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 
