habit of sitting has been 
progressive. In tropical regions it is sometimes quite 
difficult to hatch a bird’s egg under a common hen, 
because too much heat is applied, and an explosion 
usually results ; and, as we shall see in the next chap- 
ter, birds of southern origin build cooler nests than 
those from the north. Some birds sit longer than 
others on eggs of nearly the same size. As a rule, the 
time of incubation is roughly in proportion to the size 
of the eggs, and it may vary from seven days in the 
smaller birds to forty in the largest. It is well known 
also that some birds sit much closer than others. 
Likewise the second elutches, where more than one 
brood is laid, hatch a little the quicker often, doubt- 
less because of the greater warmth of the more ad- 
vanced season. 
This varying amount of heat now also has some 
relation to the condition of the nestling when hatched, 
whether downy or naked, but that bears on another 
topic (see Chapter XX). Our space precludes the 
discussion of the incubating habits of the various spe- 
cies, but some of them have been referred to in other 
chapters. Others will come up later, 
