her models, adding to and changing as experience would seem to
dictate! She has developed her higher and more complex forms as man
has developed his printing-press, or steam-engine, from rude,
simple beginnings. From the two-chambered heart of the fish she made
the treble-chambered heart of the frog, and then the four-
chambered heart of the mammal. The first mammary gland had no
nipples; the milk oozed out and was licked off by the young. The
nipple was a great improvement, as was the power of suckling in the
young.

Experimenting and experimenting endlessly, taking a forward step
only when compelled by necessity,--this is the way of
Nature,--experimenting with eyes, with ears, with teeth, with limbs,
with feet, with toes, with wings, with bladders and lungs, with
scales and armors, hitting upon the backbone only after long trials
with other forms, hitting upon the movable eye only after long ages
of other eyes, hitting on the mammal only after long ages of
egg-laying vertebrates, hitting on the placenta only
recently,--experimenting all around the circle, discarding and
inventing, taking ages to perfect the nervous system, ages and ages
to develop the centralized ganglia, the brain. First life was like a
rabble, a mob, without thought or head, then slowly organization
went on, as it were, from family to clan, from clan to tribe, from
tribe to nation, or centralized government--the brain of man--all
parts duly subordinated and directed,--millions of cells organized
and working on different functions to one grand end,--cooperation,
fraternization, division of labor, altruism, etc.

The cell was the first invention; it is the unit of life,--a speck
of protoplasm with a nucleus. To educate this cell till it could