a cell, and is itself a composite of cells, and that the
cell is the unit of all life. Secondly, that _the big and complex
organisms have through long ages developed out of simpler forms_, the
organic life of today being the result of an age-long process of
evolution.

What, then, is the cell, and what part has it played in this process of
evolution?

To begin with, a cell is visible only through a microscope. A human
blood cell is about one-three-thousandth of an inch across, while a
bacterial cell may be no more than one-twenty-five-thousandth of an inch
in diameter.

[Sidenote: Characteristics of Living Cells]

Yet, small as it is, the cell exhibits all of the customary phenomena of
independent life; that is to say, it nourishes itself, it grows, it
reproduces its kind, it moves about, and _it feels_. It is a _living,
breathing, feeling, moving, feeding thing_.

The term "cell" suggests a walled-in enclosure. This is because it was
originally supposed that a confining wall or membrane was an invariable
and essential characteristic of cell structure. It is now known,
however, that while such a membrane may exist, as it does in most plant
cells, it may be lacking, as is the case in most animal cells.

The only absolutely essential parts of the cell are the inner _nucleus_
or kernel and the tiny mass of living jelly surrounding it, called the
_protoplasm_.

[Sidenote: The Brain of the Cell]

The most powerful microscopes disclose in this protoplasm a certain
definite structure, a very fine, thread-like network spreading from the
nucleus throughout the semi-fluid albuminous protoplasm. It is certainly
in line with the broad analogies of life, to suppose that in each cell
the nucleus with its network is the brain and nervous system of that
individual cell._

All living organisms consist, then simply of cells. Those consisting of
but one cell are termed unicellular; those comprising more than one cell
are called pluricellular.

The unicellular organism is the unit of life on this earth. Yet tiny and
ultimate as it is, every unicellular organism is possessed of an
independent and "free living" existence.

[Sidenote: Mind Life of One Cell]

To be convinced of this fact, just consider for a moment the scope of
development and range of activities of one of these tiny bodies.

"We see, then," says Haeckel, "that it performs all the essential life
functions which the entire organism accomplishes. Every one of t