tice that
the machines have been built up step by step, one feature being added to
another with the slowly progressing ages. Thirdly, we notice that in one
respect this construction of the living machine by nature's processes
has been different from our ordinary method of building machines. Our
method of building puts the parts gradually into place in such a way
that until the machine is finished it is incapable of performing its
functions. The half-built engine is as useless and as powerless as so
much crude iron. Its power of action only appears after the last part is
fitted into place and the machine finished. But nature's process in
machine building is different. Every step in the process, so far as we
can trace it at least, has produced a complete machine. So far back as
we can follow this history we find that at every point the machine was
so complete as to be always endowed with motion and life activity.
Nature's method has been to take simpler types of machines and slowly
change them into more complicated ones without at any moment impairing
their vigour. It is something as if the steam engine of Watt should be
slowly changed by adding piece after piece until there was finally
produced the modern quadruple expansion engine, but all this change
being made upon the original engine without once stopping its motion.

[Illustration: FIG. 45. A group of cells resulting from division,
representing the first step in machine making.]

This gradual construction of the living machines has been called
_Organic Evolution_, or the _Theory of Descent_. It will be necessary
for us, in order to comprehend the problem which we have before us, to
briefly outline the course of this evolution. Our starting point in this
history must be the cell, for such is the earliest and simplest form of
living thing of which we have any trace. This cell is, of course,
already a machine, and we must presently return to the problem of its
origin. At present we will assume this cell as a starting point endowed
with its fundamental vital powers. It was sensitive, it could feel,
grow, and reproduce itself. From such a simple machine, thus endowed,
the history has been something as follows: In reproducing itself this
machine, as we have already seen, simply divided itself into two halves,
each like the other. At first all the parts thus arising separated from
each other and remained independent. But so long as this habit continued
there could be litt