ly the light directed in the line of their axes to reach
the nervous membrane,"[25] etc. etc. In reply, the advocate of final
causes has been invited to assume the evolutionist hypothesis.
Everything is marvelous, indeed, if one consider an eye like ours, in
which thousands of elements are coordinated in a single function. But
take the function at its origin, in the Infusorian, where it is reduced
to the mere impressionability (almost purely chemical) of a pigment-spot
to light: this function, possibly only an accidental fact in the
beginning, may have brought about a slight complication of the organ,
which again induced an improvement of the function. It may have done
this either directly, through some unknown mechanism, or indirectly,
merely through the effect of the advantages it brought to the living
being and the hold it thus offered to natural selection. Thus the
progressive formation of an eye as well contrived as ours would be
explained by an almost infinite number of actions and reactions between
the function and the organ, without the intervention of other than
mechanical causes.

The question is hard to decide, indeed, when put directly between the
function and the organ, as is done in the doctrine of finality, as also
mechanism itself does. For organ and function are terms of different
nature, and each conditions the other so closely that it is impossible
to say _a priori_ whether in expressing their relation we should begin
with the first, as does mechanism, or with the second, as finalism
requires. But the discussion would take an entirely different turn, we
think, if we began by comparing together two terms of the same nature,
an organ with an organ, instead of an organ with its function. In this
case, it would be possible to proceed little by little to a solution
more and more plausible, and there would be the more chance of a
successful issue the more resolutely we assumed the evolutionist
hypothesis.

Let us place side by side the eye of a vertebrate and that of a mollusc
such as the common Pecten. We find the same essential parts in each,
composed of analogous elements. The eye of the Pecten presents a retina,
a cornea, a lens of cellular structure like our own. There is even that
peculiar inversion of retinal elements which is not met with, in
general, in the retina of the invertebrates. Now, the origin of molluscs
may be a debated question, but, whatever opinion we hold, all are agreed
that mollus