1. The Appropriative Period
2. The Adaptive Period
3. The Creative Period

These eras overlap, and the human race, or rather its vanguard,
civilized man, may be passing into the third stage in one field of human
endeavor while still lingering in the second or first in some other
respect. But in any particular line this sequence is followed. The
primitive man picks up whatever he can find available for his use. His
successor in the next stage of culture shapes and develops this crude
instrument until it becomes more suitable for his purpose. But in the
course of time man often finds that he can make something new which is
better than anything in nature or naturally produced. The savage
discovers. The barbarian improves. The civilized man invents. The first
finds. The second fashions. The third fabricates.

The primitive man was a troglodyte. He sought shelter in any cave or
crevice that he could find. Later he dug it out to make it more roomy
and piled up stones at the entrance to keep out the wild beasts. This
artificial barricade, this false façade, was gradually extended and
solidified until finally man could build a cave for himself anywhere in
the open field from stones he quarried out of the hill. But man was not
content with such materials and now puts up a building which may be
composed of steel, brick, terra cotta, glass, concrete and plaster, none
of which materials are to be found in nature.

The untutored savage might cross a stream astride a floating tree trunk.
By and by it occurred to him to sit inside the log instead of on it, so
he hollowed it out with fire or flint. Later, much later, he constructed