128 THE STORY: OF THE BIRDS. 
rather) may have laid all these eggs at a single sitting. 
At least there is the possibility of such a thing; and 
this faculty is largely developed in tree - haunting 
birds. Many ground builders, however, as our Bob- 
white, lay great numbers of eggs at one sitting as 
often as three times a year. 
But the mere habit of tree building or other safe 
nesting habits may of itself be sufficient to account 
for the altricial condition, except perhaps the naked- 
ness. Those nestlings that stayed longest in the nest 
would be most likely to be reared; hence heredity and 
selection would tend to maintain the altricial condition 
when once started. Few things now are so destruc- 
tive of little birds as their premature escape from 
the nest. 
Likewise the nakedness may have come about 
gradually without the influence of the small egg or 
premature hatching. It is well known that many 
nestlings that are long helpless in the nest are well 
covered with down, as young hawks, owls, herons, 
ete., and that this downy state, as noted in Chapter 
VI, disappears almost exclusively as soon as (coming 
up the line of the bird’s development) the hole-build- 
ing, Picarian group is reached (see diagram, Chapter 
XXX). It does not seem improbable, therefore, that 
the down has been lost by disuse, because in holes 
there is no need for it. It is especially noticeable 
that the young of hole builders in this group are the 
nudest known. Since the higher birds (perchers) 
come out of the region of these white-egged, hole- 
homing ancestors, the naked nestlings in them are 
