ers may even
occasionally be inherited, then the direct effect of the environment
upon the individual will serve as a decided assistance in our problem.

Here, then, we have before us the factors which have been concerned in
the building of the living machine under nature's hands. Reproduction
keeps in existence a constantly active, unstable, readily modified
organism as a basis upon which to build. Variation offers constantly new
modifications of the type, while heredity insures that the modifications
produced in the machine by the influences which give rise to the
variations shall be permanently fixed.

==Method of Machine Building.==--_Natural Selection._ The method by which
these factors have worked together to build up the living machines is
easily understood in its general aspects, although there are many
details as yet unsolved. The general facts connected with the evolution
of animals are matters of common knowledge. We need do no more than
outline the subject, since it is well understood by all. The basis of
the method is _natural selection_, which acts in this machine building
something as follows:

The law of reproduction, as we have seen, produces new individuals with
extraordinary rapidity, and as a result more individuals are born than
can possibly find sustenance in the world. Hence only a few of the
offspring of any animal or plant can live long enough to produce
offspring in turn. The many must die that the few may live; and there
is, therefore, a constant struggle among the individuals that are born
for food or for room in the world. In this _struggle for existence_ of
course the weakest will go to the wall, while those that are best
adapted for their place in life will be the ones to get food, live, and
reproduce their kind. This is at all events true among the lower
animals, although with mankind the law hardly applies. Now, among the
individuals that are born there will be no two exactly alike, since
variations are universal, many of which are congenital and thus born
with the individual and transmitted by inheritance. Clearly enough those
animals that have a variation which makes them a little better adapted
for the struggle will be the ones to live and hence to produce
offspring, while those without such advantage will be the ones to die.
We may suppose, for example, that some of the individuals had longer
necks than the average. In time of scarcity of food these individuals
would be able to ge