Every plant, then, or every animal, whatever its species, however simple or complicated its structure, is in the last analysis either a single cell or a confederated group of cells.

All life, whether it be the life of a single cell or of an unorganized group of cells or of a republic of cells, has as its basis the life of the cell.

For all the animate world, two great principles stand established.  First, that every living organism, plant or animal, big or little, develops from 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.