126 THE STORY OF THE BIRDS. 
the oviduct, and hence a greater rate of egg deposition 
might occur. The result, from whatever standpoint 
considered, is a smaller egg, which produces a prema- 
ture hatching and much subsequent provision of food 
by the parent while the young is yet helpless. Another 
little fact bears on our hypothesis. In precocial birds 
the ege tooth (or limelike “ pip” on the tip of the beak 
by which, in hatching, the shell is broken) is shed in a 
few days after they are out; but in the altricials it 
persists much longer, showing that the period during 
which its use was normal was once perhaps much 
longer or later after incubation first began than now. 
So likewise the naked and helpless condition hints the 
same, since the hotbed-hatched megapode (brush tur- 
key), as we have seen, is hatched not only with the 
ability to run, but passes the downy state within the 
egg and is able at once to fly feebly also. 
What then could have brought about this decrease 
in the relative size of the egg or change in proportion 
of white to yolk if such exists? It is a difficult ques- 
tion, since progress of the bird seems to imply a less- 
ening of the relative size of the egg and a degener- 
ation of the nestling. It is noticed in a general way 
(of course with exceptions) that the relative size of a 
bird’s egg varies with the number in the clutch or 
with the capacity of the bird to cover many. Often 
perhaps it varies with both, or one condition may 
limit the other. The largest egg (for the size of the 
parent) is that of the Apteryx perhaps, and it is single. 
The number of eggs in a clutch appears to de- 
pend—other things being equal—upon the danger 

