rom the
first sufficient material to act upon. It being of advantage to a lowly
creature that it should distinguish with more and more delicacy, or with
more and more rapidity, between light and darkness by means of its
thermal sensations, the pigment spots in the skin would be rendered
permanent by natural selection, while the nerves in that region would by
the same agency be rendered more and more specialized as organs adapted
to perceive changes of temperature, until from the stage of responding
to the thermal rays of the non-luminous spectrum alone, they become
capable of responding also to luminous.

So much, then, for the first consideration which serves to invalidate
the Duke's premiss. The second consideration is, that very often an
organ which began by being useful for the performance of one function,
after having been fully developed for the performance of that function,
finds itself, so to speak, accidentally fitted to the performance of
some other and even more important function, which it thereupon begins
to discharge, and so to undergo a new course of adaptive development. In
such cases, and so far as the new function is concerned, the difficulty
touching the first inception of an organ does not apply; for here the
organ has already been built up by natural selection for one purpose,
before it begins to discharge the other. As an example of such a case we
may take the lung of an air-breathing animal. Originally the lung was a
swim-bladder, or float, and as such it was of use to the aquatic
ancestors of terrestrial animals. But as these ancestors gradually
became more and more amphibious in their habits, the swim-bladder began
more and more to discharge the function of a lung, and so to take a
wholly new point of departure as regards its developmental history. But
clearly there is here no difficulty with regard to the inception of its
new function, because the organ was already well developed for one
purpose before it began to serve another. Or, to take only one
additional example, there are few structures in the animal kingdom so
remarkable in respect of adaptation as is the wing of a bird or a bat;
and at first sight it might well appear that a wing could be of no
conceivable use until it had already acquired enormous proportional
dimensions, as well as an immense amount of special elaboration as to
its general form, size of muscle, amount of blood-supply, and so on.
For, obviously, not until it had attai