f 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. Hence a very common definition of life is the continual
adjustment of internal relations to external relations or
conditions. To a very slight extent man can rise superior to certain
of the ruder elements of his surroundings, but he gains this victory
only by learning and following the laws of the very environment
which he succeeds in subjecting to himself. Indeed his higher
development and finer build bring him into touch with an
indefinitely wider range of surroundings than even the lower animal.
Forces, conditions, and relations which never enter the sphere of
life of lower forms, crowd and press upon him and he cannot escape
them. His higher position, instead of freeing him from dependence
upon environment and subjection to law, makes him thus more
sensitive, as well as more capable of exact conformity to an
environment of almost infinite complexity; and more sure of absolute
ruin, if ignorant, negligent, or disobedient. The words of the
German poet are literally true:

"Nach ehernen, eisernen, grossen Gesetzen,
Muessen wir alle unseres Daseins
Kreise vollenden."

But man is an animal. And the principal characteristic of an animal
is that it eats a certain amount of solid foo