354 Darwin, and after Darwin. 
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 fur 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 
