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 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 



