to some sort of effort, but to an effort of
far greater depth than the individual effort, far more independent of
circumstances, an effort common to most representatives of the same
species, inherent in the germs they bear rather than in their substance
alone, an effort thereby assured of being passed on to their
descendants.

* * * * *

So we come back, by a somewhat roundabout way, to the idea we started
from, that of an _original impetus_ of life, passing from one generation
of germs to the following generation of germs through the developed
organisms which bridge the interval between the generations. This
impetus, sustained right along the lines of evolution among which it
gets divided, is the fundamental cause of variations, at least of those
that are regularly passed on, that accumulate and create new species. In
general, when species have begun to diverge from a common stock, they
accentuate their divergence as they progress in their evolution. Yet, in
certain definite points, they may evolve identically; in fact, they must
do so if the hypothesis of a common impetus be accepted. This is just
what we shall have to show now in a more precise way, by the same
example we have chosen, the formation of the eye in molluscs and
vertebrates. The idea of an "original impetus," moreover, will thus be
made clearer.

Two points are equally striking in an organ like the eye: the complexity
of its structure and the simplicity of its function. The eye is composed
of distinct parts, such as the sclerotic, the cornea, the retina, the
crystalline lens, etc. In each of these parts the detail is infinite.
The retina alone comprises three layers of nervous elements--multipolar
cells, bipolar cells, visual cells--each of which has its individuality
and is undoubtedly a very complicated organism: so complicated, indeed,
is the retinal membrane in its intimate structure, that no simple
description can give an adequate idea of it. The mechanism of the eye
is, in short, composed of an infinity of mechanisms, all of extreme
complexity. Yet vision is one simple fact. As soon as the eye opens, the
visual act is effected. Just because the act is simple, the slightest
negligence on the part of nature in the building of the infinitely
complex machine would have made vision impossible. This contrast between
the complexity of the organ and the unity of the function is what gives
us pause.

A mechanistic theory