e is the adaptation, where is the pressure of external
circumstances? There is no striking utility in sexual generation; it
has been interpreted in the most diverse ways; and some very acute
enquirers even regard the sexuality of the plant, at least, as a luxury
which nature might have dispensed with.[23] But we do not wish to dwell
on facts so disputed. The ambiguity of the term "adaptation," and the
necessity of transcending both the point of view of mechanical causality
and that of anthropomorphic finality, will stand out more clearly with
simpler examples. At all times the doctrine of finality has laid much
stress on the marvellous structure of the sense-organs, in order to
liken the work of nature to that of an intelligent workman. Now, since
these organs are found, in a rudimentary state, in the lower animals,
and since nature offers us many intermediaries between the pigment-spot
of the simplest organisms and the infinitely complex eye of the
vertebrates, it may just as well be alleged that the result has been
brought about by natural selection perfecting the organ automatically.
In short, if there is a case in which it seems justifiable to invoke
adaptation, it is this particular one. For there may be discussion about
the function and meaning of such a thing as sexual generation, in so far
as it is related to the conditions in which it occurs; but the relation
of the eye to light is obvious, and when we call this relation an
adaptation, we must know what we mean. If, then, we can show, in this
privileged case, the insufficiency of the principles invoked on both
sides, our demonstration will at once have reached a high degree of
generality.

Let us consider the example on which the advocates of finality have
always insisted: the structure of such an organ as the human eye. They
have had no difficulty in showing that in this extremely complicated
apparatus all the elements are marvelously co-ordinated. In order that
vision shall operate, says the author of a well-known book on _Final
Causes_, "the sclerotic membrane must become transparent in one point of
its surface, so as to enable luminous rays to pierce it;... the cornea
must correspond exactly with the opening of the socket;... behind this
transparent opening there must be refracting media;... there must be a
retina[24] at the extremity of the dark chamber;... perpendicular to the
retina there must be an innumerable quantity of transparent cones
permitting on