n" is to many men a metaphor that carries many meanings,
and sometimes different meanings to different men. While I heartily agree
with my fellow biologists in ascribing to Darwin himself, and to his work,
the first place in biological philosophy, yet recognition of this claim
should not deter us from a careful analysis of the situation in the light
of work that has been done since Darwin's time.

THE THEORY OF NATURAL SELECTION

In his great book on the _Origin of Species_, Darwin tried to do two
things: first, to show that the evidence bearing on evolution makes that
explanation probable. No such great body of evidence had ever been brought
together before, and it wrought, as we all know, a revolution in our modes
of thinking.

Darwin also set himself the task of showing _how_ evolution might have
taken place. He pointed to the influence of the environment, to the effects
of use and disuse, and to natural selection. It is to the last theory that
his name is especially attached. He appealed to a fact familiar to
everyone, that no two individuals are identical and that some of the
differences that they show are inherited. He argued that those individuals
that are best suited to their environment are the most probable ones to
survive and to leave most offspring. In consequence their descendants
should in time replace through competition the less well-adapted
individuals of the species. This is the process Darwin called natural
selection, and Spencer the survival of the fittest.

Stated in these general terms there is nothing in the theory to which
anyone is likely to take exception. But let us examine the argument more
critically.

[Illustration: FIG. 71. Series of leaves of a tree arranged according to
size. (After de Vries.)]

If we measure, or weigh, or classify any character shown by the individuals
of a population, we find differences. We recognize that some of the
differences are due to the varied experiences that the individuals have
encountered in the course of their lives, i.e. to their environment, but we
also recognize that some of the differences may be due to individuals
having different inheritances--different germ plasms. Some familiar
examples will help to bring home this relation.

If the leaves of a tree are arranged according to size (fig. 71), we find a
continuous series, but there are more leaves of medium size than extremes.
If a lot of beans be sorted out according to their weights, and thos