WHY TWO KINDS OF NESTLINGS? 125 
room within the shell or upon the presence of the 
necessary food for further growth, or upon both. 
Possibly the latter may be merely a lessening of the 
proportion of the white of the egg to the yolk, since 
it is well known that precocial birds have a surplus 

English land rails—a typical precocial family—young solid colored, 
adults striped. 
of yolk at hatching, which is drawn directly within 
the body to sustain the bird till it can provide for 
itself ; but all that of the altricial bird is used up 
while yet within the shell. Since the white is used 
up first, it may be a matter of decrease in the white, 
especially since it is known that the size of the yolk is 
complete before the white begins to form. All this 
would imply a more rapid transit of the egg through 
