_adaptation_, second, the _origination of new parts_, and third, the
_retention of the better invention_.

Are the creatures of the living world so constituted that biological
equivalents of these three essential elements of mechanical evolution can
be found? Are organisms adapted to the circumstances controlling their
lives, and are they capable of changing naturally from generation to
generation, and of transmitting their qualities to their offspring? These
are definite questions that bring us face to face with the fundamental
problems relating to the dynamics or workings of evolution. We need not
ask for or expect to find complete answers, for we know that it is
impossible to obtain them. But we may expect to accomplish our immediate
object, which is to see that evolution is natural. Our attention must be
concentrated upon the three biological subjects of _adaptation_,
_variation,_ and _inheritance_, and we must learn why science describes
them as real organic phenomena and the results of natural causes.

* * * * *

At the very outset, when the general characteristics of living things were
considered, much was said on the subject of adaptation as a universal
phenomenon of nature. It was not contended that perfection is attained by
any living mechanism, but it was held that no place exists in nature for
an organism that is incapable of adjusting itself to the manifold
conditions of life. A _modus vivendi_ must be established and some
satisfactory degree of adaptation must be attained, or else an animal or a
species must perish. With this fundamental point as a basis, we look to
nature for two kinds of natural processes or factors, first, those which
may originate variations as _primary factors_,--the counterparts of human
ingenuity and invention in the case of locomotive evolution,--and the