Every vital activity is manifested at least through chemical and
physical forces. And the elements of the fuel for our engines we
receive through plants from the inorganic world. For the plant, as
we have seen, stores up as potential energy in its compounds the
actual energy of the sun's rays. And thus man lives and thinks by
energy, obtained originally from the sun. But man not only consumes
food and fuel. The complicated protoplasm is continually wearing out
and being replaced. Every cell in our bodies is a centre toward
which particles of material stream to be assimilated and form for a
time a part of the living substance, and then to be cast out again
as dead matter. Our very existence depends upon this continual
change. There is synthesis of simple substances into more complex
compounds, and then analysis of these complex compounds into
simpler, and from this latter process results the energy manifested
in every vital action. We are all whirlpools on the surface of
nature; when the whirling ceases we disappear. Man, like every other
living being, exists in a condition of constant interchange with
surrounding nature; he is rooted in innumerable ways in the
inorganic world.
And because of these close relations the great characteristic of
living beings is the necessity and power of conformity to
environment.
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