Fortuitous variations here and there
along the line of the series are of little or no avail. That the
whole series of variations should happen to occur in one animal is
altogether against the law of probabilities; if the favorable
variation occurs in only a part of the series it remains useless
until the corresponding variation has taken place in the other
terms. And while the variation is thus awaiting its completion, so
to speak, it is useless, and cannot be fostered by natural
selection.
Evolution by means of fortuitous variations, combined and controlled
only through natural selection, seems to me at least impossible; and
this view is, I think, steadily gaining ground.
Natural selection, while a real and very important factor in
evolution, cannot be its sole and exclusive explanation. It
presupposes other factors, which we as yet but dimly perceive. And
this does not impeach the validity of Mr. Darwin's theory any more
than Newton's theory of gravitation is impeached by the fact that it
offers no explanation as to why the apple falls or how bodies
attract one another.
For natural selection explains the survival, but not the origin, of
the fittest. Given a species or other group composed of more and
less fit individuals and the fittest will survive. How does it come
about that there are any more and less fit individuals? This brings
us to the consideration of the subject of variation.
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