So many of the objections which are still made to the theory of
evolution, and especially to that branch of it which deals with living
organisms, rest upon a misconception of what it professes to explain,
and even of what any theory can possibly explain, that a few words on
its nature and limits seem to be necessary.
Evolution, as a general principle, implies that all things in the
universe, as we see them, have arisen from other things which
preceded them by a process of modification, under the action of those
all-pervading but mysterious agencies known to us as “natural forces,”
or, more generally, “the laws of nature.” More particularly the term
evolution implies that the process is an “unrolling,” or “unfolding,”
derived probably from the way in which leaves and flowers are usually
rolled up or crumpled up in the bud and grow into their perfect form
by unrolling or unfolding. Insects in the pupa and vertebrates in the
embryo exhibit a somewhat similar condition of folding, and the word
is therefore very applicable to an extensive range of phenomena; but
it must not be taken as universally applicable, since in the material
world there are other modes of orderly change under natural laws to
which the terms development or evolution are equally applicable. The
“continuity” of physical phenomena, as illustrated by the late Sir
William Grove in 1866, has the same general meaning, but evolution
implies more than mere continuity or succession—something like growth
or definite change from form to form under the action of unchangeable
laws.
The point to be especially noted here is, that evolution, even if it
is essentially a true and complete theory of the universe, can only
explain the existing conditions of nature by showing that it has been
derived from some pre-existing condition through the action of known
forces and laws. It may also show the high probability of a similar
derivation from a still earlier condition; but the further back we
go the more uncertain must be our conclusions, while we can never
make any real approach to the absolute beginnings of things. Herbert
Spencer, and many other thinkers before him, have shown that if we
try to realize the absolute nature of the simplest phenomena, we are
inevitably landed either in a contradiction or in some unthinkable
proposition. Thus, suppose we ask, Is matter infinitely divisible, or
is it not? If we say it is, we cannot think it out, since all infinity,
however it may be stated in words, is really unthinkable.
If we say there is a limit—the ultimate atom—then, as all size is
comparative, we can imagine a being to whom this atom seems as large
as an apple or even a house does to us; and we then find it quite
unthinkable that this mass of matter should be in its nature absolutely
indivisible even by an infinite force. It follows that all explanations
of phenomena can only be partial explanations. They can inform us of
the last change or the last series of changes which brought about the
actual conditions now existing, and they can often enable us to predict
future changes to a limited extent; but both the infinite past and the
remote future are alike beyond our powers. Yet the explanations that
the theory of evolution gives us are none the less real and none the
less important, especially when we compare its teachings with the wild
guesses or the total ignorance of the thinkers of earlier ages.