arallel changes in muscles, nerves, and
nerve-centres; though what is true of these is true, in greater or
less degree, of all the other organs.

You may answer that this is to be explained by the law of
correlation of organs; that when changes in one organ demand
corresponding changes in another, these two change similarly and
more or less at the same time and rate. But this is evidently not an
explanation but a restatement of the fact. The question remains,
What makes the organs vary simultaneously so as to always correspond
to each other? The whole series of changes must to some extent be
effected at once and in the same individual, if it is to be
preserved by natural selection. 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.

Let us begin with a simple case of change in the adult body. The
workman grasps his tools day after day, and his hands become horny.
The skin has evidently thickened, somewhat as on the soles of the
feet. This is no mere mechanical result of pressure alone.
Continuous pressure would produce the opposite result. But under the
stimulus of intermitte