_Look where you will among living things, plant or animal, you will find that all are mere assemblages of cellular tissues._ Extend your investigation further, and examine into forms of life so minute that they can be seen only with the most powerful microscope and you will come upon a _whole universe of tiny creatures consisting of a single cell_. [Sidenote: The Unit of Life] Indeed, it is a demonstrable fact that these tiny units of life consisting of but a single cell are far more numerous than the forms of life visible to the naked eye. You will have some idea of their size and number when we tell you that millions may live and die and reproduce their kind in a single thimbleful of earth. _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._