CO-ORDINATED STRUCTURES 63 



Two blind boys examined by Spencer were both found 

 to be able to distinguish with the tips of their fingers 

 points separated by only ^ inch. And two skilled 

 compositors could both distinguish in this way points 

 placed no more than T \ inch apart, so that a person 

 with a trained sense of touch acquires a considerably 

 finer development of this faculty than an ordinary 

 individual. 



If, then, acquired characters of this kind are in- 

 herited, even to an extremely minute extent, such as 

 would be scarcely perceptible in a single generation, 

 the account of the origin of the observed phenomena 

 would be complete. 



As a second argument, Herbert Spencer points out 

 the difficulty of accounting for the development of 

 co-ordinated sets of structures by the action of natural 

 selection upon separate minute variations of the several 

 parts concerned. 



The enormous horns of the ancient Irish elk, weigh- 

 ing in some cases over a hundredweight, required 

 specially strong neck muscles, bones, and ligaments, 

 and strong fore legs for their support. But an increase 

 in the strength of a single muscle following increased 

 weight of the horns would be useless if unaccompanied 

 by a corresponding increase in many other structures, 

 and, if useless, could not be selected. The chance of 

 all the parts concerned varying simultaneously in a 

 corresponding direction is very small if these variations 

 are really independent, and the chance of their doing 

 so repeatedly is in such a case infinitesimal. 



Let us take another case of a similar nature. The 



