continuous variation in a constant
direction. Of course it is unlikely that the eye of the vertebrate and
that of the mollusc have been built up by a series of variations due to
simple chance. Admitting even that light enters into the case as an
instrument of selection, in order to allow only useful variations to
persist, there is no possibility that the play of chance, even thus
supervised from without, should bring about in both cases the same
juxtaposition of elements coordinated in the same way. But it would be
different supposing that light acted directly on the organized matter so
as to change its structure and somehow adapt this structure to its own
form. The resemblance of the two effects would then be explained by the
identity of the cause. The more and more complex eye would be something
like the deeper and deeper imprint of light on a matter which, being
organized, possesses a special aptitude for receiving it.

But can an organic structure be likened to an imprint? We have already
called attention to the ambiguity of the term "adaptation." The gradual
complication of a form which is being better and better adapted to the
mold of outward circumstances is one thing, the increasingly complex
structure of an instrument which derives more and more advantage from
these circumstances is another. In the former case, the matter merely
receives an imprint; in the second, it reacts positively, it solves a
problem. Obviously it is this second sense of the word "adapt" that is
used when one says that the eye has become better and better adapted to
the influence of light. But one passes more or less unconsciously from
this sense to the other, and a purely mechanistic biology will strive to
make the _passive_ adaptation of an inert matter, which submits to the
influence of its environment, mean the same as the _active_ adaptation
of an organism which derives from this influence an advantage it can
appropriate. It must be owned, indeed, that Nature herself appears to
invite our mind to confuse these two kinds of adaptation, for she
usually begins by a passive adaptation where, later on, she will build
up a mechanism for active response. Thus, in the case before us, it is
unquestionable that the first rudiment of the eye is found in the
pigment-spot of the lower organisms; this spot may indeed have been
produced physically, by the mere action of light, and there are a great
number of intermediaries between the simple spot of