iations, nor even in the theory that
assigns definite directions to the evolution of the various organs by a
kind of mechanical composition of the external with the internal forces.
So we come to the only one of the present forms of evolution which
remains for us to mention, viz., neo-Lamarckism.

* * * * *

It is well known that Lamarck attributed to the living being the power
of varying by use or disuse of its organs, and also of passing on the
variation so acquired to its descendants. A certain number of biologists
hold a doctrine of this kind to-day. The variation that results in a new
species is not, they believe, merely an accidental variation inherent in
the germ itself, nor is it governed by a determinism _sui generis_ which
develops definite characters in a definite direction, apart from every
consideration of utility. It springs from the very effort of the living
being to adapt itself to the circumstances of its existence. The effort
may indeed be only the mechanical exercise of certain organs,
mechanically elicited by the pressure of external circumstances. But it
may also imply consciousness and will, and it is in this sense that it
appears to be understood by one of the most eminent representatives of
the doctrine, the American naturalist Cope.[39] Neo-Lamarckism is
therefore, of all the later forms of evolutionism, the only one capable
of admitting an internal and psychological principle of development,
although it is not bound to do so. And it is also the only evolutionism
that seems to us to account for the building up of identical complex
organs on independent lines of development. For it is quite conceivable
that the same effort to turn the same circumstances to good account
might have the same result, especially if the problem put by the
circumstances is such as to admit of only one solution. But the question
remains, whether the term "effort" must not then be taken in a deeper
sense, a sense even more psychological than any neo-Lamarckian supposes.

For a mere variation of size is one thing, and a change of form is
another. That an organ can be strengthened and grow by exercise, nobody
will deny. But it is a long way from that to the progressive development
of an eye like that of the molluscs and of the vertebrates. If this
development be ascribed to the influence of light, long continued but
passively received, we fall back on the theory we have just criticized.
If,