This is the universal and essential thing that a living creature
does, because it is the means by which life continues. Higher organisms
undergo a cycle of birth, sexual reproduction, and death. Many lower
organisms propagate by fissioning, budding, or subdividing in some
way, and experience individual death only by accident. Viruses reproduce,
but only with the aid of other kinds of organisms. All living things
propagate in some way, and life goes on.
This continuation of life in the family of organisms or in the
individual is different from a static enduring. Rocks and minerals
endure, and the material within them remains unchanged. A living
creature, in contrast, maintains the same form amid a continuous
exchange of molecules with its surroundings. Its individual molecules
come and go, but its structure and organization persist. It maintains
its identity in the midst of a constant flow-through of matter.
Living creatures generally increase in size and complexity with
the passage of time. They go through a controlled, predictable life
cycle or pattern. This pattern of development is not a product only
of simple physical forces (as is the "healing" of a broken
crystal), but of programmed, prestored information contained in
DNA molecules. The proper analogy is not with a bubbling pot or
a growing crystal, but with a programmed digital computer, although
the computer analogy is grossly insulting to even the simplest bacterium.
Living organisms take chemical substances and free energy from
their environment and modify both for their own particular needs.
These processes involve chemical transformations: both spontaneous
breakdowns that release free energy, and nonspontaneous syntheses
that must be driven by some other free-energy source. For the analogy
between cell growth and crystal growth to be valid, one would have
to propose that a crystal of calcium carbonate (limestone), if dropped
into a calcium chloride solution, could grow by ignoring the chloride
ions around it and taking C02 from the atmosphere to
make carbonate ions.