THE MARKINGS AND SHAPES OF BIRDS’ EGGS. ss ar 
theory that these contrasting markings (such as nearly 
black heavy blotches on a white ground) were in- 
tended to make the egg hard to detect when viewed 
through the branches of trees from above, claiming 
that the light and dark markings imitated light and 
shadow spots caused by the spaces between the 
leaves. It seems more probable, however, that these 
markings are now an inheritance from a past con- 
dition, when they were imitative, and that the bird 
has changed its building habit, and of course can not 
so soon change its physiological habit by laying eggs 
to suit the surroundings. It is well known that the 
standard theory is that the ancestors of our birds here 
in the temperate zone were once resident in regions far 
north or far south of us; that ours is a region that 
has been colonized and not a creative or developing 
center. Now, the bird that once built its nest near 
the pole when that region was tropical may have had 
its eggs marked well in keeping with the lining ma- 
terial there used; but when the ice-cap drove it south 
it could no longer match its nest to its eggs because 
the old material was not at hand. 
Birds, as we have seen, are usually very constant 
in the use of the same lining material when it can be 
obtained, but they are also easily influenced by con- 
venience. A bird which formerly, as it advanced 
from the arctics, lined its nest with grayish fern, 
wool or lichens, may now use flax fiber in Dakota, 
black roots or horsehair in Missouri, and sheep’s wool 
or cotton in Texas. Some such instances are actually 
known to oceur. 
10 
