356 Darwin, and after Darwin. 



limbs, and thus allowing the skin-extensions to act 

 after the manner of a parachute. Here, of course, we 

 have not yet got a wing, any more than we have in 

 the case of the flying-fish ; but we have the founda- 

 tions laid for the possible development of a future wing, 

 upon a somewhat similar plan as that which has been 

 so wonderfully perfected in the case of bats. And 

 through all the stages of progressive expansion which 

 the skin of the squirrel has undergone, the expansion 

 has been of use, even though it has not yet so much 

 as begun to acquire the distinctive functions of a wing. 

 Here, then, there is obviously nothing " prophetic " in 

 the matter, any more than there was in the case of the 

 swim-bladder and the lung, or in that of the nerve- 

 ending and the eye. In short, it is the business .of 

 natural selection to secure the highest available degree 

 of adaptation for the time being ; and, in doing this, 

 it not unfrequently happens that an extreme develop- 

 ment of a structure in one direction (produced by 

 natural selection for the sake of better and better 

 adapting the structure to perform some particular 

 function) ends by beginning to adapt it to the perform- 

 ance of some other function. And, whenever this 

 happens to be the case, natural selection forthwith 

 begins to act upon the structure, so to speak, from a 

 new point of departure. 



So much, then, for the Duke's premiss namely, 

 that " every modification of structure must have been 

 functionless at first, when it began to appear." This 

 premiss is clearly opposed to observable fact But 

 now, the second position is that, even if this were not 

 so, the Duke's conclusion would not follow. This 

 conclusion, it will be remembered, is, that if incipient 



