blacksmith wields
a hammer for an hour, he uses up the number of cells necessary to
yield that amount of muscular force. When a girl studies Latin for an
hour, she uses up the number of brain-cells necessary to yield that
amount of intellectual force. As fast as one cell is destroyed,
another is generated. The death of one is followed instantly by the
birth of its successor. This continual process of cellular death and
birth, the income and outgo of cells, that follow each other like the
waves of the sea, each different yet each the same, is metamorphosis
of tissue. This is life. It corresponds very nearly to Bichat's
definition that, "life is organization in action." The finer sense of
Shakspeare dictated a truer definition than the science of the French
physiologist,--

"What's yet in this
That bears the name of life? Yet in this life
Lie hid more thousand deaths."

_Measure for Measure_, Act iii. Scene 1.

No physical or psychical act is possible without this change. It is a
process of continual waste and repair. Subject to its inevitable
power, the organization is continually wasting away and continually
being repaired.

The old notion that our bodies are changed every seven years, science
has long since exploded. "The matter," said Mr. John Goodsir, "of the
organized frame to its minutest parts is in a continual flux." Our
bodies are never the same for any two successive days. The feet that
Mary shall dance with next Christmas Eve will not be the same feet
that bore her triumphantly through the previous Christmas holidays.
The brain that she learns German with to-day does not contain a cell
in its convolutions that was spent in studying French one year ago.
Whether her present feet can dance better or worse than those of a
year ago, and whether her present brain can _do_ more or less German
and French than the one of the year before, depends upon how she has
used her feet and brain during the intervening time, that is, upon the
metamorphosis of her tissue.

From birth to adult age, the cells of muscle, organ, and brain that
are spent in the activities of life, such as digesting, growing,
studying, playing, working, and the like, are replaced by others of
better quality and larger number. At least, such is the case where
metamorphosis is permitted to go on normally. The result is growth and
development. This growing period or formative epoch extends from birth
to the age of twenty or twen