of the
living machine by slow steps and through long ages of time. When these
three sources of history so accurately agree with each other, it is as
impossible to disbelieve in the existence of such history as it is to
disbelieve in the existence of the ancient Hittite nation, after its
history has been told to us by two different sources of record.

Now all this is very germane to our subject. We are trying to learn how
this living machine, with its wonderful capabilities, was built. The
history which we have outlined is undoubtedly the history of the
building of this machine, and the knowledge that these complicated
machines have been produced as the result of slow growth is of the
utmost importance to us. This knowledge gives us at the very start some
idea of the nature of the forces which have been at work. It tells us
that in searching for these forces we must look for those which have
been acting constantly. We must look for forces which produce their
effects not by sudden additions to the complication of the machine. They
must be constant forces whose effect at any one time is comparatively
slight, but whose total effect is to increase the complexity of the
machine. They must be forces which produce new types through the
modification of the old ones. We must look for forces which do not adapt
the machine for its future, but only for its present need. Each step in
the history has been a complete animal with its own fully developed
powers. We are not to expect to find forces which planned the perfect
machine from the start, nor forces which were engaged in constructing
parts for future use. Each step in the building of the machine was taken
for the good of the machine at the particular moment, and the forces
which we are to look for must therefore be only such as can adapt the
organisms for its present needs. In other words, nothing has been
produced in this machine for the purpose of being developed later into
something of value, but all parts that have been produced are of value
at the time of their appearance. We must, in short, look for forces
constantly in action and always tending in the same direction of
greater complexity of structure.

Is it possible to discover these forces and comprehend their action?
Before the modern development of evolution this question would
unhesitatingly have been answered in the negative. To-day, under the
influence of the descent theory, stimulated, in the first place, by
Darw