CHAPTER XVI. 
WHY DID BIRDS BEGIN TO INCUBATE ? 
We have followed the bird’s story thus far by the 
development of changes in its structure slightly and 
in its outer covering more extensively. This latter 
led us on into its emotions (and their reflex effect 
upon structure and color of plumage) and into the 
development of a love for beauty and music and some 
other general esthetic feelings till we have reached 
some of its tendencies toward the unselfish. From this 
standpoint we shall now go again to near the bird’s 
origin, and note its progress away from the reptiles 
through incubation, nest building, egg coloring, young 
rearing, and kindred topics. 
This may be styled another account of the genesis 
of the birds, parallel with and corroborative of the 
first. 
After this a few miscellaneous side lights will 
follow, and our task is done. 
Since all reptiles have eggs, many of which are 
hatched outside of the body, as are those of the birds, 
and since only one quite unbirdlike family of reptiles 
—the large constricting snakes—show any tendency 
96 
