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.
