106 THE STORY OF THE BIRDS. 
was a hollowed-out place in soil that was not neces- 
sarily sandy, especially if the early birds, as is quite 
likely, laid many eggs. Under this condition a de- 
pression was necessary to hold the eggs well together 
while being incubated. 
Conditions of migrations or change of tempera- 
ture may also have arisen wherein such eggs as had 
something under them were more likely to be hatched 
out, and this sort of natural selection might have in- 
tensified the tendency to seek grassy places and set up 
structure, or at the least shaping of the material at 
hand. We shall see something of a hint of this prog- 
ress in the color of the bird’s egg. 
But it was not until birds began to build in trees 
that much progress was likely made in intricate nest 
structure. Such birds as we now find building neat 
nests on the ground belong to families that are largely 
tree builders, and their ground-nesting habit is evi- 
dently comparatively recent. 
It is probable that even the first nests in trees 
were merely rude platforms as we find yet among dow 
birds that build there. If we except one bird among 
the stork forms (Scopus), few if any elaborate nests 
belong below the perchers and their near relatives. 
Even these complete nests often have the primitive 
platform first, then the cup on that, and lastly the 
lining. : 
That the lining is comparatively recent is shown 
by this sequence and by the fact that it is usually the 
last thing that a bird changes; but the platform may 
often be omitted. Some birds are so concerned about 
