compounds of high complexity. In doing this they
use the energy of sunlight, which they then store away in the compounds
formed. They thus produce starches, oils, proteids, woods, etc., and
these stores of energy now may be used by artificial machines. The
living machine builds up, other machines pull down. The living machine
stores sunlight in complex compounds, other machines take it out and use
it. The living organism is therefore to be compared to a sun engine,
which obtains its energy directly from the sun, rather than to the
ordinary engine. While this does not in the slightest militate against
the idea of the living body as a machine, it does indicate that it is a
machine of quite a different character from any other, and has powers
possessed by no other machine. _Living machines alone increase the
amount of chemical compounds of high complexity._

We must notice, however, that this power of construction in distinction
from destruction, is possessed only by one special class of living
machines. _Green plants_ alone can thus increase the store of organic
compounds in the world. All colourless plants and all animals, on the
other hand, live by destroying these compounds and using the energy thus
liberated; in this respect being more like ordinary artificial machines.
The animal does indeed perform certain constructive operations,
manufacturing complex material out of simpler bodies; as, for example,
making fats out of starches. But in this operation it destroys a large
amount of organic material to furnish the energy for the construction,
so that the total result is a degradation of chemical compounds rather
than a construction. Constructive processes, which increase the amount
of high compounds in nature, are confined to the living machine, and
indeed to one special form of it, viz., the green plant. This
constructive power radically separates the living from other machines;
for while constructive processes are possible to the chemist, and while
engines making use of sunlight are possible, the living machine is the
only machine that increases the amount of high chemical compounds in the
world.

==The Vital Factor.==--With all this explanation of life processes it can
not fail to be apparent that we have not really reached the centre of
the problem. We have explained many secondary processes, but the primary
ones are still unsolved. In studying digestion we reach an understanding
of everything until we come to the